Publications
2021

(2021) Progress in Optics. 66, Chapter 4, p. 131170 Abstract
During the evolution of coupled nonlinear oscillators on a lattice, with dynamics dictated by the discrete nonlinear Schrödinger equation (DNLSE systems), two quantities are conserved: system energy (Hamiltonian) and system density (number of particles). If the number of system oscillators is large enough, a significant portion of the array can be considered to be an “open system,” in intimate energy and density contact with a “bath”—the rest of the array. Thus, as indicated in previous works, the grand canonical formulation can be exploited in order to determine equilibrium statistical properties of thermalized DNLSE systems. In this work, given the values of the two conserved quantities, we have calculated the necessary values of the two Lagrange parameters (typically designated β, μ) associated with the grand canonical partition function in two different ways. One is numerical and the other is analytic, based on a published approximate entropy expression. In addition, we have accessed a purposelyderived approximate PDF expression of sitedensities. Applying these mathematical tools, we have generated maps of temperatures, chemical potentials, and field correlations for DNLSE systems over the entire thermalization zone of the DNLSE phase diagram, subjected to all systemnonlinearity levels. The end result is a rather complete picture, characterizing equilibrated large DNLSE systems.

(2021) Progress in Optics. 66, Chapter 3, p. 91130 Abstract
The dynamics of coupled nonlinear oscillator systems is often described by the classical discrete nonlinear Schrödinger equation (DNLSE). In its simplest version, the DNLSE is made up of two terms—a nearestneighbor hopping term and an onsite cubic nonlinear term. Each of the terms is preceded by a coefficient that can take on either a positive or a negative sign. Each of the DNLSE versions is derived from a corresponding equivalent Hamiltonian. The result is a small family of four versions of the DNLSE Hamiltonian, each with its own associated ground state, all indeed scattered in myriad of scientific publications.
Here we present a comprehensive picture for the ground states of DNLSE systems, summarize existing results and provide new insights.
First we classify the four DNLSE Hamiltonians into two pairs according to the sign of the nonlinear term—a “positive/negative Hamiltonian pair” if the sign of the nonlinear term is positive/negative respectively. Ground states of the positive Hamiltonian pair are discrete plane waves in either a ferromagneticlike or an antiferromagneticlike configuration, depending on the sign of the hopping term. Ground states of the negative Hamiltonian pair are either unstaggered or staggered sitecentered discrete breathers.
The instantaneous state of a DNLSE system is described by a set of oneparameter complex functions each with its own amplitude and phase. We show that except for the sign of the phase, a ground state associated with a positive/negative Hamiltonian is the maximum energy state associated with the signreversed (negative/positive) Hamiltonian.
Next we discuss some properties of the ground states associated with the positiveHamiltonian pair—entropy, temperature, correlations and stability. We extend our ground state stability discussion to include excited plane waves. We propose to engineer a specific perturbation that preserves both density and energy—the two conserved quantities of a DNLSE system—and to test plane wave's stability based on entropy change. We show that under such conservedquantitiespreserved perturbation, all excited plane waves are entropyunstable.
For sitecentered discrete breathers—the ground states of the negativeHamiltonian pair—we have divided system nonlinearity into two ranges and wrote very good analytic approximations for the breathers in each range.
Lastly, in a dedicated section, we very briefly discuss the specific implementation of the DNLSE in the fields of magnetism, optics, and ultracold atoms, emphasizing ground states. For example, following a 2002 article, we show that the dynamics of a 1d opticallytrapped ultracold bosonic atoms, in a rather wide range of system densities and system nonlinearities, can be described by a particular version of the herediscussed classical DNLSEs.
2020

(2020) Optica. 7, 8, p. 864871 Abstract
Efforts to understand the physics of rogue waves have motivated the study of mechanisms that produce rare, extreme events, often through analogous optical setups. As many studies have reported nonlinear generation mechanisms, recent work has explored whether optical rogue events can be produced in linear systems. Here we report the observation of linear rogue events with tunable height, generated from light imprinted with a nonMarkovian wavefront. Moreover, if the nonMarkovian wavefront is allowed to propagate through a nonlinear medium, extraordinarily longtailed intensity distributions are produced, which do not conform to the statistics previously observed in optical rogue wave experiments

(2020) Nature Physics. 16, 3, p. 328333 Abstract[All authors]
Echoes occur in many physical systems, typically in inhomogeneously broadened ensembles of nonlinear objects. They are often used to eliminate the effects of dephasing caused by interactions with the environment as well as to enable the observation of proper, inherent object properties. Here, we report the experimental observation of quantum wavepacket echoes in a single, isolated molecule. The entire dephasingrephasing cycle occurs without any inhomogeneous spread of molecular properties, or any interaction with the environment, and offers a way to probe the internal coherent dynamics of single molecules. In our experiments, we impulsively excite a vibrational wave packet in an anharmonic molecular potential and observe its oscillations and eventual dispersion with time. A second, delayed pulse gives rise to an echoa partial recovery of the initial coherent oscillations. The vibrational dynamics of single molecules is visualized by a timedelayed probe pulse dissociating them, one at a time. Two mechanisms for the echo formation are discussed: a.c. Starkinduced molecular potential shaking and creation of a depletioninduced 'hole' in the nuclear spatial distribution. The singlemolecule wavepacket echoes may lead to the development of new tools for probing ultrafast intramolecular processes in various molecules.Following an impulsive laser excitation of a single molecule, a dispersed vibrational wavepacket is partially rephased by a second pulse, and a wavepacket echo is observed. This wavepacket echo probes ultrafast intramolecular processes in the isolated molecule.

(2020) Optics Express. 28, 3, p. 38033810 Abstract
Low frequency Raman spectroscopy resolves the slow vibrations resulting from collective motions of molecular structures. This frequency region is extremely challenging to access via other multidimensional methods such as 2DIR. In this paper, we describe a new scheme which measures 2D Raman spectra in the low frequency regime. We separate the pulse into a spectrally shaped pump and a transformlimited probe, which can be distinguished by their polarization states. Low frequency 2D Raman spectra in liquid tetrabromoethane are presented, revealing coupling dynamics at frequencies as low as 115 cm−1. The experimental results are supported by numerical simulations which replicate the key features of the measurement. This method opens the door for the deeper exploration of vibrational energy surfaces in complex molecular structures.
2019

(2019) Adv. Opt. Photon.. 11, 4, p. 828891 Abstract
Since the development of laser light sources in the early 1960s, laser beams are everywhere. Laser beams are central in many industrial applications and are essential in ample scientific research fields. Prime scientific examples are optical trapping of ultracold atoms, optical levitation of particles, and laserbased detection of gravitational waves. Mathematically, laser beams are well described by Gaussian beam expressions. Rather well covered in the literature to date are basic expressions for scalar Gaussian beams. In the past, however, higher accuracy mathematics of scalar Gaussian beams and certainly highaccuracy mathematics of vectorial Gaussian beams were far less studied. The objective of the present review then is to summarize and advance the mathematics of vectorial Gaussian beams. When a weakly diverging Gaussian beam, approximated as a linearly polarized twocomponent plane wave, say (Ex,By), is tightly focused by a highnumericalaperture lens, the wave is x201C;depolarized.x201D; Namely, the prelens (practically) missing electric field Ey,Ez components suddenly appear. This is similar for the prelens missing Bx,Bz components. In fact, for any divergence angle (x03B8;dlt;1), the ratio of maximum electric field amplitudes of a Gaussian beam Ex:Ez:Ey is roughly 1:x03B8;d2:x03B8;d4. It follows that if a research case involves a tightly focused laser beam, then the case analysis calls for the mathematics of vectorial Gaussian beams. Gaussianbeamlike distributions of the six electricx2013;magnetic vector field components that nearly exactly satisfy Maxwellx2019;s equations are presented. We show that the nearfield distributions with and without evanescent waves are markedly different from each other. The herepresented nearly exact six electricx2013;magnetic Gaussianbeamlike field components are symmetric, meaning that the crosssectional amplitude distribution of Ex(x,y) at any distance (z) is similar to the By(x,y) distribution, Ey(x,y) is similar to Bx(x,y), and a 90x00B0; rotated Ez(x,y) is similar to Bz(x,y). Componentsx2019; symmetry was achieved by executing the steps of an outlined symmetrization procedure. Regardless of how tightly a Gaussian beam is focused, its divergence angle is limited. We show that the fullcone angle to full width at halfmaximum intensity of the dominant vector field component does not exceed 60x00B0;. The highest accuracy field distributions to date of the less familiar higherorder Hermitex2013;Gaussian vector components are also presented. Hermitex2013;Gaussian EB vectors only approximately satisfy Maxwellx2019;s equations. We have defined a Maxwellx2019;sresidual power measure to quantify the approximation quality of different vector sets, and each set approximately (or exactly) satisfies Maxwellx2019;s equations. Several vectorial x201C;applications,x201D; i.e., research fields that involve vector laser beams, are briefly discussed. The mathematics of vectorial Gaussian beams is particularly applicable to the analysis of the physical systems associated with such applications. Two userfriendly x201C;Mathematicax201D; programs, one for computing six highaccuracy vector components of a Hermitex2013;Gaussian beam, and the other for computing the six practically Maxwellx2019;sequationssatisfying components of a focused laser beam, supplement this review.

(2019) Optics Letters. 44, 15, p. 36373640 Abstract
Coherent antiStokes Raman scattering (CARS) has found wide applications in biomedical research. Compared with alternatives, singlebeam CARS is especially attractive at low frequencies. Yet, currently existing schemes necessitate a relatively complicated setup to perform highresolution spectroscopy. Here we show that the spectral sharp edge formed by an ultrasteep longpass filter is sufficient for performing CARS spectroscopy, simplifying the system significantly. We compare the sensitivity of the presented methodology with available counterparts both theoretically and experimentally. Importantly, we show that this method, to the best of our knowledge, is the simplest and most suitable for vibrational imaging and spectroscopy in the very lowfrequency regime (

(2019) Optics Express. 27, 15, p. 2177921787 Abstract
We demonstrate focusing and imaging through a scattering medium without access to the fluorescent object by using wavefront shaping. Our concept is based on utilizing the spatial fluorescence contrast which naturally exists in the hidden target object. By scanning the angle of incidence of the illuminating laser beam and maximizing the variation of the detected fluorescence signal from the object, as measured by a bucket detector at the front of the scattering medium, we are able to generate a tightly focused excitation spot. Thereafter, an image is obtained by scanning the focus over the object within the memory effect range. The requirements for applicability of the method and the comparison with specklecorrelation based focusing methods are discussed. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

(2019) StatPhys 27 Conference. Abstract
The evolution dynamics of a large number of physical systems can be approximated by the discrete nonlinear Schrödinger equation (DNLSE). These include coupled springmass oscillators, Coupled pendulums, LC electrical circuits, polymer chains, trapped ultracold atoms, optical waveguides, and more. Typically the DNLSE is applied to a slowlyvarying envelope – a large set of discrete complex amplitudes multiplying identical local wavefunctions.Two constants of motion characterize the DNLSE – the system’s energy (Hamiltonian) and the system’s density (number of particles). Siteaveraged values of these two quantities place the system as a point on a siteaveraged energydensity phase diagram. It turns out that systems on a welldefined area of the phase diagram thermalize. This thermalization property allows the application of statistical mechanics methods to predict equilibrium thermodynamic parameters of systems on the “thermalization zone” [1]. For example – their equilibrium temperatures.A first paper showing a map of temperatures of systems on the high nonlinearity part of the thermalization zone was already published [2]. We calculated these temperatures through suitably defined temperatureentropy relations.Since then we have extended our temperature analysis of DNLSEgoverned systems to the entire thermalization zone, including the low nonlinearity portion. This was done by adopting a mathematically far more challenging yet rigorous approach – derivations through a suitably defined partition function [1]. Equilibrium distributions of sitedensities are numerically calculated through the properties of a transfer integral operator associated with the partition function. The partitionfunction approach then allows the association of an equilibrium temperature to an equilibrium probability distribution of site densities for thermalized systems at high as well as at low nonlinearities.Indeed we find somewhat unexpected thermodynamic behavior of low nonlinearity systems. For example  very pronounced rise of temperatures everywhere on the thermalization zone, breaking the variancetemperature relations. Not including the zerotemperature ground state (equal amplitudes alternating signs) that stays at zero temperature at all nonlinearities.[1] Rasmussen, K. Ø., T. Cretegny, P. G. Kevrekidis, and Niels GrønbechJensen, "Statistical mechanics of a discrete nonlinear system." Physical review letters 84, no. 17 (2000): 3740.[2] Levy Uri, and Yaron Silberberg, "Equilibrium temperatures of discrete nonlinear systems." Physical Review B 98, no. 6 (2018): 060303.

(2019) Optica. 6, 6, p. 753757 Abstract
Entangled coherent states are a fundamentally interesting class of quantum states of light, with important implications in quantum information processing, for which robust schemes to generate them are required. Here, we show that entangled coherent states emerge, with high fidelity, when mixing coherent and squeezed vacuum states of light on a beam splitter. These maximally entangled states, where photons bunch at the exit of a beam splitter, are measured experimentally by Fockstate projections. Entanglement is examined theoretically using a Belltype nonlocality test and compared with ideal entangled coherent states. We experimentally show nearly perfect similarity with entangled coherent states for an optimal ratio of coherent and squeezed vacuum light. In our scheme, entangled coherent states are generated deterministically with small amplitudes, which could be beneficial, for example, in deterministic distribution of entanglement over long distances.

(2019) Nature Photonics. 13, 2, p. 116122 Abstract
The principles of quantum optics have yielded a plethora of ideas to surpass the classical limitations of sensitivity and resolution in optical microscopy. While some ideas have been applied in proofofprinciple experiments, imaging a biological sample has remained challenging, mainly due to the inherently weak signal measured and the fragility of quantum states of light. In principle, however, these quantum protocols can add new information without sacrificing the classical information and can therefore enhance the capabilities of existing superresolution techniques. Image scanning microscopy, a recent addition to the family of superresolution methods, generates a robust resolution enhancement without reducing the signal level. Here, we introduce quantum image scanning microscopy: combining image scanning microscopy with the measurement of quantum photon correlation allows increasing the resolution of image scanning microscopy up to twofold, four times beyond the diffraction limit. We introduce the QISM principle and obtain superresolved optical images of a biological sample stained with fluorescent quantum dots using photon antibunching, a quantum effect, as a resolutionenhancing contrast mechanism.

(2019) Physical Review Letters. 122, 1, 010404. Abstract
The theory of Hawking radiation can be tested in laboratory analogues of black holes. We use light pulses in nonlinear fiber optics to establish artificial event horizons. Each pulse generates a moving perturbation of the refractive index via the Kerr effect. Probe light perceives this as an event horizon when its group velocity, slowed down by the perturbation, matches the speed of the pulse. We have observed in our experiment that the probe stimulates Hawking radiation, which occurs in a regime of extreme nonlinear fiber optics where positive and negative frequencies mix.

(2019) OPTICAL, OPTOATOMIC, AND ENTANGLEMENTENHANCED PRECISION METROLOGY. Scheuer J. & Shahriar SM.(eds.). (trueProceedings of SPIE). Abstract
Technological advancements in the creation, manipulation and detection of quantum states of light have motivated the application of such states to overcome classical limits in sensing and imaging. In particular, there has been a surge of recent interest in superresolution imaging based on principles of quantum optics. However, the application of such schemes for practical imaging of biological samples is demanding in terms of signaltonoise ratio, speed of acquisition and robustness with respect to sample labeling. Here, we reintroduce the concept of quantum image scanning microscopy (QISM), a superresolution method that enhances the classical image scanning microscopy (ISM) method by measuring photon correlations. QISM was already utilized to achieve superresolved images of a biological sample labeled with fluorescent nanoscrystals whose contrast is based entirely on a quantum optical phenomenon, photon antibunching. We present here an experimental demonstration of the method and discuss with further details its prospects for application in life science microscopy.

(2019) Physical Review A. 99, 1, 013825. Abstract
We report an experimental demonstration of complementarity between path information and interference in secondorder correlation with thermal light. The key apparatus is a Sagnac interferometer in which an offaxis slit is inserted. Two orthogonal thermal light beams travel through the slit clockwise and anticlockwise in the interferometer. The secondorder interference fringes vanish when the path information is acquired. The interference fringes are revived when the path information is effaced.

(2019) Optica. 6, 1, p. 5255 Abstract
Vibrational spectroscopic imaging is useful and important in biological and medical studies. Yet, vibrational imaging in the terahertz region (
2018

(2018) Optics Express. 26, 17, p. 2220822217 Abstract
In recent years, wavefront shaping has been utilized to control and correct distorted light for enhancing a bright spot, generation of a Bessel beam or darkening a complete area at the output of a scattering system. All these outcomes can be thought of as enhancing a particular mode of the output field. In this letter, we study the relation between the attainable enhancement factor, corresponding to the efficiency of mode conversion, and the field distribution of the target mode. Working in the limit of a thin diffuser enables not only a comparison between experimental and simulated results, but also allows for derivation of an analytic formula. These results shed light on the ability to use a scattering medium as a mode converter and on the relationship between the desired shape and the efficiency. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

(2018) Physical Review B. 98, 6, 060303. Abstract
During the evolution of discrete nonlinear systems with dynamics dictated by the discrete nonlinear Schrodinger equation, two quantities are conserved: system energy (Hamiltonian) and system density (number of particles). It is then possible to analyze system evolution in relation to an energydensity phase diagram. Previous works have identified a "thermalization zone" on the phase diagram where regular statistical mechanics methods apply. Based on these statistical mechanics methods we have now assigned a specific equilibrium temperature to every point of the thermalization zone. Temperatures were derived in the grand canonical picture through an entropytemperature relation, modified to suit the nonlinear lattice systems. Generally, everywhere in the thermalization zone of the phase diagram, temperatures along a fixed systemdensity line, grow monotonously from zero to infinity. Isotherms on the phase diagram are concave.

(2018) APL Photonics. 3, 9, 092501. Abstract
We present a new method for the measurement of the stimulated Raman spectrum based on timedependent spatial modulation of a laser beam as it passes through a Raman active medium. This effect is similar to the instantaneous Kerr lensing and Kerr deflection yet involves resonant vibrations which result in a timedependent refractive index change. We use subnanojoule pulses together with a sensitive pumpprobe measurement apparatus to excite and detect the fine (10(5)10(4)) temporal and spatial variations in intensity resulting from the Ramaninduced Kerr effect. We demonstrate the effect by changing the spatial overlap between the pump and probe at the sample and measuring the timedependent deformation of the probe beam's cross section. This method is particularly useful for detection of lowfrequency Raman lines, as we demonstrate by measuring the Raman spectrum of neat liquids in a cuvette.

(2018) Optica. 5, 2, p. 204207 Abstract
Light scattering due to interaction with a material has long been known to create speckle patterns. We have demonstrated that even though speckle patterns from different objects are very similar, they contain minute dissimilarities that can be used to differentiate between the originating scatterers. We first approached this problem using a convolutional neural networka deep learning algorithmto show that indeed specific speckle patterns can be linked to the respective materials creating them. We then progressed to use recorded speckle patterns created from different materials in order to measure statistical parameters that possess a welldefined physical meaning. Using these parameters gave similar scatterer recognition abilities while gaining insight on the physical reasons for these materialdependent statistical deviations. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Impulsive Raman spectroscopy via precision measurement of frequency shift with low energy excitation
Stimulated Raman scattering (SRS) has recently become useful for chemically selective bioimaging. It is usually measured via modulation transfer from the pump beam to the Stokes beam. Impulsive stimulated Raman spectroscopy, on the other hand, relies on the spectral shift of ultrashort pulses as they propagate in a Raman active sample. This method was considered impractical with low energy pulses since the observed shifts are very small compared to the excitation pulse bandwidth, spanning many terahertz. Here we present a new apparatus, using tools borrowed from the field of precision measurement, for the detection of lowfrequency Raman lines via stimulatedRamanscatteringinduced spectral shifts. This method does not require any spectral filtration and is therefore an excellent candidate to resolve lowlying Raman lines (

(2018) Journal of Physics B: Atomic, Molecular and Optical Physics. 51, 3, 035401. Abstract
The evolution of random fields with known statistical properties is relatively straightforward to analyze in the linear regime, but becomes considerably more involved when nonlinearity, or interactions, are dominant. Previous works have shown that statistical physics techniques can be applied to predict the evolution of such systems. Here we study the evolution of random fields in a onedimensional lattice of optical waveguides in the presence of strong nonlinearities, using the discrete nonlinear Schrödinger equation. Extending the 2009 work by Silberberg et al (Phys. Rev. Lett. 102 233904), we assume input fields with random amplitudes and phases. We derive analytic expressions for the system's statistical properties at thermodynamic equilibrium. Specifically, expressions for the probability density functions of field intensities, of fields' phase differences, and an expression for the field correlations. We express these properties in terms of the moments of the assumed statistical excitations, and verify the results with simulations. Most interestingly, we find that at thermodynamic equilibrium, correlations are formed through the interaction between sites. These exponentially decaying fields' correlations take a universal form that is essentially independent of excitation amplitudes but visibly shrink with increased spread of the exciting amplitudes. Our results are valid not only to nonlinear discrete optical systems, but extend also to the evolution of bosonic atoms in optical lattices in the highoccupancy limit that are governed by the equivalent GrossPitaevskii equation.

(2018) J. Phys. B: At. Mol. Opt. Phys.. 51, p. 035401 Abstract
The evolution of random fields with known statistical properties is relatively straightforward to analyze in the linear regime, but becomes considerably more involved when nonlinearity, or interactions, are dominant. Previous works have shown that statistical physics techniques can be applied to predict the evolution of such systems. Here we study the evolution of random fields in a onedimensional lattice of optical waveguides in the presence of strong nonlinearities, using the discrete nonlinear Schrödinger equation. Extending the 2009 work by Silberberg et al (Phys. Rev. Lett. 102 233904), we assume input fields with random amplitudes and phases. We derive analytic expressions for the system's statistical properties at thermodynamic equilibrium. Specifically, expressions for the probability density functions of field intensities, of fields' phase differences, and an expression for the field correlations. We express these properties in terms of the moments of the assumed statistical excitations, and verify the results with simulations. Most interestingly, we find that at thermodynamic equilibrium, correlations are formed through the interaction between sites. These exponentially decaying fields' correlations take a universal form that is essentially independent of excitation amplitudes but visibly shrink with increased spread of the exciting amplitudes. Our results are valid not only to nonlinear discrete optical systems, but extend also to the evolution of bosonic atoms in optical lattices in the highoccupancy limit that are governed by the equivalent Gross–Pitaevskii equation.
2017

(2017) Light: Science and Applications. 6, 12, 17115. Abstract
Twodimensional (2D) spectroscopy is used to study the interactions between energy levels in both the field of optics and nuclear magnetic resonance (NMR). Conventionally, the strength of interaction between two levels is inferred from the value of their common offdiagonal peak in the 2D spectrum, which is termed the cross peak. However, stronger diagonal peaks often have long tails that extend into the locations of the cross peaks and alter their values. Here, we introduce a method for retrieving the true interaction strengths by using sparse signal recovery techniques and apply our method in 2D Raman spectroscopy experiments.

(2017) Optics Express. 25, 23, p. 2820128209 Abstract
Coherent antiStokes Raman scattering (CARS) spectroscopy and microscopy have many potential applications in biology and medicine. Among many variants of the technique itself, the method of singlepulse CARS spectroscopy and microscopy is attractive for its simplicity and quick implementation. Singlepulse CARS microscopy can be performed by shaping the excitation spectrum using a notch filter, yet the resonant signal rides on a large background caused by a nonresonant signal, a background which is usually removed by lockin detection. Here, we show that the background can be reduced significantly by adding a small chirp to the pulse and can even be made smaller than the resonant signal. In order to enhance the CARS signal and thus the contrast further, doublenotch shaping is introduced. The doublenotch induces two sets of CARS features shifted by the frequency difference between the two notches, thereby coherently enhancing a particular CARS feature. The experimental results agree well with theoretical simulations. We applied this scheme to perform lockin free CARS microscopy of bone tissue with enhanced contrast. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

(2017) Physical Review A. 96, 5, Abstract
We investigate a quasionedimensional periodic array of coupled waveguides, with one extended and one bound dimension, incorporating both firstand secondorder coupling. We study the evolution of optical fields in this system, and measure quantum correlations when pathentangled photon pairs are launched into them. We observe a surprisingly large and nontrivial effect of secondorder coupling on these correlationswhile quantum correlations are symmetric when only firstorder coupling is present, the introduction of nextnearestneighbor coupling often breaks the symmetry to reflections.

(2017) Optica. 4, 9, p. 10731079 Abstract
Wavefront shaping is a powerful technique that can be used to focus light through scattering media, which can be important for imaging through scattering samples such as tissue. This method is based on the assumption that the field at the output of the medium is a linear superposition of the modes traveling through different paths in the medium. However, when the scattering medium also exhibits nonlinearity, as may occur in multiphoton microscopy, this assumption is violated and the applicability of wavefront shaping becomes unclear. Here, using a simple model system with a scattering layer followed by a nonlinear layer, we show that with adaptive optimization of the wavefront, light can still be controlled and focused through a scattering medium in the presence of nonlinearity. Notably, we find that moderate positive nonlinearity can serve to significantly increase the focused fraction of power, whereas negative nonlinearity reduces it. (C) 2017 Optical Society of America

(2017) 2017 CONFERENCE ON LASERS AND ELECTROOPTICS (CLEO). p. 12 (trueConference on Lasers and ElectroOptics). Abstract
We present a method that utilizes quantum correlation measurements for multiemitter subdiffraction localization in a timedependent scene. This is demonstrated using a newly developed imaging configuration based on fiber bundle coupled singlephoton avalanche detectors.

(2017) Nature Communications. 8, 14786. Abstract
Despite advances in lowlightlevel detection, singlephoton methods such as photon correlation have rarely been used in the context of imaging. The few demonstrations, for example of subdiffractionlimited imaging utilizing quantum statistics of photons, have remained in the realm of proofofprinciple demonstrations. This is primarily due to a combination of low values of fill factors, quantum efficiencies, frame rates and signaltonoise characteristic of most available singlephoton sensitive imaging detectors. Here we describe an imaging device based on a fibre bundle coupled to singlephoton avalanche detectors that combines a large fill factor, a high quantum efficiency, a low noise and scalable architecture. Our device enables localizationbased superresolution microscopy in a nonsparse nonstationary scene, utilizing information on the number of active emitters, as gathered from nonclassical photon statistics.

(2017) Journal Of The Optical Society Of America AOptics Image Science And Vision. 34, 3, p. 331334 Abstract
The always divergingconverging laser beams, more rigorously referred to as Gaussian beams, are part of many physical and electrooptical systems. Obviously, a single set of analytic expressions describing these beams in a large span of divergenceconvergence angles at the focal plane, and at any distance away from the focal plane, will prove very handy. We have recently published three such analytic sets, one set for linearly polarized beams and two sets for radially polarized beams. However, our published analytic set for linearly polarized beams describes nonsymmetric electricmagnetic field components. Specifically, the strong transverse magnetic field component does not become elliptic at very large divergence angles as it should be, and the other transverse magnetic component, indeed very weak, is missing altogether. Here we present an analytic set of expressions symmetrically describing linearly polarized Gaussian beams. The symmetry applies to the xelectric ymagnetic components and vice versa and to the two electricmagnetic zcomponents. An important property of the presented set of expressions is power conservation. That is, the electromagnetic power crossing a plane transverse to the propagation direction in a unit time is conserved. Power conservation assures beam description accuracy at any axial distance. The presented analytic expressions, although not strictly satisfying Maxwell's equations, describe Gaussian beams with very reasonable accuracy from low divergence angles up to divergence angles as large as 0.8 rad in a medium with refractive index of 1.5, i.e., up to a NA of 1.1. These expressions should then readily assist in the design of practically all laserrelated systems and in the research of diverse physics and electrooptic fields. (C) 2017 Optical Society of America.

(2017) Light Sci Appl . 6, p. e17115 Abstract
Twodimensional (2D) spectroscopy is used to study the interactions between energy levels in both the field of optics and nuclear magnetic resonance (NMR). Conventionally, the strength of interaction between two levels is inferred from the value of their common offdiagonal peak in the 2D spectrum, which is termed the cross peak. However, stronger diagonal peaks often have long tails that extend into the locations of the cross peaks and alter their values. Here, we introduce a method for retrieving the true interaction strengths by using sparse signal recovery techniques and apply our method in 2D Raman spectroscopy experiments.

(2017) Optics Express . 25, 23 , p. 2820128209 Abstract
Coherent antiStokes Raman scattering (CARS) spectroscopy and microscopy have many potential applications in biology and medicine. Among many variants of the technique itself, the method of singlepulse CARS spectroscopy and microscopy is attractive for its simplicity and quick implementation. Singlepulse CARS microscopy can be performed by shaping the excitation spectrum using a notch filter, yet the resonant signal rides on a large background caused by a nonresonant signal, a background which is usually removed by lockin detection. Here, we show that the background can be reduced significantly by adding a small chirp to the pulse and can even be made smaller than the resonant signal. In order to enhance the CARS signal and thus the contrast further, doublenotch shaping is introduced. The doublenotch induces two sets of CARS features shifted by the frequency difference between the two notches, thereby coherently enhancing a particular CARS feature. The experimental results agree well with theoretical simulations. We applied this scheme to perform lockin free CARS microscopy of bone tissue with enhanced contrast.
2016

(2016) Optica. 3, 10, p. 11041106 Abstract
Scattering of light by random media limits many optical imaging and sensing applications, either by scrambling waves that carry images or by generating glarebackground noise that hinders detection. In recent years, it was shown that the shaping of the wavefront of light before it enters the scattering environment allows an unexpected degree of control over the scattered fields and enables various imaging techniques that are being applied in microscopy and other demanding imaging tasks. Here, we show that similar ideas can be applied to reduce glare and enable imaging under tough conditions. (C) 2016 Optical Society of America

(2016) Journal Of The Optical Society Of America AOptics Image Science And Vision. 33, 10, p. 19992009 Abstract
Analytic expressions describing all vector components of Gaussian beams, linearly polarized as well as radially polarized, are presented. These simple expressions, to high powers in divergence angle, were derived from a singlecomponent vector potential. The vector potential itself, as in the 1979 work of Davis [Phys. Rev. A 19, 1177 (1979)], was approximated by the first two terms of an infinite series solution of the Helmholtz equation. The expressions presented here were formulated to emphasize the dependence of the amplitude of the various field components on the beam's divergence angle. We show that the amplitude of the axial component of a linearly polarized Gaussian beam scales as the divergence angle squared, whereas the amplitude of the crosspolarized component of a linearly polarized Gaussian beam scales as the divergence angle to the fourth power. Weakly diverging Gaussian beams as well as strongly focused Gaussian beams can be described by exactly the same set of mathematical expressions, up to normalization constant. For a strongly focused linearly polarized Gaussian beam, the ellipticity of the dominant electric field component, typically calculated by the DebyeWolf integral, is reproduced. For yet higher accuracy, terms with higher powers in divergence angle are presented, but the inclusion of these terms is limited to low divergence angles and short axial distances. (C) 2016 Optical Society of America

(2016) Nature Photonics. 10, 2, p. 7779 Abstract
The quantum concepts of entanglement and interactionfree measurements are applied to spectroscopy to successfully sense carbon dioxide in air.

(2016) Applied Physics BLasers And Optics. 122, 2, p. 15 25. Abstract
We experimentally demonstrate a method for creating broad bandwidth photon pairs in the visible spectral region, centered at a frequency that is higher than that of the initial pump source. Spontaneous down conversion of a narrowband 1053 nm pulsed Nd:YLF laser is followed by highly efficient upconversion in adiabatic nonlinear frequencyconversion process. Photon pairs are generated from 693 to 708 nm, and the complete conversion process occurs within a single monolithic 5cmlong stoichiometric lithium tantalate nonlinear crystal. We have characterized the dependence of this structure with respect to pump intensity and crystal temperature.

(2016) J. Opt. Soc. Am. A. 33 , 10, p. 19992009


2015

(2015) Optics Express. 23, 21, p. 2779527805 Abstract
Harmonic generation by tightlyfocused Gaussian beams is finding important applications, primarily in nonlinear microscopy. It is often naively assumed that the nonlinear signal is generated predominantly in the focal region. However, the intensity of Gaussianexcited electromagnetic harmonic waves is sensitive to the excitation geometry and to the phase matching condition, and may depend on quite an extended region of the material away from the focal plane. Here we solve analytically the amplitude integral for second harmonic and third harmonic waves and study the generated harmonic intensities vs. focalplane position within the material. We find that maximum intensity for positive wavevector mismatch values, for both second harmonic and third harmonic waves, is achieved when the fundamental Gaussian is focused few Rayleigh lengths beyond the front surface. Harmonicgeneration theory predicts strong intensity oscillations with thickness if the material is very thin. We reproduced these intensity oscillations in glass slabs pumped at 1550nm. From the oscillations of the 517nm thirdharmonic waves with slab thickness we estimate the wavevector mismatch in a Sodalime glass as Delta k(H) = 0.249 mu m(1). (C) 2015 Optical Society of America

(2015) Journal of Physics B: Atomic, Molecular and Optical Physics. 48, 20, 201001. Abstract[All authors]
We demonstrate the ability to control the molecular dissociation rate using femtosecond pulses shaped with thirdorder dispersion (TOD). Explicitly, a significant 50% enhancement in the dissociation yield for the low lying vibrational levels (v similar to 6) of an H2(+) ionbeam target was measured as a function of TOD. The underlying mechanism responsible for this enhanced dissociation was theoretically identified as nonadiabatic alignment induced by the prepulses situated on the leading edge of pulses shaped with negative TOD. This control scheme is expected to work in other molecules as it does not rely on specific characteristics of our testcase H2(+) molecule.

(2015) Physical Review Letters. 115, 13, 133602. Abstract
We measure ensembleaveraged quantum correlations of pathentangled photons, propagating in a disordered lattice and undergoing Anderson localization. These result in intriguing patterns, which show that quantum interference leads to unexpected dependencies of the location of one particle on the location of the other. These correlations are shared between localized and nonlocalized components of the twophoton wave function, and, moreover, yield information regarding the nature of the disorder itself. Such effects cannot be reproduced with classical waves, and are undetectable without ensemble averaging.

(2015) 2015 CONFERENCE ON LASERS AND ELECTROOPTICS (CLEO). p. 1551p (trueConference on Lasers and ElectroOptics). Abstract
We present a singlepulse twodimensional Raman spectroscopy scheme. Our scheme offers not only a major simplification of the conventional setup but also an inherent favoring of the direct fifthorder signal over the cascaded signal, the latter being a signal that carries no coupling information.

(2015) Nature Photonics. 9, 5, p. 339343 Abstract
Vibrational modes are often localized in certain regions of a molecule, and so the coupling between these modes is sensitive to the molecular structure. Twodimensional vibrational spectroscopy can probe the strength of this coupling in a manner analogous to twodimensional NMR spectroscopy, but on ultrafast timescales. Here, we demonstrate how twodimensional Raman spectroscopy, based on fifthorder optical nonlinearity, can be performed with a single beam of shaped femtosecond optical pulses. Our spectroscopy scheme offers not only a major simplification of the conventional setup, but also an inherent elimination of a competing nonlinear signal, which overwhelms the desired signal in other schemes and carries no coupling information.

(2015) Optics Express. 23, 10, p. 1308213098 Abstract[All authors]
Noninvasive labelfree imaging of biological systems raises demand not only for highspeed threedimensional prescreening of morphology over a widefield of view but also it seeks to extract the microscopic functional and molecular details within. Capitalizing on the unique advantages brought out by different nonlinear optical effects, a multimodal nonlinear optical microscope can be a powerful tool for bioimaging. Bringing together the intensitydependent contrast mechanisms via second harmonic generation, third harmonic generation and fourwave mixing for structuralsensitive imaging, and singlebeam/singlepulse coherent antiStokes Raman scattering technique for chemical sensitive imaging in the fingerprint region, we have developed a simple and nearly alignmentfree multimodal nonlinear optical microscope that is based on a single wideband Ti:Sapphire femtosecond pulse laser source. Successful imaging tests have been realized on two exemplary biological samples, a canine femur bone and collagen fibrils harvested from a rat tail. Since the ultrabroad bandwidth femtosecond laser is a suitable source for performing highresolution optical coherence tomography, a widefield optical coherence tomography arm can be easily incorporated into the presented multimodal microscope making it a versatile optical imaging tool for noninvasive labelfree bioimaging. (C) 2015 Optical Society of America

(2015) Journal Of The Optical Society Of America AOptics Image Science And Vision. 32, 4, p. 647653 Abstract
The electric and magnetic components of an electromagnetic wave in free space are believed by many to be perpendicular to each other. We outline a procedure by which electromagnetic potentials are constructed, and we derive freespace nonperpendicular electricmagnetic fields from these potentials. We show, for example, that in freespace Besselrelated fields, at a small region near the origin, the angle between these components spans a range of 7 degrees173 degrees,that is, they are far from being perpendicular. This can be contrasted with plane waves, where, following the same procedure, we verify that the electric field strength (E(x, y, z, t)) and the magnetic flux density (B(x, y, z, t)) are indeed perpendicular to each other and to the direction of propagation. (C) 2015 Optical Society of America

(2015) Physical Review B. 91, 6, 064201. Abstract
Quasiperiodic lattices have recently been shown to be a nontrivial topological phase of matter. Charge pumpingone of the hallmarks of topological states of matterwas recently realized for photons in a onedimensional offdiagonal Harper model implemented in a photonic waveguide array. However, if the relationship between topological pumps and quasiperiodic systems is generic, one might wonder how to observe it in the canonical and most studied quasicrystalline system in one dimensionthe Fibonacci chain. This chain is expected to facilitate a similar phenomenon, yet its discrete nature hinders the experimental study of such topological effects. Here, we overcome this obstacle by utilizing the topological equivalence of a family of quasiperiodic models which ranges from the Fibonacci chain to the Harper model. Implemented in photonic waveguide arrays, we observe the topological properties of this family, and perform a topological pumping of photons across a Fibonacci chain.

(2015) Physical Review Letters. 115, 7, 073901. Abstract
We introduce a simple and flexible method to generate spatially nonMarkovian light with tunable coherence properties in one and two dimensions. The unusual behavior of this light is demonstrated experimentally by probing the far field and by recording its diffraction pattern after a double slit: In both cases we observe, instead of a central intensity maximum, a line or crossshaped dark region, whose width and profile depend on the nonMarkovian coherence properties. Because these properties can be controlled and easily reproduced in experiment, the presented approach lends itself to serving as a test bed to study and gain a deeper understanding of nonMarkovian processes.

(2015) arXiv. 1506.08586,


2014

(2014) Optics Letters. 39, 19, p. 57095712 Abstract
We demonstrate a multimodal optical coherence tomography (OCT) and online Fourier transform coherent antiStokes Raman scattering (FTCARS) platform using a single sub12 femtosecond (fs) Ti: sapphire laser enabling simultaneous extraction of structural and chemical ("morphomolecular") information of biological samples. Spectral domain OCT prescreens the specimen providing a fast ultrahigh (4 x 12 mu m axial and transverse) resolution wide field morphologic overview. Additional complementary intrinsic molecular information is obtained by zooming into regions of interest for fast labelfree chemical mapping with online FTCARS spectroscopy. Backgroundfree CARS is based on a Michelson interferometer in combination with a highly linear piezo stage, which allows for quick pointtopoint extraction of CARS spectra in the fingerprint region in less than 125 ms with a resolution better than 4 cm(1) without the need for averaging. OCT morphology and CARS spectral maps indicating phosphate and carbonate bond vibrations from human bone samples are extracted to demonstrate the performance of this hybrid imaging platform. (C) 2014 Optical Society of America

(2014) Optica. 1, 3, p. 170174 Abstract
Diffractionlimited imaging through complex scattering media is a longsoughtafter goal with important applications in biomedical research. In recent years, highresolution wavefront shaping has emerged as a powerful approach to generate a sharp focus through highly scattering, visually opaque samples. However, it requires a localized feedback signal from the target point of interest, which necessitates an invasive procedure in alloptical techniques. Here, we show that by exploiting optical nonlinearities, a diffractionlimited focus can be formed inside or through a complex sample, even when the feedback signal is not localized. We prove our approach theoretically and numerically, and experimentally demonstrate it with a twophoton fluorescence signal through highly scattering biological samples. We use the formed focus to perform twophoton microscopy through highly scattering, visually opaque layers. (C) 2014 Optical Society of America

(2014) Physical Review Letters. 112, 10, 103604. Abstract
A quantum polarized light microscope using entangled NOON states with N = 2 and N = 3 is shown to provide phase supersensitivity beyond the standard quantum limit. We constructed such a microscope and imaged birefringent objects at a very low light level of 50 photons per pixel, where shot noise seriously hampers classical imaging. The NOON light source is formed by combining a coherent state with parametric downconverted light. We were able to show improved phase images with sensitivity close to the Heisenberg limit.

(2014) Journal of Modern Optics. 61, 10, p. 872876 Abstract
A new coherent antiStokes Raman spectroscopy (CARS) technique is reported for realtime detection and classification of several chemical constituents, utilizing a single detector and a single beam of shaped femtosecond pulses. The technique is based on rapidly switching between differently shaped pulses that either maximize or minimize the targeted vibrational lines excitation, thus creating temporally modulated 'bright' and 'dark' profiles in the total CARS signal that are measured by a single photomultiplier tube and demodulated by a multichannel lockin amplifier. Using a twodimensional spatial light modulator displaying 24 different pulse shapes, we demonstrate pulse shaping at 80 kHz and chemically specific microscopy with pixel dwell times of less than 0.5 ms.

2013

(2013) Nature Photonics. 7, 11, p. 919924 Abstract
Controlling light through dynamically varying heterogeneous media is a soughtafter goal with important applications ranging from freespace communication to nanosurgery. The underlying challenge is to control a large number of degrees of freedom of the optical wavefront, at timescales shorter than the medium dynamics. Many advances have been reported recently following the demonstration of focusing through turbid samples by wavefront shaping, where spatial light modulators with more than 1,000 degrees of freedom were used. Unfortunately, spatial light modulatorbased wavefront shaping requires feedback from a detector or camera and is currently limited to slowly varying samples. Here, we demonstrate a novel approach for wavefront shaping utilizing alloptical feedback. We show that the complex wavefront required to focus light scattered by turbid samples (including thin biological tissues) can be generated at submicrosecond timescales by the process of field selforganization inside a multimode laser cavity, without requiring electronic feedback, spatial light modulators or phaseconjugation crystals.


(2013) Optics Express. 21, 20, p. 2401524024 Abstract
We present a scheme for recovering the complex input field launched into a waveguide array, from partial measurements of its output intensity, given advance knowledge that the input is sparse. In spite of the fact that in general the inversion problem is illconditioned, we demonstrate experimentally and in simulations that the prior knowledge of sparsity helps overcome the loss of information. Our method is based on GESPAR, a recently proposed efficient phase retrieval algorithm. Possible applications include optical interconnects and quantum state tomography, and the ideas are extendable to other multiple input and multiple output (MIMO) communication schemes. (C)2013 Optical Society of America

(2013) New Journal of Physics. 15, Abstract
We report the experimental observation and theoretical analysis of a novel beamsteering effect in periodic waveguide arrays that arises from the interplay between discrete diffraction, Kerr nonlinearity and any mechanism that effectively weakens the nonlinear part of the beam. In this regime the propagation direction shows increased sensitivity to the input angle and for a certain angular range around normal incidence a nonlinear beam may be guided to a direction opposite to that initially inserted. For continuous wave beams the role of this mechanism is played by absorption of any kind, such as three photon absorption, two photon absorption or even linear absorption. For pulsed beams we show that the same dynamics can arise due to strong normal temporal dispersion, while absorption is not necessary and can be a further enhancing or alternative factor. This observation falls under a more general dissipationassisted beam velocity control mechanism in nonlinear optical lattices, which is also theoretically predicted by the effective particle approach.

(2013) Nature Photonics. 7, 3, p. 197204 Abstract
Over the past decade, the Anderson localization of light and a wide variety of associated phenomena have come to the forefront of research. Numerous investigations have been made into the underlying physics of how disorder affects transport in a crystalline lattice incorporating disorder. The physics involved relies on the analogy between the paraxial equation for electromagnetic waves and the Schrodinger equation describing quantum phenomena. Experiments have revealed how wavefunctions evolve during the localization process, and have led to discoveries of new physics that are universal to wave systems incorporating disorder. This Review summarizes the phenomena associated with the transverse localization of light, with an emphasis on the history, new ideas and future exploration of the field.

(2013) Physical Review Letters. 110, 7, Abstract
Topological insulators and topological superconductors are distinguished by their bulk phase transitions and gapless states at a sharp boundary with the vacuum. Quasicrystals have recently been found to be topologically nontrivial. In quasicrystals, the bulk phase transitions occur in the same manner as standard topological materials, but their boundary phenomena are more subtle. In this Letter we directly observe bulk phase transitions, using photonic quasicrystals, by constructing a smooth boundary between topologically distinct onedimensional quasicrystals. Moreover, we use the same method to experimentally confirm the topological equivalence between the Harper and Fibonacci quasicrystals. DOI: 10.1103/PhysRevLett.110.076403

(2013) CLEO. (trueConference on Lasers and ElectroOptics). Abstract
We present a scheme for recovering the input signal launched into a waveguide array from partial measurements of its output intensity, given that the input is sparse. Possible applications include optical interconnects, and quantum tomography.
2012

(2012) Optics Letters. 37, 22, p. 46634665 AbstractPolarization control of multiply scattered light through random media by wavefront shaping
We show that the polarization state of coherent light propagating through an optically thick multiple scattering medium can be controlled by wavefront shaping, that is, by controlling only the spatial phase of the incoming field with a spatial light modulator. Any polarization state of light at any spatial position behind the scattering medium can be attained with this technique. Thus, transforming the random medium to an arbitrary optical polarization component becomes possible. (C) 2012 Optical Society of America

(2012) Physical Review A. 86, 4, 043418. Abstract[All authors]
We present a method to control photodissociation by manipulating the bondsoftening mechanism occurring in strong shaped laser fields, namely by varying the chirp sign and magnitude of an ultrashort laser pulse. Manipulation of bond softening is experimentally demonstrated for strongfield (10121013 W/cm2) photodissociation of H2+, exhibiting a substantial increase of dissociation by positively chirped pulses with respect to both negatively chirped and transformlimited pulses. The measured kinetic energy release and angular distributions are used to quantify the degree of dissociation control. The control mechanism is attributed to the interplay of dynamic alignment and chirped light induced potential curves.

(2012) Physical Review Letters. 109, 10, Abstract
Quantum lithography achieves phase superresolution using fragile, experimentally challenging entangled states of light. We propose a scalable scheme for creating features narrower than classically achievable with reduced use of quantum resources and, consequently, enhanced resistance to loss. The scheme is an implementation of interferometric lithography using a mixture of a spontaneous parametric downconverted entangled state with intense classical coherent light. We measure coincidences of up to four photons mimicking multiphoton absorption. The results show a narrowing of the interference fringes of up to 30% with respect to the best analogous classical scheme using only 10% of the nonclassical light required for creating NOON states.

(2012) Physical Review A. 86, 2, Abstract
It is shown that a classical optical Fourier processor can be used for the shaping of quantum correlations between two or more photons, and the class of filter functions applicable in the multiphoton Fourier space is identified. This concept is experimentally demonstrated by using two types of periodic phase Fourier filters to manipulate the quantum correlations between pathentangled photon pairs. As many more types of filter functions are applicable, this opens up an alternative avenue for the manipulation of correlations between photons, a key ingredient in optical quantum information processing.

(2012) Nature Photonics. 6, 8, p. 549553 Abstract
Imaging with optical resolution through turbid media is a long soughtafter goal with important applications in deep tissue imaging. Although extensively studied(1), this goal was considered impractical until recently. Adaptiveoptics techniques(2,3), which can correct weak aberrations, are inadequate for turbid samples, where light is scattered to complex speckle patterns with a number of modes greatly exceeding the number of degrees of control(4). This conception changed after the demonstration of coherent focusing through turbid media by wavefrontshaping, using spatial light modulators(57). Here, we show that wavefrontshaping enables widefield imaging through turbid layers with incoherent illumination, and imaging of occluded objects using light scattered from diffuse walls. In contrast to the recently introduced schemes for imaging through turbid media(815), our technique does not require coherent sources(814), interferometric detection(1014), rasterscanning(810,14,15) or offline reconstruction(1115). Our results bring wavefrontshaping closer to practical applications and realize the vision of looking through 'walls' and around corners(16).

(2012) Optics Letters. 37, 16, p. 34293431 Abstract
A random medium can serve as a controllable arbitrary spectral filter with spectral resolution determined by the inverse of the interaction time of the light in the medium. We use wavefront shaping to implement an arbitrary spectral response at a particular point in the scattered field. We experimentally demonstrate this technique by selecting either a narrow band or dual bands with a width of 5.5 nm each. (C) 2012 Optical Society of America

(2012) Physical Review A. 86, 1, Abstract
We study the effect of interactions on the propagation of quantum correlations in the bosonic twobody quantum walk. The combined effect of interactions and Hanbury BrownTwiss interference results in unique spatial correlations which depend on the strength of the interaction, but not on its sign. We experimentally measure the weak interaction limit of these effects using light propagating in a highly nonlinear photonic lattices. Finally, we propose an experimental approach to observe the strong interaction limit using few atoms in optical lattices.

(2012) Physical Review Letters. 108, 15, Abstract
We experimentally show that twophoton pathentangled states can be coherently manipulated by multimode interference in multimode waveguides. By measuring the output twophoton spatial correlation function versus the phase of the input state, we show that multimode waveguides perform as nearly ideal multiport beam splitters at the quantum level, creating a large variety of entangled and separable multipath twophoton states.

(2012) Optics Express. 20, 5, p. 51895195 Abstract
We analyze the spatiotemporal distortions of an ultrashort pulse focused through a thin scattering surface. We show and experimentally verify that in such a scenario temporal distortions are proportional to the distance from the optical axis and are present only outside the focal point, as result of geometrical path length differences. We use wavefront shaping to correct for the spatiotemporal distortions and to temporally compress chirped input pulses through the scattering medium. (C) 2012 Optical Society of America

(2012) Physical Review A. 85, 2, Abstract
We have performed experimental quantum state tomography of NOON states with up to four photons. The measured states are generated by mixing photons from a classical coherent state with downconverted photon pairs. We show that the fidelity between the produced states and the ideal NOON states is high. The fidelity is limited by the overlap of the twophoton downconverted state with any two photons originating from the coherent state, for which we introduce and measure a figure of merit. A second limitation on the fidelity set by the total setup transmission is discussed. We also apply the same tomography procedure for characterizing correlated photon hole states.

(2012) Optics Express. 20, 4, p. 36133619 Abstract
We study the case of two simultaneous threewavemixing processes, where one frequency is converted to another through an intermediate frequency. The common assumption is that these processes can occur only when the material is transparent at all participating frequencies. Here we show experimentally that, under appropriate conditions, the intermediate frequency remains dark throughout the interaction. This means that even if the material is opaque at the intermediate frequency, the conversion will remain efficient. New possibilities of frequency conversion are therefore available, e.g. through absorptive bands in the ultraviolet or midinfrared. Moreover, though it was hitherto assumed that the phase mismatch value is governed only by dispersion, we show here that phase matching also depends on light intensity. These findings promise novel all optical switching techniques. (C) 2012 Optical Society of America

(2012) Journal of Physics: Conference Series. 388, PART 3, 032017. Abstract
The competition between dissociation paths of I_{2}^{+} and NO^{+} molecules was studied using femtosecond laser pulses with different intensities. It was found, both for moderate fields and for strong fields, that the dissociation path strongly prefers the higher energy dissociation path with smaller kinetic energy rather than the lower energy path with higher kinetic energy.
[All authors] 
(2012) Applied Physics Letters. 100, 5, Abstract
We demonstrate singlebeam coherent antiStokes Raman spectroscopy (CARS), for detecting and identifying traces of solids, including minute amounts of explosives, from a standoff distance (> 50 m) using intense femtosecond pulses. Until now, singlebeam CARS methods relied on pulseshapers in order to obtain vibrational spectra. Here, we present a simple and easytoimplement detection scheme, using a commercially available notch filter that does not require the use of a pulseshaper. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681365]
2011

(2011) Applied Physics BLasers And Optics. 105, 4, p. 697702 Abstract
A new method for efficient, broadband sum and difference frequency generation of ultrafast pulses is demonstrated. The principles of the method follow from an analogy between frequency conversion and coherent optical excitation of a twolevel system. For conversion of ultrafast pulses, the concepts of adiabatic conversion are developed further in order to account for dispersion and group velocity mismatch. The scheme was implemented using aperiodically poled nonlinear crystals and a single step nonlinear mixing process, leading to conversion of nearIR (∼790 nm) ultrafast pulses into the blue (∼450 nm) and midIR (∼3.15 μm) spectral regions. Conversion bandwidths up to 15 THz FWHM and efficiencies up to 50% are reported.

(2011) Applied Physics BLasers And Optics. 105, 2, p. 203211 Abstract
Molecular alignment of linear molecules (O _{2}, N _{2}, CO _{2} and CO) is measured photoacoustically in the gas phase. The rotational excitation is accomplished using a simple femtosecond stimulated Raman excitation scheme, employing two femtosecond pulses with variable delay between the pulses. Molecular alignment is determined directly by measuring the energy dumped into the gas by quartzenhanced photoacoustic spectroscopy (QEPAS), utilizing a quartz tuning fork as a sensitive photoacoustic transducer. The experimental results demonstrate for the first time the use of a tuning fork for resonant photoacoustic detection of Raman spectra excited by femtosecond double pulses and match both simulation and literature values.

(2011) Physical Review A. 84, 4, 041806. Abstract
When light waves propagate through disordered photonic lattices, they can eventually become localized due to multiple scattering effects. Here we show experimentally that while the evolution and localization of the photon density distribution is similar in the two cases of diagonal and offdiagonal disorder, the densitydensity correlation carries a distinct signature of the type of disorder. We show that these differences reflect a symmetry in the spectrum and eigenmodes that exists in offdiagonally disordered lattices but is absent in lattices with diagonal disorder.

(2011) Journal of the Korean Physical Society. 59, 41, p. 5 Abstract
We report the experimental findings of a systematic study of the effect of linear chirp on strong field photodissociation of H_{2}^{+}. For vibrational levels around or above the one photon crossing, the effect manifests itself in terms of a shift in the kinetic energy release (KER) peaks. The peaks shift up for negative chirp whereas they shift down for positive chirp. The measurements are carried out by varying two of the three laser pulse characteristics, energy, pulse peak intensity and linear chirp, while keeping the third constant. The shifts in the KER peaks are found to be intensity dependent for a given value of chirp. However, in the last two cases (i.e., fixed pulsed energy and fixed pulse peak intensity), they are found to be independent of the chirp magnitude. The results are understood on the basis of saturation of photodissociation probabilities for these levels.
[All authors] 
(2011) Physical Review A. 84, 4, 041805. Abstract
We consider the propagation of classical and nonclassical light in multimode optical waveguides. We focus on the evolution of the fewphoton correlation functions, which, much as the lightintensity distribution in such systems, evolve in a periodic manner, culminating in the "revival" of the initial correlation pattern at the end of each period. It is found that when the input state possesses nontrivial symmetries, the correlation revival period can be longer than that of the intensity, and thus the same intensity pattern can display different correlation patterns. We experimentally demonstrate this effect for classical, pseudothermal light, and compare the results with the predictions for nonclassical, quantum light.

(2011) Physical Review A. 84, 1, 013414. Abstract
Complete population transfer in a fourcoupledmodes system is analyzed from a geometrical point of view. An analytical solution of the dynamics is written by the use of two distinct frequencies, the generalization of the single Rabi frequency of the twostate dynamics. We also present its visualization on two separate Bloch spheres with two independent torque equations. With this scheme we analytically derive the requirements for complete population transfer in a fourstate quantum system. Interestingly, the solutions are found to be linked to fundamental number theory, whereas complete population transfer occurs only if the ratios between coupling coefficients exactly match a set of Pythagorean triples.

(2011) Nature Photonics. 5, 6, p. 372377 Abstract
Light scattering in inhomogeneous media induces wavefront distortions that pose an inherent limitation in many optical applications. Examples where this occurs include microscopy, nanosurgery and astronomy. In recent years, ongoing efforts have made the correction of spatial distortions possible using wavefrontshaping techniques. However, when ultrashort pulses are used, scattering also induces temporal distortions, which hinder the use of such pulses in nonlinear processes such as multiphoton microscopy and quantum control experiments. Here, we show that correction of both spatial and temporal distortions can be achieved by manipulating only the spatial degrees of freedom of the incident wavefront. By optimizing a nonlinear signal, we demonstrate spatiotemporal focusing and compression of chirped ultrashort pulses through scattering media, and refocusing in both space and time of 100 fs pulses through thick brain and bone samples. Our results open up new possibilities for optical manipulation and nonlinear imaging in scattering media.

(2011) Optics Letters. 36, 7, p. 12481250 AbstractSinglepulse stimulated Raman scattering spectroscopy
We demonstrate the acquisition of stimulated Raman scattering spectra with the use of a single femtosecond pulse. Highresolution vibrational spectra are obtained by shifting the phase of a narrow band of frequencies within the input pulse spectrum, using spectral shaping. The vibrational lines are resolved via amplitude features formed in the spectrum after interaction with the sample. Using this technique, lowfrequency Raman lines (

(2011) Annals of Physics. 326, 3, p. 626633 Abstract
We propose a technique for efficient midrange wireless power transfer between two coils, by adapting the process of adiabatic passage for a coherently driven twostate quantum system to the realm of wireless energy transfer. The proposed technique is shown to be robust to noise, resonant constraints, and other interferences that exist in the neighborhood of the coils. (C) 2010 Elsevier Inc. All rights reserved.

(2011) Physical Review A. 83, 1, 013806. Abstract
We show that a periodic twodimensional (2D) photonic lattice with Kerr nonlinearity exhibits a BerezinskiiKosterlitzThouless (BKT) crossover associated with a vortexunbinding transition. We find that averaging over random initial conditions is equivalent to Boltzmann thermal averaging with the discrete nonlinear Schrodinger Hamiltonian. By controlling the initial randomness we can continuously vary the effective temperature. Since this Hamiltonian is in the 2D XY universality class, a BKT transition ensues. We verify this prediction using experimentally accessible observables and find good agreement between theory and simulations. This opens the possibility of experimental access to interesting phase transitions known in condensed matter using nonlinear optics.
2010

(2010) Physical Review Letters. 105, 26, 263604. Abstract
We show that when photons in Nparticle pathentangled vertical bar N, 0 > + vertical bar 0, N > or N00N states undergo Bloch oscillations, they exhibit a periodic transition between spatially bunched and antibunched states. The period of the bunchingantibunching oscillation is N times faster than the period of the oscillation of the photon density, manifesting the unique coherence properties of N00N states. The transition occurs even when the photons are well separated in space.

(2010) Nature Photonics. 4, 10, p. 721726 Abstract
Five decades ago, Hanbury Brown and Twiss (HBT) demonstrated that the angular size of stars can be measured by correlating the intensity fluctuations measured by two detectors at two different locations. Since then, nonlocal correlation measurements have become ubiquitous in many areas of physics and have also been applied, beyond photons, to electrons, matter waves and subatomic particles. An important assumption in HBT interferometry is that the particles do not interact on their way from the source to the detectors. However, this assumption is not always valid. Here, we study the effects of interactions on HBT interferometry by considering the propagation of light fields in a nonlinear medium that induces interactions between the photons. We show that interactions affect multipath interference, limiting the ability to extract information on the source. Nevertheless, we find that proper analysis of the intensity fluctuations can recover the size of the source, even in the presence of interactions.

(2010) Physical Review Letters. 105, 16, 163905. Abstract
We predict quantum correlations between noninteracting particles evolving simultaneously in a disordered medium. While the particle density follows the singleparticle dynamics and exhibits Anderson localization, the twoparticle correlation develops unique features that depend on the quantum statistics of the particles and their initial separation. On short time scales, the localization of one particle becomes dependent on whether or not the other particle is localized. On long time scales, the localized particles show oscillatory correlations within the localization length. These effects can be observed in Anderson localization of nonclassical light and ultracold atoms.

(2010) Optics Express. 18, 22, p. 2269322701 Abstract
We present a simple and easily implementable scheme for multiplexed Coherent AntiStokes Raman Scattering (CARS) spectroscopy and microscopy using a single femtosecond pulse, shaped with a narrow spectral notch. We show that a tunable spectral notch, shaped by a resonant photonic crystal slab, can serve as a narrowband, optimally timedelayed probe, resolving a broad vibrational spectrum with high spectral resolution in a singleshot measurement. Our singlesource, singlebeam scheme allows the simple transformation of any multiphoton microscope with adequate bandwidth into a nearly alignmentfree CARS microscope. (C) 2010 Optical Society of America

(2010) Science. 329, 5998, p. 15001503 Abstract[All authors]
Quantum walks of correlated particles offer the possibility of studying largescale quantum interference; simulating biological, chemical, and physical systems; and providing a route to universal quantum computation. We have demonstrated quantum walks of two identical photons in an array of 21 continuously evanescently coupled waveguides in a SiOxNy chip. We observed quantum correlations, violating a classical limit by 76 standard deviations, and found that the correlations depended critically on the input state of the quantum walk. These results present a powerful approach to achieving quantum walks with correlated particles to encode information in an exponentially larger state space.

(2010) Physical Review Letters. 105, 9, 93603. Abstract
We generate bipartite states of light which exhibit an absence of multiphoton coincidence events between two modes amid a constant background flux. These "correlated photon holes" are produced by mixing a coherent state and relatively weak spontaneous parametric downconversion by using a balanced beam splitter. Correlated holes with arbitrarily high photon numbers may be obtained by adjusting the relative phase and amplitude of the inputs. We measure states of up to five photons and verify their nonclassicality. The scheme provides a route for observation of highphotonnumber nonclassical correlations without requiring intense quantum resources.

(2010) Physical Review A. 81, 6, 063410. Abstract
Simple analytical approaches for implementing strong field coherent control schemes are often elusive due to the complexity of the interaction between the intense excitation field and the system of interest. Here, we demonstrate control over multiphoton excitation in a threelevel resonant system using simple, analytically derived ultrafast pulse shapes. We utilize a twodimensional spatiotemporal control technique, in which temporal focusing produces a spatially dependent quadratic spectral phase, while a second, arbitrary phase parameter is scanned using a pulse shaper. In the current work, we demonstrate weaktostrong field excitation of Rb85, with a π phase step and the quadratic phase as the chosen control parameters. The intricate dependence of the multilevel dynamics on these parameters is exhibited by mapping the data onto a twodimensional control landscape. Further insight is gained by simulating the complete landscape using a dressedstate, timedomain model, in which the influence of individual shaping parameters can be extracted using both exact and asymptotic timedomain representations of the dressedstate energies.

(2010) Science. 328, 5980, p. 879881 Abstract
Precision measurements can be brought to their ultimate limit by harnessing the principles of quantum mechanics. In optics, multiphoton entangled states, known as NOON states, can be used to obtain highprecision phase measurements, becoming more and more advantageous as the number of photons grows. We generated "highNOON" states (N = 5) by multiphoton interference of quantum downconverted light with a classical coherent state in an approach that is inherently scalable. Superresolving phase measurements with up to five entangled photons were produced with a visibility higher than that obtainable using classical light only.

(2010) Optics Letters. 35, 10, p. 15901592 Abstract
We experimentally demonstrate efficient tunable upconversion by cascading optical oscillation and wideband adiabatic sumfrequency generation in a single KTiOPO(4) crystal, yielding red light tunable over a 6.2 nm wavelength band. The conversion efficiency of the 1064 nm pump to the red output was up to 4.7%, and with the highest pump power of 1.5 W we obtained 71 mW of average power at 637 nm. (C) 2010 Optical Society of America

(2010) Physical Review Letters. 104, 12, 123602. Abstract
The employment of pathentangled multiphoton states enables measurement of phase with enhanced precision. It is common practice to demonstrate the unique properties of such quantum states by measuring superresolving oscillations in the coincidence rate of a MachZehnder interferometer. Similar oscillations, however, have also been demonstrated in various configurations using classical light only; making it unclear what, if any, are the classical limits of this phenomenon. Here we derive a classical bound for the visibility of superresolving oscillations in a MachZehnder interferometer. This provides an easy to apply, fundamental test of nonclassicality. We apply this test to experimental multiphoton coincidence measurements obtained using photon number resolving detectors. MachZehnder superresolution is found to be a highly distinctive quantum effect.

(2010) Physical Review A. 81, 2, 023401. Abstract[All authors]
The temporal evolution of the dissociation probabilities for various vibrational levels of H2+ is observed in terms of shifts in the kinetic energy release dissociation spectra, induced by linearly chirped intense laser pulses. In contrast to previous observations, in which no dependence on the chirp sign was observed, the energy spectrum reported here shows peak shifts, up for negative chirp and down for positive chirp. For some vibrational levels, dissociation takes place early on in the pulse; hence, care must be taken while interpreting the effect of pulse duration in photodissociation studies. This interpretation is supported by numerical solutions of the timedependent Schrödinger equation.

(2010) International Conference on Ultrafast Phenomena, UP 2010. Abstract
A method for efficient frequency conversion of ultrafast pulses is demonstrated using an adiabatic aperiodically poled KTP crystal. We produce broadband blue pulses centered at 450 nm by upconverting 30 fs pulses in the nearIR.
2009

(2009) Optics Communications. 282, 23, p. 45244526 Abstract
We present an analytical description of a new class of threecore adiabatic following directional couplers. Using a multiplescale WKB method we obtain closedform expressions describing the optical field dynamics in such structures. The adiabatic evolution occurring in this particular threecore configuration can lead to a spatial switch over of local supermodes and to an irreversible power transfer. (C) 2009 Elsevier B.V. All rights reserved.

(2009) Optics Express. 17, 21, p. 1865918668 Abstract
We introduce an illumination configuration which is a spatiotemporal analog of a nondiffracting Xwave. By interfering multiple ultrashort converging plane waves, we generate a tight central spot at which a transform limited ultrashort pulse is formed. Outside this tight focus a spatiotemporal speckle field with longer duration and reduced peak power is created. We investigate this spatiotemporal Xwave configuration analytically, numerically, and experimentally demonstrate the effect using two photon excitation fluorescence.

(2009) Applied Physics Letters. 95, 13, Abstract
We describe an advanced image reconstruction algorithm for pseudothermal ghost imaging, reducing the number of measurements required for image recovery by an order of magnitude. The algorithm is based on compressed sensing, a technique that enables the reconstruction of an Npixel image from much less than N measurements. We demonstrate the algorithm using experimental data from a pseudothermal ghostimaging setup. The algorithm can be applied to data taken from past pseudothermal ghostimaging experiments, improving the reconstruction's quality. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3238296]


(2009) Optics Express. 17, 15, p. 1273112740 Abstract
We discuss theoretically and demonstrate experimentally the robustness of the adiabatic sum frequency conversion method. This technique, borrowed from an analogous scheme of robust population transfer in atomic physics and nuclear magnetic resonance, enables the achievement of nearly full frequency conversion in a sum frequency generation process for a bandwidth up to two orders of magnitude wider than in conventional conversion schemes. We show that this scheme is robust to variations in the parameters of both the nonlinear crystal and of the incoming light. These include the crystal temperature, the frequency of the incoming field, the pump intensity, the crystal length and the angle of incidence. Also, we show that this extremely broad bandwidth can be tuned to higher or lower central wavelengths by changing either the pump frequency or the crystal temperature. The detailed study of the properties of this converter is done using the LandauZener theory dealing with the adiabatic transitions in two level systems.

(2009) Physical review letters. 103, 1, 013901. Abstract
We report the observation of the signature of a localization phase transition for light in onedimensional quasiperiodic photonic lattices, by directly measuring wave transport inside the lattice. Below the predicted transition point an initially narrow wave packet expands as it propagates, while above the transition expansion is fully suppressed. In addition, we measure the effect of focusing nonlinear interaction on the propagation and find it increases the width of the localized wave packets.

(2009) Physical Review Letters. 102, 25, Abstract
We study quantum and classical Hanbury BrownTwiss correlations in waveguide lattices. We develop a theory for the propagation of photon pairs in the lattice, predicting the emergence of nontrivial quantum interferences unique to lattice systems. Experimentally, we observe the classical counterpart of these interferences using intensitycorrelation measurements. We discuss the correspondence between the classical and quantum correlations, and consider pathentangled input states which do not have a classical analogue. Our results demonstrate that waveguide lattices can be used as a robust and highly controllable tool for manipulating quantum states, and offer new ways of studying the quantum properties of light.

(2009) Physical Review Letters. 102, 23, Abstract
When a periodic 1D system described by a tightbinding model is uniformly initialized with equal amplitudes at all sites, yet with completely random phases, it evolves into a thermal distribution with no spatial correlations. However, when the system is nonlinear, correlations are spontaneously formed. We find that for strong nonlinearities, the intensity histograms approach a narrow Gaussian distributed around their mean and phase correlations are formed between neighboring sites. Sites tend to be out of phase for a positive nonlinearity and in phase for a negative one. Most impressively, the field correlation takes a universal shape independent of parameters. These results are relevant to bosonic gas in 1D optical lattices as well as to nonlinear optical waveguide arrays, which are used to demonstrate experimentally some of the features of this equilibrium state.

(2009) Physical Review A. 79, 5, Abstract
We experimentally demonstrate pseudothermal ghost imaging and ghost diffraction using only a single detector. We achieve this by replacing the highresolution detector of the reference beam with a computation of the propagating field, following a recent proposal by Shapiro [ Phys. Rev. A 78, 061802(R) (2008)]. Since only a single detector is used, this provides experimental evidence that pseudothermal ghost imaging does not rely on nonlocal quantum correlations. In addition, we show the depthresolving capability of this ghost imaging technique.

(2009) Physical Review A. 79, 4, Abstract
The characterization and conditional preparation of multiphoton quantum states require the use of photonnumber resolving detectors. We study the use of detectors based on multiple avalanche photodiode pixels in this context. We develop a general model that provides the positive operator value measures for these detectors. The model incorporates the effect of cross talk between pixels which is unique to these devices. We validate the model by measuring coherentstate photonnumber distributions and reconstructing them with high precision. Finally, we evaluate the suitability of such detectors for quantum state tomography and entanglementbased quantum state preparation, highlighting the effects of dark counts and cross talk between pixels.

(2009) Annual Review of Physical Chemistry. 60, p. 277292 Abstract
The field of quantum coherent control, initially formulated with the goal of modifying and manipulating molecular systems, has had a number of applications in atomic and molecular spectroscopy in recent years. This review demonstrates how carefully designed femtosecond pulses could be used to enhance resolution and improve detection in several areas of nonlinear spectroscopy. The two effects that arc most intensively studied in this context arc twophoton absorption and coherent antiStokes Raman scattering. This,article discusses the principles of the control of such processes and several possible applications in microscopy and remote sensing.

(2009) Ultrafast Phenomena Xvi. 92, p. 985987 AbstractSinglepulse standoff nonlinear Raman spectroscopy using shaped femtosecond pulses
We demonstrate fast standoff (>10 m) singlepulse coherent antiStokes Raman spectroscopy (CARS), from trace amounts of solids, under ambient light conditions, using phase or amplitude shaped femtosecond pulses.

(2009) Ultrafast Phenomena Xvi. 92, p. 457459 AbstractStrong Field Coherent Control Using 2D SpatioTemporal Mapping
Multiphoton excitation in Rubidium can be effectively controlled using simple pulse shaping parameters. Interplay between ionization and dynamic Stark shifts is revealed by mapping onto 2D landscapes using a recently developed spatiotemporal coherent control technique.

(2009) Journal of Modern Optics. 56, 1819, p. 20492054 Abstract
We consider a twodimensional liquid crystal spatial light modulator (SLM) for femtosecond pulse shaping. Novel shaping schemes enabling a drastic speed increase (about four orders of magnitude as compared with conventional liquidcrystal SLMbased pulse shapers) and complex (phase and amplitude) femtosecond pulse shaping are discussed and experimentally demonstrated. In the first case, while a horizontal resolution of 1920 addressable pixels provided superior fidelity for generating complex waveforms, scanning across the vertical dimension (1080 pixels) has been used to facilitate an update rate in excess of 100 kHz. In the second case, we use the pixel count redundancy in the vertical direction and encode a spectrallydependent diffraction grating for modulation of both spectral phases and amplitudes.
2008

(2008) Physical Review A. 78, 6, 063821. Abstract
We present a geometrical representation of the process of sum frequency generation in the undepleted pump approximation, in analogy with the known optical Bloch equations. We use this analogy to propose a technique for achieving both high efficiency and large bandwidth in sum frequency conversion using the adiabatic inversion scheme. The process is analogous with rapid adiabatic passage in NMR, and adiabatic constraints are derived in this context. This adiabatic frequency conversion scheme is realized experimentally using an aperiodically poled potassium titanyl phosphate (KTP) device, where we achieved high efficiency signaltoidler conversion over a bandwidth of 140 nm.

(2008) Optics Letters. 33, 23, p. 28302832 Abstract
The breakup of highorder spatial solitons propagating in an AlGaAs slab waveguide is studied. We experimentally observe the breakup of such beams into multiple fragments and identify the mechanism of this breakup as the combined effect of two and threephoton absorption. We show that the multiple breakup persists even when the value of twophoton absorption is reduced by an order of magnitude owing to the high value of threephoton absorption of AlGaAs at the halfbandgap. The experimental results extend known mechanisms of soliton breakup induced by twophoton absorption and agree well with numerical beampropagation simulations. (C) 2008 Optical Society of America

(2008) Physical Review Letters. 101, 19, Abstract
We investigate the effect of nonlinearity in a system described by an adiabatically evolving Hamiltonian. Experiments are conducted in a threecore waveguide structure that is adiabatically varying with distance, in analogy to the stimulated Raman adiabatic passage process in atomic physics. In the linear regime, the system exhibits an adiabatic power transfer between two waveguides which are not directly coupled, with negligible power recorded in the intermediate coupling waveguide. In the presence of nonlinearity the adiabatic light passage is found to critically depend on the excitation power. We show how this effect is related to the destruction of the dark state formed in this configuration.

(2008) Physics ReportsReview Section Of Physics Letters. 463, 3Jan, p. 1126 Abstract
We provide an overview of recent experimental and theoretical developments in the area of optical discrete solitons. By nature, discrete solitons represent selftrapped wavepackets in nonlinear periodic structures and result from the interplay between lattice diffraction (or dispersion) and material nonlinearity. In optics, this class of selflocalized states has been successfully observed in both one and twodimensional nonlinear waveguide arrays. In recent years such photonic lattices have been implemented or induced in a variety of material systems, including those with cubic (Kerr), quadratic, photorefractive, and liquidcrystal nonlinearities. In all cases the underlying periodicity or discreteness leads to altogether new families of optical solitons that have no counterpart whatsoever in continuous systems. We first review the linear properties of photonic lattices that are key in the understanding of discrete solitons. The physics and dynamics of the fundamental discrete and gap solitons are then analyzed along with those of many other exotic classes  e.g. twisted, vector and multiband, cavity, spatiotemporal, randomphase, vortex, and nonlocal lattice solitons, just to mention a few. The possibility of alloptically routing optical discrete solitons in 2D and 3D periodic environments using soliton collisions is also presented. Finally, soliton formation in optical quasicrystals and at the boundaries of waveguide array structures are discussed. (C) 2008 Elsevier B.V. All rights reserved.

(2008) Physical Review Letters. 100, 17, Abstract
Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems (similar to 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects.

(2008) Applied Physics Letters. 92, 17, Abstract
We demonstrate a singlebeam, standoff (>10 m) detection and identification of various materials including minute amounts of explosives under ambient light conditions. This is obtained by multiplex coherent antiStokes Raman scattering spectroscopy (CARS) using a single femtosecond phaseshaped laser pulse. We exploit the strong nonresonant background for amplification of the backscattered resonant CARS signals by employing a homodyne detection scheme. The simple and highly sensitive spectroscopic technique has a potential for hazardous materials standoff detection applications. (C) 2008 American Institute of Physics.

(2008) JOURNAL OF PHYSICS BATOMIC MOLECULAR AND OPTICAL PHYSICS. 41, 7, 074008. Abstract
Coherent control of resonant and nonresonant twophoton absorption processes was examined using a spatiotemporal pulseshaping technique. By utilizing a combination of temporal focusing and femtosecond pulseshaping techniques, we spatially control multiphoton absorption processes in a completely deterministic manner. Distinctive symmetry properties emerge through twodimensional mapping of spatiotemporal data. These symmetries break down in the transition to strong fields, revealing details of strongfield effects such as power broadenings and dynamic Stark shifts. We also present demonstrations of chirpdependent population transfer in atomic rubidium, as well as the spatial separation of resonant and nonresonant excitation pathways in atomic caesium.

(2008) Physical Review Letters. 100, 1, Abstract
We experimentally investigate the evolution of linear and nonlinear waves in a realization of the Anderson model using disordered onedimensional waveguide lattices. Two types of localized eigenmodes, flatphased and staggered, are directly measured. Nonlinear perturbations enhance localization in one type and induce delocalization in the other. In a complementary approach, we study the evolution on short time scales of deltalike wave packets in the presence of disorder. A transition from ballistic wave packet expansion to exponential (Anderson) localization is observed. We also find an intermediate regime in which the ballistic and localized components coexist while diffusive dynamics is absent. Evidence is found for a faster transition into localization under nonlinear conditions.
2007

(2007) Journal Of The Optical Society Of America BOptical Physics. 24, 12, p. 29402947 Abstract
We consider a programmable, phase, and amplitude femtosecond pulse shaper based on a twodimensional (2D) reflective liquidcrystal (LC) spatial light modulator (SLM). A new zeroorder pulse shaping scheme is introduced and compared to the firstorder scheme, both theoretically and experimentally, using liquid crystal on silicon 2D SLM. While the spectral components of the pulse are spread across the horizontal dimension, we use the vertical direction for modulation of both spectral phases and amplitudes. It was found that while zeroorder approach provided better light efficiency (67% versus 43%), the firstorder scheme has superior dynamic range of amplitude modulation. (C) 2007 Optical Society of America.

(2007) Journal of Structural Biology. 159, 1, p. 103110 Abstract
The tectorial membrane (TM) is a highly hydrated noncellular matrix situated over the sensory cells of the cochlea. It is widely accepted that the mechanical coupling, between the TM and outer hair cells stereocilia bundles, plays an important role in the cochlea energy transduction mechanism. Recently, we provided supporting evidence for the existence of mechanical coupling by demonstrating that the mechanical properties of the TM change along its longitudinal direction. Since the biochemical composition of the TM is similar throughout its entire length, it is likely that structural differences induce the observed material properties changes. Presently, however, the structure of the TM under physiological environments remains unknown. In this work, the 3D structure of native TM samples is shown by using twophoton secondharmonic imaging microscopy. We find that the collagen fibers at the basal region are arranged in a parallel orientation while being tilted in an angle with respect to the plane of the TM surface at the apical region. Moreover, we find an intensified marginal band at the basal OHC zone which forms a shelllike structure which engulfs the stereocilium imprints surface of the TM. In supports of our previous mechanical characterization, the analysis presented here provides a structural basis for the changes in TM's mechanical properties. (c) 2007 Elsevier Inc. All rights reserved.

(2007) Optics Express. 15, 14, p. 87608769 Abstract
We describe a novel nonlinear detection method for optical tomography that does not rely on detection of interference fringes and is free of optical background. The method exploits temporally coherent broadband illumination such as ultrashort pulses, and a nonlinear twophoton detection process such as sumfrequency generation (SFG). At the detection stage, the reference beam and the sample beam are mixed in a thick nonlinear crystal, and only the mixing term, which is free of optical background, is detected. Consequently, the noise limitations posed by the background in standard OCT (excess and shot noise), do not exist here. Due to the nonlinearity, the signal to noise ratio scales more favorably with the optical power compared to standard OCT, yielding an inherent improvement for high speed tomographic scans. Careful design of phase matching in the crystal enables nonlinear mixing which is both highly efficient and broadband, yielding both high sensitivity and high depth resolution.

(2007) Optics Letters. 32, 11, p. 13841386 Abstract
We introduce a programmable, highrate scanning femtosecond pulse shaper based on a twodimensional liquid crystal on a silicon spatial light modulator (SLM). While horizontal resolution of 1920 addressable pixels provides superior fidelity for generating complex waveforms, scanning across the vertical dimension (1080 pixels) has been used to facilitate at least 3 orders of magnitude speed increase as compared with typical liquidcrystal SLMbased pulse shapers. An update rate in excess of 100 kHz is demonstrated. (c) 2007 Optical Society of America.

(2007) Physical Review A. 75, 4, 043804. Abstract
We demonstrate a scheme to spectrally manipulate a collinear, continuous stream of time and energy entangled photons to generate beamlike, bandwidthlimited fluxes of polarizationentangled photons with nearly degenerate wavelengths. Utilizing an ultrashortpulse shaper to control the spectral phase and polarization of the photon pairs, we tailor the shape of the HongOuMandel interference pattern, demonstrating the rules that govern the dependence of this interference pattern on the spectral phases of the photons. We then use the pulse shaper to generate all four polarization Bell states. The singlet state generated by this scheme forms a very robust decoherencefree subspace, extremely suitable for longdistance fiberopticsbased quantum communication.

(2007) Journal of Dentistry. 35, 2, p. 150155 Abstract
Objectives: We present a novel way to create highresolution threedimensional images of tooth dentin by harmonic generation scanning laser microscopy. Methods: The images were taken using a pulsed infrared laser. Threedimensional reconstruction enables the visualization of individual tubules and the collagen fibrils mesh around them with an optical resolution of ∼1 μm. Results: The images show micromorphological details of the dentinal tubules as well as the collagen fibrils at a depth of up to about 200 μm. The data show that while collagen fibrils are organized in planes perpendicular to the tubules, close to the dentin enamel junction they lie also along the long axis of the tubules. Conclusions: The unique 3D information opens the opportunity to study the collagen fibril arrangement in relation to the tubule orientation within the dentin matrix, and may be applied to study the micromorphology of normal versus altered dentin.

(2007) Physical Review Letters. 98, 2, Abstract
We present experimental evidence for the spontaneous formation of discrete X waves in AlGaAs waveguide arrays. This new family of optical waves has been excited, for the first time, by using the interplay between discrete diffraction and normal temporal dispersion, in the presence of Kerr nonlinearity. Our experimental results are in good agreement with theoretical predictions.
2006

(2006) Optics Letters. 31, 23, p. 35293531 Abstract
We have theoretically and experimentally studied the local temporal and spectral characteristics of an ultrashort pulse passing a simple thin diffusive element. We show that as one moves away from the diffuser the pulse evolves into a spatiotemporal speckle. (c) 2006 Optical Society of America.

(2006) Physical Review A. 74, 5, Abstract
We present the design and experimental proof of principle of a low threshold optical parametric oscillator (OPO) that continuously oscillates over a large bandwidth allowed by phase matching. The large oscillation bandwidth is achieved with a selective twophoton loss that suppresses the inherent mode competition, which tends to narrow the bandwidth in conventional OPOs. Our design performs pairwise mode locking of many frequency pairs, in direct equivalence to passive mode locking of ultrashort pulsed lasers. The ability to obtain high powers of continuous and broadband downconverted light enables the optimal exploitation of the correlations within the downconverted spectrum, thereby strongly affecting twophoton interactions even at classically high power levels, and opening new venues for applications such as twophoton spectroscopy and microscopy and optical spread spectrum communication.

(2006) Optics Communications. 264, 2, p. 482487 Abstract
The transition probability of a twophoton absorption (TPA) process in atomic Cesium, excited by phasecontrolled temporally focused ultrashort pulses is shown to be spatially modulated in a controlled manner. In particular, we demonstrate the generation of a dark nonlinear focus. By controlling the excitation pulse shape along the propagation coordinate we create a region in space where the TPA rate vanishes which is flanked by bright regions.

(2006) Physical Review A. 73, 6, Abstract
We study nonlinear relaxation of the excited state in a system with two levels and a continuum. The process is first simulated in a onedimensional waveguide array described by the discrete nonlinear Schrodinger equation. The results are interpreted in terms of degenerate fourwave mixing between the eigenmodes and diffraction properties of the array. We also show analytically that the role of the continuum can be played by a third bound state, with linear loss that replaces the diffraction in the continuum. This model enables derivation of the energy transfer rate and other parameters of the process.

(2006) Optics Letters. 31, 5, p. 631633 Abstract
The spectral polarization of an ultrashort pulse is fully controlled by a novel pulse shaper design, including three liquidcrystal spatial light modulator arrays at three different orientations. The added degree of controllability permits generation of previously unattainable pulse shapes, with possible applications in multidimensional spectroscopy, coherent control, and ultrafast polarization gating.

(2006) Journal Of The Optical Society Of America BOptical Physics. 23, 1, p. 6266 Abstract
We investigate, theoretically and experimentally, how linear modes evolve around discrete solitons. If the shape of the exciting beam is not matched to a soliton state, linear modes are excited, resulting in significant oscillations of the beam amplitude. Linear modes can cause a power increase in a single guide, where the effect of nonlinear absorption is enhanced tremendously. As a result we observe a splitting of the soliton, which is highly asymmetric in case of a tilted input beam. In the latter case most of the emerging field distribution finally moves opposite to the initial tilt. (c) 2006 Optical Society of America.

(2006) 2006 Ieee Leos Annual Meeting Conference Proceedings, Vols 1 And 2. p. 9495 Abstract
We present experimental evidence for the formation of Xwaves in nonlinear AlGaAs waveguide arrays. These results agree with numerical simulations based on the discrete nonlinear Schrodinger equation with an appropriate temporal dispersion term.
2005

(2005) Journal Of The Optical Society Of America BOptical Physics. 22, 12, p. 26602663 Abstract
The properties of harmonic generation with temporally focused ultrashort pulses are explored both theoretically and experimentally. Analyzing the phasematching conditions for harmonic generation we find a correspondence between temporal focusing and spatial focusing along one dimension. In particular, temporally focused pulses experience a π phase shift in passing through the temporal focus, similar to the Guoy phase shift experienced by spatially focused beams. This correspondence is confirmed by measurements of thirdharmonic generation induced by temporally focused pulses.

(2005) Optics Letters. 30, 23, p. 31743176 Abstract
We have observed the incoherent interaction between a highly confined (blocker) soliton and wide, moving signal beams of a different wavelength in a onedimensional discrete Kerr medium. Digital switching of the blocker solitons to successive adjacent channels was measured with increasing signal power via both one and two cascaded interactions in an AlGaAs waveguide array, operations equivalent to a reconfigurable threeoutput router. (c) 2005 Optical Society of America.

(2005) Physical Review A. 72, 6, 063816. Abstract
Polarization gating of highorder harmonic generation takes advantage of the significant reduction of harmonic generation efficiency for elliptically polarized excitation fields, in order to generate short bursts of harmonic radiation from relatively long pulses. We show that the currently used method for generation of polarization gated pulses using waveplate combinations is inefficient, and propose an alternative method based on polarization pulse shaping techniques. This method is shown to be significantly more efficient and to enable significant shortening of the gate duration. Using this scheme, isolated attosecond pulses should be achievable with excitation pulses of a duration as long as 20 fs.

(2005) Chemical Physics. 318, 12, p. 163169 Abstract
Using a single shaped femtosecond pulse, a molecular wavepacket is both tailored and monitored. Several toluene vibrational levels are excited in a controlled manner, both in amplitude and phase, through a quantum coherently controlled nonresonant Raman process. The entire coherent antiStokes Raman spectroscopy (CARS) process, that includes excitation and probing of the Ramaninduced wavepacket, is enabled by polarization and phase shaping. We achieve a wavepacket detection sensitive to the modes relative phase and further extend the capabilities of the singlepulse CARS technique.

(2005) Optics Express. 13, 24, p. 99039908 Abstract
In most coherent control experiments with femtosecond pulses, the temporal shape of the pulses is maintained throughout the interaction region. Here we show how pulses can be controlled such that their shapes vary rapidly even when propagating very short distances of a few micrometers. This changing pulse shape has a significant effect on coherent nonlinear optical processes. Here we study thirdharmonic generation induced by a coherently controlled excitation pulse whose temporal profile changes along the axial coordinate. We show how such manipulations can be used to improve the axial resolution in a multiphoton optical microscope.

(2005) Optics Letters. 30, 20, p. 27962798 Abstract
We report highrate, computercontrolled femtosecond pulse shaping by use of an electrooptical gallium arsenide optical phasedarray modulator with 2304 controlled waveguides. It provides fast modulation speed of both spectral phases and amplitudes. Limited by the driving electronics of our current setup, we were able to update a pulse shape in similar to 30 ns. This technique paves the way toward individual shaping of every single pulse of typical femtosecond modelocked oscillators. (c) 2005 Optical Society of America

(2005) Physical Review Letters. 95, 7, Abstract
We report the experimental demonstration of a nonlinear process in a twolevel system, in which the amplitude of the excited state decays, transferring irreversibly a large fraction of its energy to the ground state, while shedding a part of it into radiation states. The experiments where preformed in a nonlinear optical waveguide, supporting two or three modes. The process is general, and is expected to occur in other nonlinear few level systems such as nonlinear quantum wells and BoseEinstein condensates.

(2005) Journal Of The Optical Society Of America BOptical Physics. 22, 7, p. 14321436 Abstract
The excitation and propagation of both scalar and vector optical waves in onedimensional nonlinear arrays of channel waveguides was investigated both theoretically and experimentally. In this arrangement vector discrete solitons are also possible through the coexistence of two orthogonally polarized fields. At high input power levels a rapid collapse of the power back to the incidence channel occurred over a small power range for both scalar and vector inputs. (c) 2005 Optical Society of America.

Improved depth resolution in videorate linescanning multiphoton microscopy using temporal focusing
By introducing spatiotemporal pulse shaping techniques to multiphoton microscopy it is possible to obtain videorate images with depth resolution similar to pointbypoint scanning multiphoton microscopy while mechanically scanning in only one dimension. This is achieved by temporal focusing of the illumination pulse: The pulsed excitation field is compressed as it propagates through the sample, reaching its shortest duration (and highest peak intensity) at the focal plane before stretching again beyond it. This method is applied to produce, in a simple and scalable setup, videorate twophoton excitation fluorescence images of Drosophila egg chambers with nearly 100,000 effective pixels and 1.5 μm depth resolution.

(2005) Optics Letters. 30, 9, p. 10271029 Abstract
We investigate experimentally and numerically the interaction of a highly localized, singlechannel discrete soliton (blocker) with a wide, tilted beam in a onedimensional AlGaAs array. In agreement with theory the blocker is observed to discretely shift its position by multiple channels, depending on the intensity and relative phase of the tilted beam. (c) 2005 Optical Society of America.

(2005) Optics Express. 13, 6, p. 17621773 Abstract
We experimentally study polarization dependent linear and nonlinear dynamics in waveguide arrays. We found that in certain arrays, the band structure and the modal shapes of the array modes are markedly different for the two polarizations, in a manner that cannot be simply explained using the effective index approximation. Specifically, one of the gaps was found to be missing for the TM polarization. In the nonlinear regime, we observe mixedpolarization nonlinear localizations in high bands, such as Band2 FloquetBloch vector solitons. The band structure anomaly enabled the excitation of a multiband moving breather. (C) 2005 Optical Society of America.

(2005) Physical review letters. 94, 8, 083002. Abstract
Coherentcontrol schemes to manipulate weakfield interactions are generally invalid at stronger fields, since strongfield interactions are accompanied by level power broadenings and level shifts that usually elude simple analytical treatments. Here we show that a broad subgroup of weakfield solutions (those with real fields, i.e., fields with only one quadrature in the complex plane) can be extended to the strongfield regime while retaining their properties. The salient feature of these fields is a symmetry that cancels out power broadening effects. Such fields can be generated from ultrashort coherent pulses or from incoherent broadband downconverted light. Weakfield coherentcontrol approaches based on these solutions can therefore be extended to the strongfield regime as we demonstrate in a twophoton absorption experiment in atomic cesium.

(2005) Optics Express. 13, 5, p. 14681476 Abstract
The ability to perform optical sectioning is one of the great advantages of laserscanning microscopy. This introduces, however, a number of difficulties due to the scanning process, such as lower frame rates due to the serial acquisition process. Here we show that by introducing spatiotemporal pulse shaping techniques to multiphoton microscopy it is possible to obtain fullframe depth resolved imaging completely without scanning. Our method relies on temporal focusing of the illumination pulse. The pulsed excitation field is compressed as it propagates through the sample, reaching its shortest duration at the focal plane, before stretching again beyond it. This method is applied to obtain depthresolved twophoton excitation fluorescence (TPEF) images of drosophila eggchambers with nearly 10^{5} effective pixels using a standard Ti:Sapphire laser oscillator.

(2005) Optics Express. 13, 6, p. 17971807 Abstract
The interaction between parallel beams in onedimensional discrete Kerr systems has been investigated using arrays of coupled channel waveguides. The experiments were performed in AlGaAs waveguides at 1550 nm which corresponds to photon energies just below one half the semiconductor's bandgap. The input intensity and relative input phase between the input beams was varied and the output intensity patterns were recorded. Observed was behavior ranging from a linear response, to soliton interactions between moderately and then strongly localized spatial solitons. Finally the influence of multiphoton absorption and asymmetric beam inputs on these interactions was investigated at very high intensities. (C) 2005 Optical Society of America.

(2005) Physical review letters. 94, 7, 073601. Abstract
We experimentally demonstrate shaping of the twophoton wave function of entangledphoton pairs, utilizing coherent pulseshaping techniques. By performing spectralphase manipulations we tailor the secondorder correlation function of the photons exactly like a coherent ultrashort pulse. To observe the shaping we perform sumfrequency generation with an ultrahigh flux of entangled photons. At the appropriate conditions, sumfrequency generation performs as a coincidence detector with an ultrashort response time (∼100 fs), enabling a direct observation of the twophoton wave function. This property also enables us to demonstrate backgroundfree, highvisibility twophoton interference oscillations.

(2005) Physical review letters. 94, 4, 043602. Abstract
We experimentally demonstrate sumfrequency generation with entangled photon pairs, generating as many as 40000 photons per second, visible even to the naked eye. The nonclassical nature of the interaction is exhibited by a linear intensity dependence of the nonlinear process. The key element in our scheme is the generation of an ultrahigh flux of entangled photons while maintaining their nonclassical properties. This is made possible by generating the downconverted photons as broadband as possible, orders of magnitude wider than the pump. This approach can be applied to other nonlinear interactions, and may become useful for various quantummeasurement tasks.

(2005) Optoelectronics Review. 13, 2, p. 103106 AbstractOptics in non homogeneous waveguide arrays
Optical discrete solitons possess extremely interesting dynamical properties. Such properties lead to the realization of several ultrafast, alloptical switching mechanisms, based on the use of uniform and nonuniform waveguides array.

(2005) Multiphoton Microscopy In The Biomedical Sciences V. 5700, p. 3543 10. Abstract
The ability to perform optical sectioning is one of the great advantages of laserscanning microscopy. This introduces, however, a number of difficulties due to the scanning process, such as lower frame rates due to the serial acquisition process. Here we show that by introducing spatiotemporal pulse shaping techniques to multiphoton microscopy it is possible to obtain fullframe depth resolved imaging completely without scanning. Our method relies on temporal focusing of the illumination pulse. The pulsed excitation field is compressed as it propagates through the sample, reaching its shortest duration at the focal plane, before stretching again beyond it. Combining temporal focusing with linescanning microscopy results in an enhanced depth resolution, equivalent to that achieved by point scanning. Both the scanningless and the linescanning techniques are applied to obtain depthresolved twophoton excitation fluorescence (TPEF) images of drosophila eggchambers.
2004

(2004) Optics Express. 12, 26, p. 66006605 Abstract
The smallest spot in optical lithography and microscopy is generally limited by diffraction. Quantum lithography, which utilizes interference between groups of N entangled photons, was recently proposed to beat the diffraction limit by a factor N. Here we propose a simple method to obtain N photons interference with classical pulses that excite a narrow multiphoton transition, thus shifting the "quantum weight" from the electromagnetic field to the lithographic material. We show how a practical complete lithographic scheme can be developed and demonstrate the underlying principles experimentally by twophoton interference in atomic Rubidium, to obtain focal spots that beat the diffraction limit by a factor of 2.

(2004) Optics Letters. 29, 24, p. 28902892 Abstract
We report an experimental study of discrete gap solitons in binary arrays of optical waveguides. We observe selffocusing indicating soliton generation when the inclination angle of an input beam is slightly above the Bragg angle and show that the propagation direction of the emerging gap soliton is influenced by the effect of interband momentum exchange. (C) 2004 Optical Society of America.

(2004) Journal Of The Optical Society Of America BOptical Physics. 21, 11, p. 19641968 Abstract
Performing thirdharmonicgeneration (THG) microscopy with a cylindrically focused Gaussian beam results in significant differences from standard THG microscopy owing to the different phasematching geometry. These differences are characterized analytically in the slowly varying envelope approximation. It is shown that THG is not observed in samples with normal dispersion even for line illumination. We use this to perform videorate THG microscopy. (C) 2004 Optical Society of America.

(2004) Physical Review Letters. 93, 9, Abstract
We report our investigation of Kerr nonlinear beam interactions in discrete systems. The influence of power and the relative phase between two Gaussian shaped beams was investigated in detail by performing numerical simulations of the discrete nonlinear Schrodinger equation and comparing the results with experiments done in AlGaAs waveguide arrays. Good agreement between theory and experiment was obtained.


(2004) Physical Review A. 70, 2, p. 02341510234154 023415. Abstract
Alloptical processing of the entire vibrational spectrum was investigated by using a more complex and carefully designed probe pulse. The probe pulse was constructed containing contributions from several narrow spectral bands with controllable intensities and phases. Strong interference effcts were observed in the coherent antiStokes Raman spectroscopy (CARS) which resulted from the presence of several spectrally separated probe frequencies. A scheme for nonlinear spectroscopy was also suggested, where the information from the entire spectrum could be collected into a single coherent entity through coherent control of the nonlinear process.

(2004) Journal of Structural Biology. 147, 1, p. 311 Abstract
Third harmonic generation microscopy is shown to be a robust method for obtaining structural information on a variety of biological specimens. Its nature allows depthresolved imaging of inhomogeneities with virtually no background from surrounding homogeneous media. With an appropriate illumination geometry, third harmonic generation microscopy is shown to be particularly suitable for imaging of biogenic crystals, enabling extraction of the crystal orientation.

(2004) Physical review letters. 93, 2, p. 02300510230054 023005. Abstract
The twophoton absorption in rubidium was investigated with broadband downconverted light. The pulseshaping methods were used to control twophoton absorption, which demonstrated that the spectral and temporal resolutions were 35 orders of magnitude below the actual bandwidth and temporal duration of the light. The pulse shaper separates the spectral components of the beam and ultilizes a computer controlled spatial light modulator (SLM) to introduce desired spectral phase filter to the light. It was found that the broadband downconverted light induced twophoton absorption with the same sharp temporal behavior as femtosecond pulses.

(2004) Journal of Lightwave Technology. 22, 6, p. 14631471 Abstract
A novel approach for an optical directsequence spread spectrum is presented. It is based on the complementary processes of broadband parametric downconversion and upconversion. With parametric downconversion, a narrowband continuouswave (CW) optical field is transformed into two CW broadband whitenoise fields that are complex conjugates of each other. These noise fields are exploited as the key and conjugate key in optical directsequence spread spectrum. The inverse process of parametric upconversion is then used for multiplying the key by the conjugate key at the receiver in order to extract the transmitted data. A complete scheme for optical codedivision multiple access (OCDMA) based on this approach is presented. The salient feature of the approach presented in this paper is that an ideal whitenoise key is automatically generated, leading to highcapacity versatile codedivision multipleaccess configurations.

(2004) Physical Review Letters. 92, 16, Abstract
We report the first experimental observation of modulation instability in a discrete optical nonlinear array.

(2004) Optics Letters. 29, 8, p. 890892 Abstract
We demonstrate a new scanning femtosecond pulseshaping technique that allows pulse shapes to be modulated at kilohertz rates. This technique is particularly useful for lockin measurements in which the signal is synchronized with the alternating pulse shapes. The pulseshape lockin technique is demonstrated in resonant coherent antiStokes Raman scattering, where it is shown to significantly improve the ratio of the resonant signal to both the nonresonant background and to noise.

(2004) Journal of Neuroscience Methods. 133, 12, p. 153159 Abstract
Using a pulsed UV laser in a confocal scanning microscope, we present a relatively cheap, accurate and efficient method for fast UV laser flash photolysis of caged molecules in twodimensional cultured neurons. The laser light is introduced through the imaging optics, can be localized by a parallel red laser and can photolyse a sphere of less than 1μm^{2}, and evoke local fluorescence changes in the imaged neurons. Caged glutamate and caged fluorescein are used to illustrate a disparity between spines and their parent dendrites at a submicron resolution.

(2004) Physical Review Letters. 92, 9, Abstract
Bright and dark spatial gap solitons are demonstrated in waveguide arrays. These gap solitons travel across the array at zero transverse velocity, in complete analogy with stationary (immobile) temporal gap solitons. Furthermore, the launching configuration for observing these stationary gap solitons is shown to be the analog of an "ideal experiment" for observing stationary temporal gap solitons, never observed so far. A clear distinction is established between the family of FloquetBloch solitons in general and discrete solitons in particular, and the limiting case of gap solitons.

(2004) Physical review letters. 92, 10, p. 10300311030034 103003. Abstract
The polarization pulse shaping was applied to the control of twophoton absorption in atomic rubidium. It was shown that the technique enables the manipulation of the transient vector properties of a light matter interaction. It was established that control can be exerted on the angular distribution of the final state. The ability to control the final state population of a nearly degenerate system and to perform Mstate resolved spectroscopy were demonstrated.
2003

(2003) Optics Letters. 28, 23, p. 23152317 Abstract
We achieve depthresolved polarization microscopy by measuring thirdharmonic generation induced by a tightly focused circularly polarized beam. In crystals exhibiting strong birefringence this signal is dominated by positively phasematched thirdharmonic generation. This process occurs in only optically anisotropic media, in which the birefringence compensates for the phase mismatch between the fundamental and the third harmonic induced by dispersion. Both the intensity and the polarization of the emitted signal provide information on the local optical anisotropy. We demonstrate the technique by imaging biogenic crystals in sea urchin larval spicules.

(2003) Physical Review Letters. 91, 22, Abstract
We investigate the propagation of short, intense laser pulses in arrays of coupled silica waveguides, in the anomalous dispersion regime. The nonlinearity induces trapping of the pulse in a single waveguide, over a wide range of input parameters. A sharp transition is observed for single waveguide excitation, from strong diffraction at low powers to strong localization at high powers.

(2003) Physical Review Letters. 91, 14, Abstract
We report the first experimental observation of discrete vector solitons in AlGaAs nonlinear waveguide arrays. These selftrapped states are possible through the coexistence of two orthogonally polarized fields and are stable in spite of the presence of fourwave mixing effects. We demonstrate that at sufficiently high power levels the two polarizations lock into a highly localized vector discrete soliton that would have been otherwise impossible in the absence of either one of these two components.

(2003) Nature. 424, 6950, p. 817823 Abstract
Light propagating in linear and nonlinear waveguide lattices exhibits behaviour characteristic of that encountered in discrete systems. The diffraction properties of these systems can be engineered, which opens up new possibilities for controlling the flow of light that would have been otherwise impossible in the bulk: these effects can be exploited to achieve diffraction free propagation and minimize the power requirements for nonlinear processes. In twodimensional networks of waveguides, selflocalized statesor discrete solitonscan travel along 'wirelike' paths and can be routed to any destination port. Such possibilities may be useful for photonic switching architectures.

(2003) Physical Review Letters. 90, 25, Abstract
Multiphoton fluorescence is used for the direct observation of a new class of breathers in waveguide arrays, which are a coherent superposition of FloquetBloch solitons of different bands. These FloquetBloch breathers oscillate along their spatial propagation axis, and possess several novel properties. Some behavior of these breathers is readily understood intuitively in terms of the band structure of the waveguide array and the properties of discrete solitons.

(2003) Optics Express. 11, 12, p. 13851391 Abstract
A novel method for detection of noblemetal nanoparticles by their nonlinear optical properties is presented and applied for specific labeling of cellular organelles. When illuminated by laser light in resonance with their plasmon frequency these nanoparticles generate an enhanced multiphoton signal. This enhanced signal is measured to obtain a depthresolved image in a laser scanning microscope setup. Plasmonresonance images of both live and fixed cells, showing specific labeling of cellular organelles and membranes, either by twophoton autofluorescence or by thirdharmonic generation, are presented.

(2003) Journal of Chemical Physics. 118, 20, p. 92089215 Abstract
Singlepulse vibrational spectroscopy was readily achieved using coherent control techniques. By tailoring the spectral phase of an ultrashort pulse, the interference between quantum processes induced by the various spectral components of the pulse was controlled, leading either to selective population of given Raman levels or to the generation of narrow features in the CARS spectrum by all the populated Raman levels. By applying this principle, highresolution spectroscopy was demonstrated in the vibrational energy range 7001400 cm^{1}. By measuring a complex Raman spectrum of a molecule in this spectral region, the method was found to be robust and practical.

(2003) Physical Review Letters. 90, 21, Abstract
Phaseandpolarization coherent control is applied to control the nonlinear response of a quantum system. We use it to obtain highresolution backgroundfree singlepulse coherent antiStokes Raman spectra. The ability to control both the spectral phase and the spectral polarization enables measurement of a specific offdiagonal susceptibility tensor element while exploiting the different spectral response of the resonant Raman signal and the nonresonant background to achieve maximal background suppression.

(2003) Optics Letters. 28, 10, p. 834836 Abstract
We investigated the interaction of discrete solitons with defect states fabricated in arrays of coupled waveguides. We achieved attractive and repulsive defects by decreasing and increasing, respectively, the spacing of one pair of waveguides in an otherwise uniform array. Linear and nonlinear propagation in the same samples show distinctly different properties. The role of the PeierlsNabarro potential in the interaction of the soliton with the defect is discussed. (C) 2003 Optical Society of America.

(2003) IEEE Journal of Quantum Electronics. 39, 1, p. 3150 Abstract
We overview theoretical and experimental results on spatial optical solitons excited in arrays of nonlinear waveguides. First, we briefly summarize the basic properties of the discrete nonlinear Schrodinger (NLS) equation frequently employed to study spatially localized modes in arrays, the socalled discrete solitons. Then, we introduce an improved analytical model that describes a periodic structure of thinfilm nonlinear waveguides embedded into an otherwise linear dielectric medium. Such a model of waveguide arrays goes beyond the discrete NLS equation and allows studying many new features of the nonlinear dynamics in arrays, including the complete bandgap spectrum, modulational instability of extended modes, different types of gap solitons, the mode oscillatory instability, the instabilityinduced soliton dynamics, etc. Additionally, we summarize the recent experimental results on the generation and steering of spatial solitons and diffraction management in waveguide arrays. We also demonstrate that many effects associated with the dynamics of discrete gap solitons can be observed in a binary. waveguide array. Finally, we discuss the important concept of twodimensional (2D) networks of nonlinear waveguides, not yet verified experimentally, which provides a roadmap for the future developments of this field. In particular, 2D networks of nonlinear waveguides may allow a possibility of realizing useful functional operations with discrete solitons such as blocking, routing, and time gating.
2002

(2002) Journal Of The Optical Society Of America BOptical Physics. 19, 12, p. 29382944 Abstract
We investigate the generation of discrete spatial solitons in arrays of coupled waveguides. Light was launched into the center of the array, and different beam sizes and array geometries were tested. At low power, the propagating field spreads as it couples to more waveguides. When the intensity is increased, localization is observed around the input waveguides, leading to the formation of a discrete soliton. For wide input beams, exciting a few waveguides, soliton splitting, which is due to instability induced by multiphoton absorption, is observed. All of these effects are described well by a coupledmode formalism. (C) 2002 Optical Society of America.

(2002) Journal Of The Optical Society Of America BOptical Physics. 19, 11, p. 26372644 Abstract
An AlGaAs waveguide array below the halfbandgap is used to investigate experimentally basic dynamic features of discrete systems. In particular, nonlinear locking of a discrete soliton to its input waveguide was observed for certain input conditions. We also investigated the soliton dynamics as a function of the position of the initial excitation and found that small shifts from the centers of symmetries of the structure could be greatly enhanced. Both effects depend on the geometry of the array and on the beam size. (C) 2002 Optical Society of America.

(2002) Nature. 418, 6897, p. 512514 Abstract
Molecular vibrations have oscillation periods that reflect the molecular structure, and are hence being used as a spectroscopic fingerprint for detection and identification. At present, all nonlinear spectroscopy schemes use two or more laser beams to measure such vibrations. The availability of ultrashort (femtosecond) optical pulses with durations shorter than typical molecular vibration periods has enabled the coherent excitation of molecular vibrations using a single pulse. Here we perform singlepulse vibrational spectroscopy on several molecules in the liquid phase, where both the excitation and the readout processes are performed by the same pulse. The main difficulty with singlepulse spectroscopy is that all vibrational levels with energies within the pulse bandwidth are excited. We achieve high spectral resolution, nearly two orders of magnitude better than the pulse bandwidth, by using quantum coherent control techniques. By appropriately modulating the spectral phase of the pulse we are able to exploit the quantum interference between multiple paths to selectively populate a given vibrational level, and to probe this population using the same pulse. This scheme, using a single broadband laser source, is particularly attractive for nonlinear microscopy applications, as we demonstrate by constructing a coherent antiStokes Raman (CARS) microscope operating with a single laser beam.

(2002) Physical Review A. 65, 4 B, p. 434081434084 043408. AbstractQuantum control of coherent antiStokes Raman processes
Quantum control of coherent antiStokes Raman spectroscopy (CARS) was demonstrated. Raman spectra was measured with a resolution of the order of 5 cm^{1} using shaped pulses with a bandwidth of about 120 cm^{1}. Nearly complete suppression of the nonresonant background signal was also demonstrated. Selective population of only one of the two Raman levels of pyridine, lying within the bandwidth of excitation pulses, was achieved. Simple modeling was used to derive the required spectral phases and the measured signals were found in agreement with the model predictions.

(2002) Physical review letters. 88, 12, p. 12300411230044 123004. AbstractCoherent transient enhancement of optically induced resonant transitions
The coherent transient enhancement of optically induced resonant transitions was discussed. Using pulse shaping techniques, the total transient population was enhanced by inducing constructive interference, and for 100 fs pulses, an enhancement by a factor of about 2.5 was demonstrated. The results showed that the population transient peak remains nearly constant, even when the absorption coefficient was increased by over three order of magnitude.

(2002) Physical review letters. 88, 6, p. 063004/1063004/4 063004. AbstractNarrowband coherent antistokes Raman signals from broadband pulses
A narrowband coherent antiStokes Raman spectroscopy (CARS) signal was generated from a broadband probe pulse through simple spectral phase manipulation of the probe pulse. The spectral resolution increased by an order of magnitude. The signal at a given wavelength was enhanced by almost a factor of 2 relative to the maximal signal obtained at this frequency by a transform limited pulse.

(2002) Physical review letters. 89, 27, Abstract
High spectral resolution nonlinear vibrational spectroscopy with a single ultrashort pulse is demonstrated on a variety of samples. The spectral data are obtained by shaping the excitation pulse in order to control the relative phase between the weak resonant signal and the strong nonresonant background, in analogy with phasecontrast microscopy techniques. This is unlike the more conventional approach to nonlinear spectroscopy, in which the nonresonant background is reduced to a minimum. By measuring the spectrum of the coherent antiStokes Raman signal, it is possible to infer the vibrational energy levels in a band spanning almost an entire octave.
2001


(2001) Physical Review Letters. 87, 4, Abstract
We observed simultaneous focusing in both space and time for light pulses propagating in a planar waveguide. In particular, 60 fs pulses with a width of 170 mum were injected into a planar glass waveguide in the anomalous dispersion regime. Output pulses as short as 30 fs and as narrow as 20 mum were measured. The results suggest that multiphoton absorption and intrapulse stimulated Raman scattering arrest the spatiotemporal contraction. The results were compared to the pulse evolution in zero and normal dispersion regimes and were shown to be significantly different. All of the experimental results were reproduced by a numerical model.

(2001) Physical Review Letters. 86, 15, p. 32963299 Abstract
We show that two regimes of diffraction exist in arrays of waveguides, depending upon the input conditions. At higher powers, normal diffraction leads to selffocusing and to the formation of bright solitons through the nonlinear Kerr effect. By slightly changing the input conditions, light experiences anomalous diffraction and is nonlinearly defocused. For the first time, selffocusing and selfdefocusing have been achieved for the same medium, structure, and wavelength.

(2001) Physical review letters. 86, 1, p. 4750 Abstract
Resonant multiphoton transitions were enhanced by exploiting the general spectral response of the interaction around resonance. Large enhancements of resonant twophoton absorption (TPA) were achieved by properly designing the spectral amplitude and phase of the exciting pulse.

(2001) Acta Physica Polonica A. 99, 1, p. 5765 AbstractSteering and locking of discrete solitons in optical waveguide arrays
By operating an AlGaAs waveguide array at the half band gap, we experimentally demonstrate basic features of discrete systems, which do not appear in the corresponding continuous counterpart  the slab waveguide. Under selected initial conditions, we observed nonlinearly induced locking of an initially moving soliton to the input waveguide. If the power is even higher, originally locked beams start to move again, but in a direction opposite to the initial tilt.
2000

(2000) Physical Review Letters. 85, 9, p. 18631866 Abstract
By using the diffraction properties of waveguide arrays, we propose a scheme to produce structures with designed diffraction. We fabricated arrays with reduced, canceled, and even reversed diffraction. Results of experiments with such waveguides are presented and compared with the predictions made by coupledmode theory.

(2000) Unconventional Optical Elements For Information Storage, Processing And Communications. 75, p. 155162 AbstractAdaptive techniques in ultrafast optics
Adaptive selflearning optical systems for ultrashort pulse compression and shaping are presented. In these systems, the input pulses are modified iteratively by updatable femtosecond pulse shapers, according to feedback signals derived from measurements of the output pulses, to approach desired waveforms. These adaptive techniques remove many of the difficulties associated with femtosecond pulse manipulations, such as the need for characterization of the input pulses, and optimization of the spectral filters of ultrashort pulse shapers in the presence of noise and uncertainty resulting from experimental constraints. The potential of the adaptive selflearning approach is demonstrated experimentally by compressing uncharacterized 80 fs long pulses of a modelocked Ti:Sapphire laser down to 11 fs, and by adaptively shaping uncharacterized input pulses into desired temporal intensity distributions. These adaptive techniques for ultrashort pulse manipulations can be extended towards quantum coherent control, where quantum systems can be adaptively steered towards desired final quantum states.

(2000) ElectroOptics And Microelectronics, Proceedings. 14, p. 99102 AbstractLaser scanning thirdharmonic generation microscopy in biology
A laser scanning microscope using thirdharmonic generation as a probe is shown to produce highresolution images of transparent biological specimens. Third harmonic light is generated by a tightly focused shortpulse laser beam and collected pointbypoint to form a digital image. Demonstrations with two biological samples are presented. Live neurons in a cell culture are imaged with clear and detailed images. including organelles at the threshold of optical resolution. Internal organelles of yeast cells are also imaged, demonstrating the ability of the technique for cellular and intracellular imaging.
1999

(1999) Physical Review Letters. 83, 14, p. 27262729 Abstract
By using a nonlinear waveguide array we experimentally demonstrate dynamic features of solitons in discrete systems. Spatial solitons do not exhibit these properties in continuous systems. We experimentally recorded nonlinearly induced locking of an initially moving soliton at a single waveguide. We also show that discrete solitons can acquire transverse momentum and propagate at an angle with respect to the waveguide direction, when the initial excitation is not centered on a waveguide. This is to our knowledge the first time that the effect of the PeierlsNabarro potential has been observed in a macroscopic system.

(1999) Optics Express. 5, 8, p. 169175 AbstractLaser scanning thirdharmonicgeneration microscopy in biology
A laser scanning microscope using thirdharmonic generation as a probe is shown to produce highresolution images of transparent biological specimens. Third harmonic light is generated by a tightly focused shortpulse laser beam and collected pointbypoint to form a digital image. Demonstrations with two biological samples are presented. Live neurons in a cell culture are imaged with clear and detailed images, including organelles at the threshold of optical resolution. Internal organelles of yeast cells are also imaged, demonstrating the ability of the technique for cellular and intracellular imaging. (C) 1999 Optical Society of America.

(1999) Applied Physics Letters. 75, 10, p. 13481350 Abstract
We experimentally investigate the linear and nonlinear optical properties of a nonuniform waveguide array. By reducing the width of a single waveguide, we decrease its effective index and induce waveguiding along the defect. Due to the positive nonlinearity, the index difference is reduced for increasing power levels with the result that the field escapes. Waveguiding is suppressed by the action of the nonlinearity. (C) 1999 American Institute of Physics. [S00036951(99)011365].

(1999) Physical Review A. 60, 2, p. 12871292 Abstract
Multiphoton transitions can be reached by many routes through a continuum of virtual levels. We show that the transition probability of twophoton and multiphoton processes can be controlled by tailoring the shape of an ultrashort excitation pulse, so that the many paths leading to the final state interfere to give a desired probability amplitude. We analyze the effect of pulse shapes on Nphoton absorption as well as on Raman transitions. We show theoretically that certain tailored dark pulses do not excite the system at all, while other shaped pulses induce transitions as effectively as transform limited pulses, even when their peak amplitudes are greatly reduced. These results are confirmed experimentally for twophoton transitions in atomic cesium. [S10502947(99)008082].

(1999) Physical Review Letters. 83, 3, p. 540543 Abstract
We report the numerical observation of edgetype phase defects which occur during the selffocusing of ultrashort pulses. We identified two distinct kinds of defects, farmed at the temporal or spatial edges of the pulse. We show the effect of phase defect creation on the dynamics and the eventual arrest of the selffocusing process. Phase defects often lead to the inversion of the phase fronts curvature, transforming contracting pulses into expanding pulses.

(1999) Applied Physics Letters. 74, 21, p. 31073109 Abstract
Thirdharmonic microscopy is shown to be a powerful tool for the study of liquid crystal structures. The thirdharmonic probe enables the investigation of molecular order well inside a liquid crystal cell. The observation of phase transition in various nematic liquid crystal samples is presented as a demonstration for the power of this technique. For example, nematic and isotropic regions are shown to coexist across the depth of the cell. Thermal fluctuations are observed in the nematic regions close to the isotropicnematic transition temperature. (C) 1999 American Institute of Physics. [S00036951(99)043211].

(1999) Physical Review Letters. 82, 15, p. 30463049 Abstract
We show the generation of a strong phasematched third harmonic in a simple parallel nematic liquid crystal cell. A single laser beam of femtosecond pulses is used to deform the internal structure of the molecular orientation distribution, and to generate the third harmonic from the twisted structure that is formed. Up to 10(5) of the laser power was converted to the third harmonic, in a volume as small as 5 mu m(3). [S00319007(99)088596].
1998

(1998) Nature. 396, 6708, p. 239242 AbstractCoherent quantum control of twophoton transitions by a femtosecond laser pulse
Coherent quantum control(13) has attracted interest as a means to influence the outcome of a quantummechanical interaction. In principle, the quantum system can be steered towards a desired state by its interaction with light. For example, in photoinduced transitions between atomic energy levels, quantum interference effects can lead to enhancement or cancellation of the total transition probability. The interference depends on the spectral phase distribution of the incident beam; as this phase distribution can be tuned, the outcome of the interaction can in principle be controlled. Here we demonstrate that a femtosecond laser pulse can be tailored, using ultrashort pulseshaping(47) techniques, to control twophoton transitions in caesium. By varying the spectral phases of the pulse components, we observe the predicted cancellation of the transitions due to destructive quantum interference; the power spectrum and energy of these 'dark pulses' are unchanged. We also show that the pulse shape can be modified extensively without affecting the twophoton transition probability.

(1998) Physical Review Letters. 81, 16, p. 33833386 Abstract
We report the observation of discrete spatial optical solitons in an array of 41 waveguides. Light was coupled to the central waveguide. At low power, the propagating field spreads as it couples to more waveguides. When sufficient power was injected, the field was localized close to the input waveguides and its distribution was successfully described by the discrete nonlinear Schrodinger equation. [S00319007(98)072950].

(1998) IEEE Photonics Technology Letters. 10, 10, p. 13891391 Abstract
Noiselike generation in lasers can be controlled by changing the cavity in order to obtain pulses with unique properties: Intense noiselike pulses, as narrow as a few picoseconds, were obtained, These are two orders of magnitude narrower than pulses obtained in previous work, In long cavities, coherent and incoherent twocolor noiselike generation were demonstrated. The wavelength difference between the generated pulses could be tuned in a wide wavelength range, which is much broader than the amplifier bandwidth. Highenergy approximate to 16nJ noiselike pulses with a broadband spectrum and narrow intensity autocorrelation trace were also demonstrated.

(1998) Journal Of The Optical Society Of America BOptical Physics. 15, 5, p. 16151619 Abstract
Experimental results of a practical selflearning pulseshaping system are presented. Realtime adaptive pulse shaping of uncharacterized pulses is achieved. A crosscorrelation feedback measurement of the output pulses is used by a simulatedannealing algorithm to modify the pulses iteratively toward target shapes. This scheme can readily be used for coherent control of quantum dynamics. (C) 1998 Optical Society of America [S07403224(98)013058].
1997

(1997) Optics Letters. 22, 23, p. 17601762 Abstract
We analyze the transmissivity of a nonlinear filter that is based on intensitydependent polarization rotation in a birefringent fiber. It is shown that the transmissivity of the element depends not only on the intensity of the incident light but also on the time behavior of its amplitude. Such an element can be used as a derivator, an element that transmits only variations in the input pulse. The filter can also be used for obtaining lasers that generate a train of intense noiselike pulses with a broadband spectrum and a short coherence length. (C) 1997 Optical Society of America.

(1997) Optics Letters. 22, 23, p. 17931795 Abstract
A practical adaptive method for femtosecond optical pulse compression is demonstrated experimentally for the first time to our knowledge. The method is robust and capable of handling the general case of pulse compression, in which the input pulses are completely uncharacterized or partially characterized. (C) 1997 Optical Society of America.

(1997) IEEE Journal of Quantum Electronics. 33, 11, p. 19691974 Abstract
White light dispersion measurements by one and twodimensional spectral interference are shown. Onedimensional white light spectral interference measurements allow accurate characterization of dispersion using weak optical fields. Twodimensional spectral interference allows for realtime measurements since no postprocessing is needed, and therefore can be used in situations where the optical properties of the elements under test vary in time. We demonstrate the applicability of the two methods for characterizing dispersive elements such as optical glasses and dielectric coatings.

(1997) Journal Of The Optical Society Of America BOptical Physics. 14, 11, p. 32533260 Abstract
The vector Maxwell's equations are solved to simulate propagating and colliding optical pulses in planar waveguides. The slowly varying envelope approximation is not made, and therefore the dynamics of the optical carrier are retained in the calculations. Some short optical pulses are found to be essentially unchanged over rather long propagation distances and stable over some energy variation in a manner characteristic of light bullets. With greater energy increase the pulses do not collapse, as predicted by the nonlinear Schrodinger equation; rather, after initially compressing, they undergo unlimited expansion. Because the optical pulses that are employed in these simulations are extremely short, they are beyond the limitations of the slowly varying envelope approximation that is used in the derivation of the nonlinear Schrodinger equation. The dispersive effects are modeled by a single Lorentzian resonance, and the nonlinear refraction is modeled by a Kerrlike instantaneous nonlinearity. The procedure for obtaining numerical solutions to the nonlinear Maxwell equations is described, and solutions are obtained by a finitedifference algorithm. (C) 1997 Optical Society of America.

(1997) Journal Of The Optical Society Of America BOptical Physics. 14, 8, p. 21222125 Abstract
Interferometric and intensity autocorrelations by thirdharmonic generation for the measurement of ultrashort optical pulses are demonstrated. The thirdharmonic signal is generated by tight focusing of the ultrashort pulses upon the surface of an ordinary glass slide, replacing the highopticalquality nonlinear crystals used in conventional secondharmonic generation measurements. The method is shown to be applicable to realtime measurements. (C) 1997 Optical Society of America.

(1997) Journal Of The Optical Society Of America BOptical Physics. 14, 8, p. 20952098 Abstract
Realtime linear spectral interference measurements of ultrashort pulses are shown experimentally. The technique involves measurements of the twodimensional interference pattern of the spectral interference between a reference and a signal pulse propagating at an angle with respect to each other. No postprocessing is needed to extract the spectral phase difference between the two pulses. Quadratic spectral phase distortions as well as spectral phase discontinuities are measured. The method is applicable to singleshot measurements of ultraweak pulses and is useful for identification of the critical adjustments of ultrashort pulse shapers and compressors. (C) 1997 Optical Society of America.

(1997) Optics Communications. 138, 6Apr, p. 345348 Abstract
A practical adaptive approach for ultrashort optical pulse compression and shaping is nummerically investigated. A simple single valued feedback measurement was suggested and proven useful for compression. The method is robust, and capable of handling the general case of pulse compression and shaping, where the input pulses are completely uncharacterized or partially characterized.

(1997) Optics Letters. 22, 11, p. 799801 Abstract
An erbiumdoped fiber laser that produces a train of intense noiselike pulses with a broadband spectrum and a short coherence length is reported. The noiselike behavior was observed in the amplitude as well as in the phase of the pulses. The maximum spectral width obtained was 44 nm. The high intensity and the short coherence length of the light were maintained even after propagation through a long dispersive fiber. A theoretical model indicates that this mode of operation can be explained by the internal birefringence of the laser cavity combined with a nonlinear transmission element and the gain response of the fiber amplifier. (C) 1997 Optical Society of America.

(1997) Physical Review Letters. 78, 17, p. 32903293 Abstract
Ultrashort optical solitons with different stales of polarization are used to map the polarization evolution in a fixed section of a weakly birefringent fiber. Soliton polarization evolution was compared to linear propagation. A significant change in the polarization behavior between the solitons and linear regime is observed. Although solitons contain a continuum of instantaneous intensities, they transform their polarization as a unit, demonstrating the particlelike nature of solitons.

(1997) Applied Physics Letters. 70, 8, p. 922924 Abstract
Third harmonic generation near the focal point of a tightly focused beam is used to probe microscopical structures of transparent samples. It is shown that this method can resolve interfaces and inhomogeneities with axial resolution comparable to the confocal length of the beam. Using 120 fs pulses at 1.5 mu m, we were able to resolve interfaces with a resolution of 1.2 mu m. Twodimensional crosssectional images have also been produced. (C) 1997 American Institute of Physics.

(1997) 10Th Meeting On Optical Engineering In Israel. 3110, p. 215225 Abstract
Onedimensional and twodimensional linear spectral interference measurements of ultrashort pulses are shown. The methods involve measurements of the spectral interference pattern between ultrashort reference and signal pulses, from which the spectral phase difference can be extracted. Onedimensional measurements require postprocessing to extract the phase. Twodimensional measurements require no postprocessing, and are therefore most suitable for visual interpretation. Dispersion of optical glasses, as well as crafted spectral phase discontinuities are measured. The methods are applicable to single shot measurements of extremely weak pulses.

(1997) 10Th Meeting On Optical Engineering In Israel. 3110, p. 272276 Abstract
Third harmonic generation near the focal point of a tightly focused beam is used to probe microscopical structures of transparent samples. The axial resolution at which this method can resolve interfaces and inhomogeneities is shown to be comparable with the confocal length of the beam. Using 125 femtosecond pulses at 1.5 mu m, we were able to resolve interfaces with a resolution of 1.2 mu m and to produce two dimensional crosssectional images of the samples used.
1996

(1996) Optics Letters. 21, 15, p. 11201122 Abstract
We demonstrate the extension of the photorefractive effect in BaTiO3 crystals to the 1.5mu m wavelength regime, which is obtained by twophoton absorption induced by intense femtosecond light pulses. The strong dependence of the effect on the light's intensity permits nondestructive readout of the photorefractively induced pattern by lowintensity light. One can thus use the photorefractive effect in the 1.5mu m regime to form optical components for communication applications. (C) 1996 Optical Society of America
1995

(1995) Optics Letters. 20, 3, p. 246248 Abstract
The properties of elliptically polarized solitary waves in isotropic optical fibers are investigated. Two families of polarized solitary pulses are identified, but only one of them is shown to be stable. These pulses are characterized by a fixed polarization pattern that rotates at a constant rate as the pulse propagates down the fiber. The evolution of the polarization of these pulses is important for the operation of shortpulse fiber lasers and other nonlinear devices.
1984

(1984) Journal Of The Optical Society Of America BOptical Physics. 1, 4, p. 662670 Abstract
Instabilities and selfoscillation in systems containing an optical Kerr medium are studied in detail. Nonlinear interactions, both within and without a cavity, are discussed. Instability thresholds and frequencies of selfoscillation are treated as a gainfeedback process. A detailed investigation of the instabilities in a highfinesse nonlinear ring cavity shows that both Ikeda instabilities and bistability are obtained in a rather limited regime of detuning. In most of the detuning range, the system exhibits periodτ oscillations. Counterpropagating waves in a Kerr medium are shown to become unstable above a certain threshold intensity. This results in a system that is equivalent to a Ramanlike laser that is being excited in the distributedfeedback structure generated by the two waves. A qualitative description of harmonic generation and period doubling, based on wavemixing processes, is also presented.
1983

(1983) Optics Communications. 48, 1, p. 5356 Abstract
It is shown that instabilities in a ring optical cavity emerge from a nonlinear wave mixing process. Several selfoscillating limits are analyzed, and a new type of oscillation having a period comparable to the medium lifetime is predicted. A mechanism for period doubling is also presented.

(1983) Proceedings of the IEEE. 71, 2, p. 208221 Abstract
The characteristics and limits of parallel transmission of TwoDimensional imagery through single optical fibers are reviewed, and methods for overcoming the inherent smearing that occurs at the output are discussed. These solutions indude: a) control of the fiber's refractive index profile; b) incorporation of complex filters; and c) encoding the information at the input and decoding the output after transmission.

1982

(1982) Optics Communications. 41, 6, p. 455458 Abstract
The steady state and temporal behaviour of real time holography by nondegenerate four wave mixing in organic dye saturable absorbers is analyzed and demonstrated. A transient peak in the intensity of the diffracted signal is observed and discussed. It is found that the peak is sometimes higher than the maximal steady state diffracted signal of the sample.

1981

(1981) Optics Communications. 39, 4, p. 265268 Abstract
Organic dyes with high triplet yield are used as saturable absorbers with very low saturation intensity. Phase conjugation is performed in a thin film by a cw laser. The theory of degenerate four wave mixing in saturable absorbers is verified and imaging capabilities are demonstrated.

(1981) Optics Letters. 6, 9, p. 414416 Abstract
The operation of MachZehnder and Michelson phaseconjugate interferometers is demonstrated. Phase conjugation is obtained by degenerate fourwave mixing in thin films of eosin. Highvisibility fringes are observed both in cw and pulsed modes of operation.

(1981) IEEE Journal of Quantum Electronics. 17, 9, p. 19671970 Abstract
The transient behavior of degenerate fourwave mixing (DFWM) in a threelevel saturable absorber is analyzed and verified experimentally, A sharp peak of phaseconjugated reflectivity is predicted and observed at high pump intensities for pulsed beams. The influence of the timedependent behavior of the conjugate wave signal on results of pulsedbeam experiments is studied. It is shown that transient effects could account for discrepancies between previous experimental results and steadystate theory.
1979
1975

(1975) Optics Communications. 14, 3, p. 290293 Abstract
The stability of laser resonators which contain a medium with a gain profile is investigated. Such resonators are unstable when the resonator mirrors are large (infinite aperture approximation), and the gain is lowest on the optical axis and increases with radial distance. It is shown that such resonators have welldefined regions of stability if the mirrors have gaussian reflectivity tapers.

(1975) APPLIED OPTICS. 14, 2, p. 306309 Abstract
The problem of the stability of the Gaussian modes of an optical resonator of large mirror apertures filled with a medium having a gain and index profile is considered. Recent results by other investigators are carefully examined in an attempt to clarify a discrepancy between them. It is found that the modes of the resonator are stable if the gain is highest on the resonator axis. When the gain increases with distance from the resonator axis, one can calculate a Gaussian eigenmode,: But this mode is unstable in the sense that any deviation from the exact physical parameters defining the mode will cause its divergence to an infinite spot size.