Publications

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 wave-packet echoes in a single, isolated molecule. The entire dephasing-rephasing 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 echo-a partial recovery of the initial coherent oscillations. The vibrational dynamics of single molecules is visualized by a time-delayed probe pulse dissociating them, one at a time. Two mechanisms for the echo formation are discussed: a.c. Stark-induced molecular potential shaking and creation of a depletion-induced 'hole' in the nuclear spatial distribution. The single-molecule wave-packet 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 wave-packet is partially rephased by a second pulse, and a wave-packet echo is observed. This wave-packet echo probes ultrafast intramolecular processes in the isolated molecule.

Coherent anti-Stokes Raman scattering (CARS) has found wide applications in biomedical research. Compared with alternatives, single-beam CARS is especially attractive at low frequencies. Yet, currently existing schemes necessitate a relatively complicated setup to perform high-resolution spectroscopy. Here we show that the spectral sharp edge formed by an ultra-steep long-pass 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 low-frequency regime (

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 speckle-correlation based focusing methods are discussed. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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 proof-of-principle 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 super-resolution techniques. Image scanning microscopy, a recent addition to the family of super-resolution 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 Q-ISM principle and obtain super-resolved optical images of a biological sample stained with fluores-cent quantum dots using photon antibunching, a quantum effect, as a resolution-enhancing contrast mechanism.

Vibrational spectroscopic imaging is useful and important in biological and medical studies. Yet, vibrational imaging in the terahertz region (

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 energy-density 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 entropy-temperature relation, modified to suit the nonlinear lattice systems. Generally, everywhere in the thermalization zone of the phase diagram, temperatures along a fixed system-density line, grow monotonously from zero to infinity. Isotherms on the phase diagram are concave.

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 low-frequency Raman lines via stimulated-Raman-scattering-induced spectral shifts. This method does not require any spectral filtration and is therefore an excellent candidate to resolve low-lying Raman lines (

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 one-dimensional 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 high-occupancy limit that are governed by the equivalent Gross-Pitaevskii equation.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy have many potential applications in biology and medicine. Among many variants of the technique itself, the method of single-pulse CARS spectroscopy and microscopy is attractive for its simplicity and quick implementation. Single-pulse 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 non-resonant signal, a background which is usually removed by lock-in 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, double-notch shaping is introduced. The double-notch 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 lock-in 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

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

The always diverging-converging laser beams, more rigorously referred to as Gaussian beams, are part of many physical and electro-optical systems. Obviously, a single set of analytic expressions describing these beams in a large span of divergence-convergence 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 electric-magnetic 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 x-electric y-magnetic components and vice versa and to the two electric-magnetic z-components. 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 laser-related systems and in the research of diverse physics and electro-optic fields. (C) 2017 Optical Society of America.

Scattering of light by random media limits many optical imaging and sensing applications, either by scrambling waves that carry images or by generating glare-background 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

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 frequency-conversion process. Photon pairs are generated from 693 to 708 nm, and the complete conversion process occurs within a single monolithic 5-cm-long stoichiometric lithium tantalate nonlinear crystal. We have characterized the dependence of this structure with respect to pump intensity and crystal temperature.

We measure ensemble-averaged quantum correlations of path-entangled 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 two-photon 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.

We present a single-pulse two-dimensional Raman spectroscopy scheme. Our scheme offers not only a major simplification of the conventional setup but also an inherent favoring of the direct fifth-order signal over the cascaded signal, the latter being a signal that carries no coupling information.

Vibrational modes are often localized in certain regions of a molecule, and so the coupling between these modes is sensitive to the molecular structure. Two-dimensional vibrational spectroscopy can probe the strength of this coupling in a manner analogous to two-dimensional NMR spectroscopy, but on ultrafast timescales. Here, we demonstrate how two-dimensional Raman spectroscopy, based on fifth-order 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 set-up, but also an inherent elimination of a competing nonlinear signal, which overwhelms the desired signal in other schemes and carries no coupling information.

Quasiperiodic lattices have recently been shown to be a nontrivial topological phase of matter. Charge pumping-one of the hallmarks of topological states of matter-was recently realized for photons in a one-dimensional off-diagonal 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 dimension-the 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.

Diffraction-limited imaging through complex scattering media is a long-sought-after goal with important applications in biomedical research. In recent years, high-resolution 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 all-optical techniques. Here, we show that by exploiting optical nonlinearities, a diffraction-limited 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 two-photon fluorescence signal through highly scattering biological samples. We use the formed focus to perform two-photon microscopy through highly scattering, visually opaque layers. (C) 2014 Optical Society of America

A new coherent anti-Stokes Raman spectroscopy (CARS) technique is reported for real-time 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 multi-channel lock-in amplifier. Using a two-dimensional 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.

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 ill-conditioned, 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

We report the experimental observation and theoretical analysis of a novel beam-steering 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 dissipation-assisted beam velocity control mechanism in nonlinear optical lattices, which is also theoretically predicted by the effective particle approach.

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 one-dimensional 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

We present a method to control photodissociation by manipulating the bond-softening 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 strong-field (1012-1013 W/cm2) photodissociation of H2+, exhibiting a substantial increase of dissociation by positively chirped pulses with respect to both negatively chirped and transform-limited 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.

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

Imaging with optical resolution through turbid media is a long sought-after goal with important applications in deep tissue imaging. Although extensively studied(1), this goal was considered impractical until recently. Adaptive-optics 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 wavefront-shaping, using spatial light modulators(5-7). Here, we show that wavefront-shaping enables wide-field 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(8-15), our technique does not require coherent sources(8-14), interferometric detection(10-14), raster-scanning(8-10,14,15) or off-line reconstruction(11-15). Our results bring wavefront-shaping closer to practical applications and realize the vision of looking through 'walls' and around corners(16).

We experimentally show that two-photon path-entangled states can be coherently manipulated by multimode interference in multimode waveguides. By measuring the output two-photon 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 two-photon states.

We study the case of two simultaneous three-wave-mixing 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 mid-infrared. 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

The competition between dissociation paths of I₂⁺ 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.

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 quartz-enhanced 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.

We report the experimental findings of a systematic study of the effect of linear chirp on strong field photodissociation of H₂⁺. 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.

Complete population transfer in a four-coupled-modes 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 two-state 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 four-state 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.

We demonstrate the acquisition of stimulated Raman scattering spectra with the use of a single femtosecond pulse. High-resolution 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, low-frequency Raman lines (

We show that a periodic two-dimensional (2D) photonic lattice with Kerr nonlinearity exhibits a Berezinskii-Kosterlitz-Thouless (BKT) crossover associated with a vortex-unbinding 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.

We present a simple and easily implementable scheme for multiplexed Coherent Anti-Stokes 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 time-delayed probe, resolving a broad vibrational spectrum with high spectral resolution in a single-shot measurement. Our single-source, single-beam scheme allows the simple transformation of any multiphoton microscope with adequate bandwidth into a nearly alignment-free CARS microscope. (C) 2010 Optical Society of America

We predict quantum correlations between noninteracting particles evolving simultaneously in a disordered medium. While the particle density follows the single-particle dynamics and exhibits Anderson localization, the two-particle 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.

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 down-conversion 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 high-photon-number nonclassical correlations without requiring intense quantum resources.

We experimentally demonstrate efficient tunable upconversion by cascading optical oscillation and wide-band adiabatic sum-frequency 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

The employment of path-entangled 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 Mach-Zehnder 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 Mach-Zehnder 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. Mach-Zehnder superresolution is found to be a highly distinctive quantum effect.

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 near-IR.

We introduce an illumination configuration which is a spatiotemporal analog of a non-diffracting X-wave. 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 X-wave configuration analytically, numerically, and experimentally demonstrate the effect using two photon excitation fluorescence.

We report the observation of the signature of a localization phase transition for light in one-dimensional 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.

When a periodic 1D system described by a tight-binding 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.

The characterization and conditional preparation of multiphoton quantum states require the use of photon-number 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 coherent-state photon-number distributions and reconstructing them with high precision. Finally, we evaluate the suitability of such detectors for quantum state tomography and entanglement-based quantum state preparation, highlighting the effects of dark counts and cross talk between pixels.

We demonstrate fast standoff (>10 m) single-pulse coherent anti-Stokes Raman spectroscopy (CARS), from trace amounts of solids, under ambient light conditions, using phase or amplitude shaped femtosecond pulses.

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 signal-to-idler conversion over a bandwidth of 140 nm.

We provide an overview of recent experimental and theoretical developments in the area of optical discrete solitons. By nature, discrete solitons represent self-trapped wavepackets in nonlinear periodic structures and result from the interplay between lattice diffraction (or dispersion) and material nonlinearity. In optics, this class of self-localized states has been successfully observed in both one- and two-dimensional 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 liquid-crystal 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 multi-band, cavity, spatio-temporal, random-phase, vortex, and non-local lattice solitons, just to mention a few. The possibility of all-optically routing optical discrete solitons in 2D and 3D periodic environments using soliton collisions is also presented. Finally, soliton formation in optical quasi-crystals and at the boundaries of waveguide array structures are discussed. (C) 2008 Elsevier B.V. All rights reserved.

Coherent control of resonant and non-resonant two-photon absorption processes was examined using a spatio-temporal pulse-shaping technique. By utilizing a combination of temporal focusing and femtosecond pulse-shaping techniques, we spatially control multiphoton absorption processes in a completely deterministic manner. Distinctive symmetry properties emerge through two-dimensional mapping of spatio-temporal data. These symmetries break down in the transition to strong fields, revealing details of strong-field effects such as power broadenings and dynamic Stark shifts. We also present demonstrations of chirp-dependent population transfer in atomic rubidium, as well as the spatial separation of resonant and non-resonant excitation pathways in atomic caesium.

The tectorial membrane (TM) is a highly hydrated non-cellular 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 two-photon second-harmonic 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 shell-like 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.

We introduce a programmable, high-rate scanning femtosecond pulse shaper based on a two-dimensional 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 liquid-crystal SLM-based pulse shapers. An update rate in excess of 100 kHz is demonstrated. (c) 2007 Optical Society of America.

Objectives: We present a novel way to create high-resolution three-dimensional images of tooth dentin by harmonic generation scanning laser microscopy. Methods: The images were taken using a pulsed infrared laser. Three-dimensional 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 micro-morphological 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 micro-morphology of normal versus altered dentin.

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 two-photon 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 down-converted light enables the optimal exploitation of the correlations within the down-converted spectrum, thereby strongly affecting two-photon interactions even at classically high power levels, and opening new venues for applications such as two-photon spectroscopy and microscopy and optical spread spectrum communication.

We study nonlinear relaxation of the excited state in a system with two levels and a continuum. The process is first simulated in a one-dimensional waveguide array described by the discrete nonlinear Schrodinger equation. The results are interpreted in terms of degenerate four-wave 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.

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.

We have observed the incoherent interaction between a highly confined (blocker) soliton and wide, moving signal beams of a different wavelength in a one-dimensional 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 three-output router. (c) 2005 Optical Society of America.

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 third-harmonic 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.

We report high-rate, computer-controlled femtosecond pulse shaping by use of an electro-optical gallium arsenide optical phased-array 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 mode-locked oscillators. (c) 2005 Optical Society of America

The excitation and propagation of both scalar and vector optical waves in one-dimensional 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.

We investigate experimentally and numerically the interaction of a highly localized, single-channel discrete soliton (blocker) with a wide, tilted beam in a one-dimensional 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.

The interaction between parallel beams in one-dimensional 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.

Coherent-control schemes to manipulate weak-field interactions are generally invalid at stronger fields, since strong-field interactions are accompanied by level power broadenings and level shifts that usually elude simple analytical treatments. Here we show that a broad subgroup of weak-field solutions (those with real fields, i.e., fields with only one quadrature in the complex plane) can be extended to the strong-field 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 down-converted light. Weak-field coherent-control approaches based on these solutions can therefore be extended to the strong-field regime as we demonstrate in a two-photon absorption experiment in atomic cesium.

We experimentally demonstrate sum-frequency 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 down-converted 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 quantum-measurement tasks.

We report an experimental study of discrete gap solitons in binary arrays of optical waveguides. We observe self-focusing 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.

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.

All-optical 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 anti-Stokes 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.

The two-photon absorption in rubidium was investigated with broadband down-converted light. The pulse-shaping methods were used to control two-photon absorption, which demonstrated that the spectral and temporal resolutions were 3-5 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 down-converted light induced two-photon absorption with the same sharp temporal behavior as femtosecond pulses.

We report the first experimental observation of modulation instability in a discrete optical nonlinear array.

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 two-dimensional 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 1μm², 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 sub-micron resolution.

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.

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 two-dimensional networks of waveguides, self-localized states-or discrete solitons-can travel along 'wire-like' paths and can be routed to any destination port. Such possibilities may be useful for photonic switching architectures.

Multiphoton fluorescence is used for the direct observation of a new class of breathers in waveguide arrays, which are a coherent superposition of Floquet-Bloch solitons of different bands. These Floquet-Bloch 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.

Phase-and-polarization coherent control is applied to control the nonlinear response of a quantum system. We use it to obtain high-resolution background-free single-pulse coherent anti-Stokes Raman spectra. The ability to control both the spectral phase and the spectral polarization enables measurement of a specific off-diagonal susceptibility tensor element while exploiting the different spectral response of the resonant Raman signal and the nonresonant background to achieve maximal background suppression.

An AlGaAs waveguide array below the half-bandgap 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.

Quantum control of coherent anti-Stokes 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.

A narrow-band coherent anti-Stokes Raman spectroscopy (CARS) signal was generated from a broad-band 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.

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.

Resonant multiphoton transitions were enhanced by exploiting the general spectral response of the interaction around resonance. Large enhancements of resonant two-photon absorption (TPA) were achieved by properly designing the spectral amplitude and phase of the exciting pulse.

Adaptive self-learning 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 self-learning approach is demonstrated experimentally by compressing uncharacterized 80 fs long pulses of a mode-locked 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.

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 Peierls-Nabarro potential has been observed in a macroscopic system.

Multiphoton transitions can be reached by many routes through a continuum of virtual levels. We show that the transition probability of two-photon 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 N-photon 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 two-photon transitions in atomic cesium. [S1050-2947(99)00808-2].

Third-harmonic microscopy is shown to be a powerful tool for the study of liquid crystal structures. The third-harmonic 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 isotropic-nematic transition temperature. (C) 1999 American Institute of Physics. [S0003-6951(99)04321-1].

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. [S0031-9007(98)07295-0].

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.

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 Kerr-like instantaneous nonlinearity. The procedure for obtaining numerical solutions to the nonlinear Maxwell equations is described, and solutions are obtained by a finite-difference algorithm. (C) 1997 Optical Society of America.

Real-time linear spectral interference measurements of ultrashort pulses are shown experimentally. The technique involves measurements of the two-dimensional 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 single-shot 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.

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.

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. Two-dimensional cross-sectional images have also been produced. (C) 1997 American Institute of Physics.

The characteristics and limits of parallel transmission of Two-Dimensional 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.

The operation of Mach-Zehnder and Michelson phase-conjugate interferometers is demonstrated. Phase conjugation is obtained by degenerate four-wave mixing in thin films of eosin. High-visibility fringes are observed both in cw and pulsed modes of operation.