Publications
2020

Structured beams invariant to coherent diffusion(2020) arXiv. Abstract
Bessel beams are renowned members of a wide family of nondiffracting (propagationinvariant) fields. We report on experiments showing that nondiffracting fields are also immune to diffusion. We map the phase and magnitude of structured laser fields onto the spatial coherence between two internal states of warm atoms undergoing diffusion. We measure the field after a controllable, effective, diffusion time by continuously generating light from the spatial coherence. The coherent diffusion of BesselGaussian fields and more intricate, nondiffracting fields is quantitatively analyzed and directly compared to that of diffracting fields. To elucidate the origin of diffusion invariance, we show results for nondiffracting fields whose phase pattern we flatten.

(2020) Nature Communications. 11, 1, 3854. Abstract
The synchronization of human networks is essential for our civilization and understanding its dynamics is important to many aspects of our lives. Human ensembles were investigated, but in noisy environments and with limited control over the network parameters which govern the network dynamics. Specifically, research has focused predominantly on alltoall coupling, whereas current social networks and human interactions are often based on complex coupling configurations. Here, we study the synchronization between violin players in complex networks with full and accurate control over the network connectivity, coupling strength, and delay. We show that the players can tune their playing period and delete connections by ignoring frustrating signals, to find a stable solution. These additional degrees of freedom enable new strategies and yield better solutions than are possible within current models such as the Kuramoto model. Our results may influence numerous fields, including traffic management, epidemic control, and stock market dynamics.

(2020) Physical Review A. 102, 1, 013326. Abstract
Pointsource atom interferometry is a promising approach for implementing robust, highsensitivity, rotation sensors using cold atoms. However, its scale factor, i.e., the ratio between the interferometer signal and the actual rotation rate, depends on the initial conditions of the atomic cloud, which may drift in time and result in bias instability, particularly in compact devices with short interrogation times. We present two methods to stabilize the scale factor. One relies on a modelbased correction which exploits correlations between multiple features of the interferometer output and works on a singleshot basis. The other is a selfcalibrating method where a known bias rotation is applied to every other measurement, requiring no prior knowledge of the underlying model but reducing the sensor bandwidth by a factor of two. We demonstrate both schemes experimentally with complete suppression of scalefactor drifts, maintaining the original rotation sensitivity and allowing for biasfree operation over several hours.

(2020) Optics Letters. 45, 13, p. 34313434 Abstract
The FresnelFizeau effect of transverse drag, in which the trajectory of a light beam changes due to transverse motion of the optical medium, is usually extremely small and hard to detect. We observe transverse drag in a moving hotvapor cell, utilizing slow light due to electromagnetically induced transparency (EIT). The drag effect is enhanced by a factor 400,000, corresponding to the ratio between the light speed in vacuum and the group velocity under the EIT conditions. We study the contribution of the thermal atomic motion, which is much faster than the mean medium velocity, and identify the regime where its effect on the transverse drag is negligible.

(2020) Nanophotonics. 20200137. Abstract
Recently, there has been growing interest in the utilization of physical systems as heuristic optimizers for classical spin Hamiltonians. A prominent approach employs gaindissipative optical oscillator networks for this purpose. Unfortunately, these systems inherently suffer from an inexact mapping between the oscillator network loss rate and the spin Hamiltonian due to additional degrees of freedom present in the system such as oscillation amplitude. In this work, we theoretically analyze and experimentally demonstrate a scheme for the alleviation of this difficulty. The scheme involves control over the laser oscillator amplitude through modification of individual laser oscillator loss. We demonstrate this approach in a laser network classical XY model simulator based on a digital degenerate cavity laser. We prove that for each XY model energy minimum there corresponds a unique set of laser loss values that leads to a network state with identical oscillation amplitudes and to phase values that coincide with the XY model minimum. We experimentally demonstrate an eight fold improvement in the deviation from the minimal XY energy by employing our proposed solution scheme.

(2020) Physical Review Applied. 13, 5, 054053. Abstract
Atom interferometers offer excellent sensitivity to gravitational and inertial signals but have limited dynamic range. We introduce a scheme that improves this tradeoff by a factor of 50 using composite fringes, obtained from sets of measurements with slightly varying interrogation times, as in a moire effect. We analyze analytically the performance gain in this approach and the tradeoffs it entails between sensitivity, dynamic range, and bandwidth, and we experimentally validate the analysis over a wide range of parameters. Combining compositefringe measurements with a particlefilter estimation protocol, we demonstrate continuous tracking of a rapidly varying signal over a span 2 orders of magnitude larger than the dynamic range of a traditional atom interferometer.

(2020) Physical Review A. 101, 4, 042123. Abstract
We observe a significant steadystate deviation from the generalized equipartition theorem, one of the pivotal results of classical statistical mechanics, in a system of confined, lasercooled atoms. We parametrize this deviation, measure its dynamics, and show that its steadystate value quantifies the departure of nonthermal states from thermal equilibrium even for anharmonic confinement. In particular, we find that deviations from equipartition grow as the system dynamics becomes more anomalous. We present numerical simulations that validate the experimental data and reveal an inhomogeneous distribution of the kinetic energy through the system, supported by an analytical examination of the phase space.

(2020) Physical Review Letters. 124, 16, 163401. Abstract
Interactions in an ultracold bosonfermion mixture are often manifested by elastic collisions. In a mixture of a condensed Bose gas (BEC) and spin polarized degenerate Fermi gas (DFG), fermions can mediate spinspin interactions between bosons, leading to an effective longrange magnetic interaction analogous to RudermanKittclKasuyaYosida [Phys. Rev. 96, 99 (1954); Prog. Theor. Phys. 16, 45 (1956); Phys. Rev. 106, 893 (1957)] interaction in solids. We used Ramsey spectroscopy of the hyperfine clock transition in a Rb87 BEC to measure the interaction mediated by a K40 DFG. By controlling the boson density we isolated the effect of mediated interactions from meanfield frequency shifts due to direct collision with fermions. We measured an increase of boson spinspin interaction by a factor of eta = 1.45 +/ 0.05(stat) +/ 0.13(syst) in the presence of the DFG, providing clear evidence of spinspin fennion mediated interaction. Decoherence in our system was dominated by inhomogeneous boson density shift, which increased significantly in the presence of the DFG, again indicating mediated interactions. We also measured a frequency shift due to bosonfermion interactions in accordance with a scattering length difference of a(bf2)  a(bf1) = 5.36 +/ 0.44(stat) +/ 1.43(syst)a(0) between the clocktransition states, a first measurement beyond the lowenergy elastic approximation [R. Cote, & A. Dalgarno, H. Wang, and W. C. Stwalley, Phys. Rev. A 57, R4118 (1998); A. Dalgarno and M. Rudge, Proc. R. Soc. A 286, 519 (1965)] in this mixture. This interaction can be tuned with a future use of a bosonfermion Feshbach resonance. Fermionmediated interactions can potentially give rise to interesting new magnetic phases and extend the BoseHubbard model when the atoms are placed in an optical lattice.

(2020) Physical Review Letters. 124, 13, 133901. Abstract
An arrangement based on a degenerate cavity laser for forming an array of nonlinearly coupled lasers with an intracavity saturable absorber is presented. More than 30 lasers were spatially phase locked and temporally Q switched. The arrangement with nonlinear coupling was found to be 25 times more sensitive to loss differences and converged five times faster to the lowest loss phase locked state than with linear coupling, thus providing a unique solution to problems that have several neardegenerate solutions.
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) Optica. 6, 11, p. 14061411 Abstract
Partially coherent fields are abundant in many physical systems. While the propagation of partially coherent light undergoing diffraction is well understood, its evolution in the presence of coherent diffusion (i.e., diffusion of complex fields) remains largely unknown. Here we develop an analytic model describing the diffusion of partially coherent beams and study it experimentally. Our model is based on a diffusion analog of the famous Van Cittertâ€“Zernike theorem. Experimentally, we use a fourwave mixing scheme with electromagnetically induced transparency to couple optical speckle patterns to diffusing atoms in a warm vapor. The spatial coherence properties of the speckle fields are monitored under diffusion and are compared to our model and to the familiar evolution of spatial coherence of light speckles under diffraction. We identify several important differences between the evolution dynamics of the spatial coherence under diffraction and diffusion. Our findings shed light on the propagation of partially coherent fields in media where multiple scattering or thermal motion lead to coherent diffusion.

(2019) Journal of Physics B: Atomic, Molecular and Optical Physics. 52, 20, 205401. Abstract
The dynamics of topological defects in a system of coupled phase oscillators, arranged in one and twodimensional arrays, was numerically investigated using the Kuramoto model. After a rapid decay of the number of topological defects, a longtime quasi steady state with few topological defects was detected. Two competing time scales governed the dynamics corresponding to the dissipation rate and the coupling quench rate. The density of topological defects scales as a power law function of the coupling quench rate rho similar to Cnu with nu = 0.25. Reducing the number of topological defects improves the long time coherence and order parameter of the system, enhancing the probability to reach a global minimal loss state that can be mapped to the ground state of a classical XY spin Hamiltonian.

(2019) Science advances. 5, 10, 4530. Abstract
Tailored physical systems were recently exploited to rapidly solve hard computational challenges, such as spin simulators, combinatorial optimization, and focusing through scattering media. Here, we address the phase retrieval problem where an object is reconstructed from its scattered intensity distribution. This is a key problem in many applications, ranging from xray imaging to astrophysics, and currently, it lacks efficient direct reconstruction methods: The widely used indirect iterative algorithms are inherently slow. We present an optical approach based on a digital degenerate cavity laser, whose most probable lasing mode rapidly and efficiently reconstructs the object. Our experimental results suggest that the gain competition between the many lasing modes acts as a highly parallel computer that could rapidly solve the phase retrieval problem. Our approach applies to most twodimensional objects with known compact support, including complexvalued objects, and can be generalized to imaging through scattering media and other hard computational tasks.

(2019) Physical Review A. 100, 2, 023617. Abstract
We present techniques for inertialsensing atom interferometers which produce multiple phase measurements per experimental cycle. With these techniques, we realize two types of multiport measurements, namely, quadrature phase detection and realtime systematic phase cancellation, which address challenges in operating highsensitivity coldatom sensors in mobile and field applications. We confirm experimentally the increase in sensitivity due to quadrature phase detection in the presence of large phase uncertainty, and demonstrate suppression of systematic phases on a singleshot basis.

(2019) Nature Reviews Physics. 1, 2, p. 156168 Abstract
Lasers have enabled scientific and technological progress, owing to their high brightness and high coherence. However, the high spatial coherence of laser illumination is not always desirable, because it can cause adverse artefacts such as speckle noise. To reduce spatial coherence, new laser cavity geometries and alternative feedback mechanisms have been developed. By tailoring the spatial and spectral properties of cavity resonances, the number of lasing modes, the emission profiles and the coherence properties can be controlled. In this Technical Review, we present an overview of such unconventional, complex lasers, with a focus on their spatial coherence properties. Laser coherence control not only provides an efficient means for eliminating coherent artefacts but also enables new applications in imaging and wavefront shaping. High spatial coherence of laser illumination is not always desirable, because it can cause adverse artefacts such as speckle noise. This Technical Review describes unconventional lasers that have inherently low and/or tunable spatial coherence. Key pointsHigh spatial coherence of laser emission, a common feature of conventional lasers, causes deleterious effects, including speckle noise and crosstalk, in applications such as fullfield imaging, display, materials processing, photolithography, holography and optical trapping.Fundamental changes in laser design or operation are more effective than schemes to reduce the spatial coherence outside of the laser cavity to achieve low or tunable spatial coherence.Random lasers and wavechaotic microcavity lasers support numerous lasing modes with distinct spatial profiles, producing emission of low spatial coherence suitable for specklefree, fullfield imaging and spatial coherence gating.The number of modes and thus spatial coherence of degenerate cavity laser emission can be tuned with little change in power, allowing fast switching between specklefree imaging and specklecontrast imaging.Wavefront shaping inside a degenerate cavity laser can generate propagationinvariant output beams or spindependent twisted light beams. The dynamic wavefront control can focus laser light through a random scattering medium.

(2019) Applied Optics. 58, 3, p. 485491 Abstract
We propose a new design principle for optimal concentration of light with small diffusivity based on the conservation of local brightness in passive optical transformations. A coordinate transformation is applied on the incoming rays to compensate for the variations in local brightness by the focusing stage. We apply this analytic design for a compact reflective configuration for ideal imaging concentration of diffuse light such as sunlight in one dimension on an elongated target with arbitrary crosssectional shape at the thermodynamic limit. As illustrations, we present the design for two different target geometries and verify its validity using numerical raytracing simulations. The same configuration can be used in reverse as an ideal collimator of a finite diffuse source. (C) 2019 Optical Society of America
2018

(2018) Applied Physics B: Lasers and Optics. 124, 9, 175. Abstract
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(2018) Applied Physics B: Lasers and Optics. 124, 8, 158. Abstract
Loading an ultracold ensemble into a static magnetic trap involves unavoidable loss of phase space density when the gravitational energy dominates the kinetic energy of the ensemble. In such a case the gravitational energy is transformed into heat, making a subsequent evaporation process slower and less efficient. We apply a high phase space loading scheme on a subdoppler cooled ensemble of Rubidium atoms, with a gravitational energy much higher than its temperature of . Using the regular configuration of a quadrupole magnetic trap, but driving unequal currents through the coils to allow the trap center to fall, we dissipate most of the gravitational energy and obtain a 20fold improvement in the phase space density as compared to optimal loading into a static magnetic trap. Applying this scheme, we start an efficient and fast evaporation process as a result of the subsecond thermalization rate of the magnetically trapped ensemble.

(2018) Physical Review A. 98, 2, 023812. Abstract
Spatial coherence quantifies spatial field correlations and is one of the fundamental properties of light. Here we investigate the spatial coherence of highly multimode lasers in the regime of short timescales. Counterintuitively, we show that in this regime, the temporal (longitudinal) modes play a crucial role in spatial coherence reduction. To evaluate the spatial coherence we measured the temporal dynamics of speckle fields generated by a highly multimode laser with over 10(5) lasing spatial (transverse) modes and examined the dependence of speckle contrast on the exposure time of the detecting device. We show that in the regime of short timescales, the spatial and temporal modes interact to form spatiotemporal supermodes, such that the spatial degrees of freedom are encoded onto the temporal modes. As a result, the speckle contrast is suppressed according to the number of temporal modes, and the degree of spatial coherence is reduced. Moreover, the functional form of the spatial coherence is shown to have a bimodal distribution. In the regime of long timescales, the supermodes are no longer a valid representation of the laser modal structure. Consequently, the spatial coherence is independent of the temporal modes, and the classical result, where the speckle contrast is suppressed according to the number of spatial modes, is obtained. Due to this spatiotemporal mechanism, highly multimode lasers can be used for speckle suppression in highspeed fullfield imaging applications, as we demonstrate here for imaging of a fast moving object.

(2018) Applied Optics. 57, 16, p. 45834589 Abstract
Two approaches for generating flattop beams (uniform intensity profile) with extended depth of focus are presented. One involves two diffractive optical elements (DOEs) and the other only a single DOE. The results indicate that the depth of focus of such beams strongly depends on the phase distribution at the output of the DOEs. By having uniform phase distribution, it is possible to generate flattop beams with extended depth of focus. (C) 2018 Optical Society of America

(2018) ACS Photonics. 5, 5, p. 18171821 Abstract
Incorporation of a metasurface that involves spinorbit interaction phenomenon into a laser cavity provides a route to the generation of spincontrolled intracavity modes with different topologies. By utilizing the geometric phase, PancharatnamBerry phase, we found a spinenabled selfconsistent cavity solution of a Nd:YAG laser with a siliconbased metasurface. Using this solution we generated a laser mode possessing spincontrolled orbitalangular momentum. Moreover, an experimental demonstration of a vectorial vortex is achieved by the coherent superposition of modes with opposite spin and orbital angular momenta. We experimentally achieved a high mode purity of 95% due to laser mode competition and purification. The photonic spinorbit interaction mechanism within a lasercavity can be implemented with multifunctional shared aperture nanoantenna arrays to achieve multiple intracavity topologies.

(2018) Applied Optics. 57, 12, p. 32053208 Abstract
We propose and demonstrate a method to produce a thin and highly collimated annular beam that propagates similarly to an ideal thin Gaussian ring beam, maintaining its excellent propagation properties. Our optical configuration is composed of a binary axicon, a circular binary phase grating, and a lens, making it robust and well suited for highpower lasers. It has a nearperfect circular profile with a dark center, and its large radius to waist ratio is achieved with high conversion efficiency. The measured profile and propagation are in excellent agreement with a numerical Fourier simulation we perform. (C) 2018 Optical Society of America

(2018) Optics Express. 26, 4, p. 44314439 Abstract
A rapid and efficient alloptical method for forming propagation invariant shaped beams by exploiting the optical feedback of a laser cavity is presented. The method is based on the modified degenerate cavity laser (MDCL), which is a highly incoherent cavity laser. The MDCL has a very large number of degrees of freedom (320,000 modes in our system) that can be coupled and controlled, and allows direct access to both the real space and Fourier space of the laser beam. By inserting amplitude masks into the cavity, constraints can be imposed on the laser in order to obtain minimal loss solutions that would optimally lead to a superposition of BesselGauss beams forming a desired shaped beam. The resulting beam maintains its transverse intensity distribution for relatively long propagation distances. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

(2018) Optics Express. 26, 2, p. 905916 Abstract
Novel multitasking geometric phase metasurfaces were incorporated into a modified degenerate cavity laser as an output coupler to efficiently generate spindependent twisted light beams of different topologies. Multiple harmonic scalar vortex laser beams were formed by replacing the laser output coupler with a sharedaperture metasurface. A variety of distinct wave functions were obtained with an interleaving approach  random interspersing of geometric phase profiles within sharedaperture metasurfaces. Utilizing the interleaved metasurfaces, we generated vectorial vortices by coherently superposing of scalar vortices with opposite topological charges and spin states. We also generated multiple partially coherent vortices by incorporating harmonic response metasurfaces. The incorporation of the metasurface platforms into a laser cavity opens a pathway to novel types of nanophotonic functionalities and enhanced lightmatter interactions, offering exciting new opportunities for light manipulation. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
2017

(2017) Journal of Physics B: Atomic, Molecular and Optical Physics. 50, 21, 215003. Abstract
We use a fourwave mixing process to readout light from atomic coherence which is continuously written. The light is continuously generated after an effective delay, allowing the atomic coherence to evolve during the process. Contrary to slowlight delay, which depends on the medium optical depth, here the generation delay is determined solely by the intensive properties of the system, approaching the atomic coherence lifetime at the weak driving limit. The atomic evolution during the generation delay is further manifested in the spatial profile of the generated light due to atomic diffusion. Continuous generation of light with a long intrinsic delay can replace discrete writeread procedures when the atomic evolution is the subject of interest.

(2017) Physical Review Letters. 119, 16, 163201. Abstract
We report the observation of optomechanical strain applied to thermal and quantum degenerate Rb87 atomic clouds when illuminated by an intense, far detuned homogeneous laser beam. In this regime the atomic cloud acts as a lens that focuses the laser beam. As a backaction, the atoms experience a force opposite to the beam deflection, which depends on the atomic cloud density profile. We experimentally demonstrate the basic features of this force, distinguishing it from the wellestablished scattering and dipole forces. The observed strain saturates, ultimately limiting the momentum impulse that can be transferred to the atoms. This optomechanical force may effectively induce interparticle interactions, which can be optically tuned.

(2017) Physical Review A. 96, 4, 043831. Abstract
We perform Raman spectroscopy of optically trapped noninteracting Rb87 atoms, and observe revivals of the atomic coherence at integer multiples of the trap period. The effect of coherence control methods such as echo and dynamical decoupling is investigated experimentally, analytically, and numerically, along with the effect of the anharmonicity of the trapping potential. The latter is shown to be responsible for incompleteness of the revivals. Coherent Raman control of trapped atoms can be useful in the context of freeoscillation atom interferometry and spatial multimode quantum memory.

(2017) Physical Review A. 96, 3, 32713. Abstract
The softness of elastic atomic collisions, defined as the average number of collisions each atom undergoes until its energy decorrelates significantly, can have a considerable effect on the decay dynamics of atomic coherence. In this paper we combine two spectroscopic methods to measure these dynamics and obtain the collisional softness of ultracold atoms in an optical trap: Ramsey spectroscopy to measure the energy decorrelation rate and echo spectroscopy to measure the collision rate. We obtain a value of 2.5(3) for the collisional softness, in good agreement with previously reported numerical moleculardynamics simulations. This fundamental quantity is used to determine the swave scattering lengths of different atoms but has not been directly measured. We further show that the decay dynamics of the revival amplitudes in the echo experiment has a transition in its functional decay. The transition time is related to the softness of the collisions and provides yet another way to approximate it. These conclusions are supported by Monte Carlo simulations of the full echo dynamics. The methods presented here can allow measurement of a generalized softness parameter for other twolevel quantum systems with discrete spectral fluctuations.

(2017) Physical Review Letters. 119, 6, 060602. Abstract
In this Letter, we present a measurement of the phasespace density distribution (PSDD) of ultracold Rb87 atoms performing 1D anomalous diffusion. The PSDD is imaged using a direct tomographic method based on Raman velocity selection. It reveals that the positionvelocity correlation function Cxv(t) builds up on a time scale related to the initial conditions of the ensemble and then decays asymptotically as a power law. We show that the decay follows a simple scaling theory involving the powerlaw asymptotic dynamics of position and velocity. The generality of this scaling theory is confirmed using Monte Carlo simulations of two distinct models of anomalous diffusion.

(2017) Physical Review Letters. 119, 1, 013902. Abstract
Topological defects have been observed and studied in a wide range of systems, such as cosmology, spin systems, cold atoms, and optics, as they are quenched across a phase transition into an ordered state. These defects limit the coherence of the system and its ability to approach a fully ordered state, so revealing their origin and control is becoming an increasingly important field of research. We observe dissipative topological defects in a onedimensional ring of phasedlocked lasers, and show how their formation is related to the KibbleZurek mechanism and is governed in a universal manner by two competing time scales. The ratio between these two time scales depends on the system parameters, and thus offers the possibility of enabling the system to dissipate to a fully ordered, defectfree state that can be exploited for solving hard computational problems in various fields.

(2017) Slow Light, Fast Light, and OptoAtomic Precision Metrology X. Scheuer J. & Shahriar S. M.(eds.). Abstract
This paper describes a technique that allows measurement of very small alternating accelerations. It is based on a quantum version of a lockin amplifier, 1 which filters out spectral components far from the frequency of the measured signal, improving the signaltonoise ratio of the measurement. As a proofofprinciple, a controlled experiment using microwave radiation is performed, modulating the phase of the control pulses at a given frequency. A strong response at twice the modulation frequency is observed as expected. Preliminary results of measurements taken with Raman control are also presented, in which a controlled modulation of the phase was obtained by modulating a piezoelectric actuator, causing one of the Raman mirrors to vibrate.

(2017) Applied Optics. 56, 1, p. A126A132 Abstract
Talbot diffraction, together with Fourier filtering, are incorporated into a degenerate laser cavity to demonstrate efficient and controlled phase locking of hundreds of coupled lasers formed in different geometries and having different phase distributions. Such a combined approach leads to higher efficiency, better control, and greater variety of output phase distributions than would be possible with either separately. Simulated and experimental results for square, triangular, and honeycomb laser array geometries are presented. (C) 2016 Optical Society of America
2016

(2016) Physical Review A. 94, 4, 42317. Abstract
An optimal dynamic decoupling of a quantum system coupled to a noisy environment must take into account also the imperfections of the control pulses. We present a formalism which describes, in a closedform expression, the evolution of the system, including the spectral function of both the environment and control noise. We show that by measuring these spectral functions, our expression can be used to optimize the decoupling pulse sequence. We demonstrate this approach with an ensemble of optically trapped ultracold rubidium atoms, and use quantum process tomography to identify the effect of the environment and control noise. Our approach is applicable and important for any realistic implementation of quantum information processing.
2015

(2015) Photonics Research. 3, 3, p. 7781 Abstract
A novel method for converting an array of outofphase lasers into one of inphase lasers that can be tightly focused is presented. The method exploits secondharmonic generation and can be adapted for different laser arrays geometries. Experimental and calculated results, presented for negatively coupled lasers formed in a square, honeycomb, and triangular geometries are in good agreement. (C) 2015 Chinese Laser Press

(2015) Optics Express. 23, 10, p. 1298912997 Abstract
An efficient method for controlling the spatial coherence has previously been demonstrated in a modified degenerate cavity laser. There, the degree of spatial coherence was controlled by changing the size of a circular aperture mask placed inside the cavity. In this paper, we extend the method and perform general manipulation of the spatial coherence properties of the laser, by resorting to more sophisticated intracavity masks. As predicted from the Van Cittert Zernike theorem, the spatial coherence is shown to depend on the geometry of the masks. This is demonstrated with different mask geometries: a variable slit which enables independent control of spatial coherence properties in one coordinate axis without affecting those in the other; a double aperture, an annular ring and a circular aperture array which generate spatial coherence functional forms of cosine, Bessel and comb, respectively. (C) 2015 Optical Society of America

(2015) Optics Express. 23, 10, p. 1304113050 Abstract
The effects of topological charge on phase locking an array of coupled lasers are presented. This is done with even and odd number of lasers arranged on a ring geometry. With an even number of lasers the topologicalcharge effect is negligible, whereas with an odd number of lasers the topologicalcharge effect is clearly detected. Experimental and calculated results show how the topological charge effects degrade the quality of the phase locking, and how they can be removed. Our results shed further light on the frustration and also the quality of phase locking of coupled laser arrays. (C) 2015 Optical Society of America

(2015) 2015 CONFERENCE ON LASERS AND ELECTROOPTICS (CLEO). New York: . (Conference on Lasers and ElectroOptics). Abstract
Efficient method for manipulating the spatial coherence of a laser is presented. Different mutual intensity coherence functions, such as cosine or Bessel functions, are obtained, and number of modes is controlled in 1D and 2D.

(2015) 2015 CONFERENCE ON LASERS AND ELECTROOPTICS (CLEO). New York: . (Conference on Lasers and ElectroOptics). Abstract
Efficient inphase coupling of hundreds of lasers by means of combined Talbot cavity and intracavity spatial Fourier filtering is developed. Simulated and experimental results for square, triangular and honeycomb laser arrays are presented.

(2015) 2015 CONFERENCE ON LASERS AND ELECTROOPTICS (CLEO). New York: . (Conference on Lasers and ElectroOptics). Abstract
Second harmonic generation in coupled laser arrays is exploited to convert outofphase lasers into inphase lasers and reveal hitherto unknown properties of some laser array geometries.
2014

(2014) Physical Review A. 89, 3, 33807. Abstract
The electromagnetically induced transparency (EIT) spectrum of atoms diffusing in and out of a narrow beam is measured and shown to manifest the twodimensional deltafunction anomaly in a classical setting. In the limit of smallarea beams, the EIT line shape is independent of power, and equal to the renormalized local density of states of a free particle Hamiltonian. The measured spectra for different powers and beam sizes collapses to a single universal curve with a characteristic logarithmic Van Hove singularity close to resonance.
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 Letters. 38, 20, p. 41744177 Abstract
The synchronization of chaotic lasers and the optical phase synchronization of light originating in multiple coupled lasers have both been extensively studied. However, the interplay between these two phenomena, especially at the network level, is unexplored. Here, we experimentally compare these phenomena by controlling the heterogeneity of the coupling delay times of two lasers. While chaotic lasers exhibit deterioration in synchronization as the time delay heterogeneity increases, phase synchronization is found to be independent of heterogeneity. The experimental results are found to be in agreement with numerical simulations for semiconductor lasers. (C) 2013 Optical Society of America

(2013) Optics Letters. 38, 19, p. 38583861 Abstract
An efficient method to tune the spatial coherence of a degenerate laser over a broad range with minimum variation in the total output power is presented. It is based on varying the diameter of a spatial filter inside the laser cavity. The number of lasing modes supported by the degenerate laser can be controlled from 1 to 320,000, with less than a 50% change in the total output power. We show that a degenerate laser designed for low spatial coherence can be used as an illumination source for specklefree microscopy that is nine orders of magnitude brighter than conventional thermal light. (C) 2013 Optical Society of America

(2013) Laser Physics. 23, 10, Abstract
We propose and investigate theoretically a new concept for singlelargemode amplification in doubleclad active fibers. The concept is based on exploiting a very small fiber core, guiding only a single transverse mode that has large overlap with a doped active cladding. We show that the guided mode can have very large area with good modal discrimination. This simple structured fiber can be used in an 'allfiber' configuration. We further investigate the sensitivity to small refractive index changes of the doped area and to bending of the fiber. Our proposed fiber concept could lead to fibers with very large mode area with advantages over current commercial fibers.

(2013) Reviews of Modern Physics. 85, 3, p. 941960 Abstract
Coherent diffusion pertains to the motion of atomic dipoles experiencing frequent collisions in vapor while maintaining their coherence. Recent theoretical and experimental studies on the effect of coherent diffusion on key Raman processes, namely, Raman spectroscopy, slow polariton propagation, and stored light, are reviewed in this Colloquium.

(2013) Physical Review Letters. 110, 18, Abstract
Geometric frustration, the inability of an ordered system to find a unique ground state plays a key role in a wide range of systems. We present a new experimental approach to observe largescale geometric frustration with 1500 negatively coupled lasers arranged in a kagome lattice. We show how dissipation drives the lasers into a phaselocked state that directly maps to the classical XY spin Hamiltonian ground state. In our system, frustration is manifested by the lack of long range phase ordering. Finally, we show how nextnearestneighbor coupling removes frustration and restores order.

(2013) Optics Letters. 38, 8, p. 12031205 Abstract
The close relation between the processes of paraxial diffraction and coherent diffusion is reflected in the similarity between their shapepreserving solutions, notably the Gaussian modes. Differences between these solutions enter only for highorder modes. Here we experimentally study the behavior of shapepreserving highorder modes of coherent diffusion, known as "elegant" modes, and contrast them with the nonshapepreserving evolution of the corresponding "standard" modes of optical diffraction. Diffusion of the light field is obtained by mapping it onto the atomic coherence field of a diffusing vapor in a storageoflight setup. The growth of the elegant mode fits well the theoretical expectations. (C) 2013 Optical Society of America
2012

(2012) Optics Express. 20, 27, p. 2816328170 Abstract
A novel configuration for phase locking two ring lasers with selfstabilized minimal exchange of power between them is presented. We show experimentally that losses introduced between the lasers are self compensated in order to maintain minimal power exchange between them. The experimental results are in good agreement with numerical results. (C) 2012 Optical Society of America

(2012) Physical Review E. 86, 4, Abstract
We measure the statistics of phase locking levels of coupled fiber lasers with fluctuating cavity lengths. We found that the measured distribution of the phase locking level of such coupled lasers can be described by the generalized extreme value distribution. For large number of lasers the distribution of the phase locking level can be approximated by a Gumbel distribution. We present a simple model, based on the spectral response of coupled lasers, and the calculated results are in good agreement with the experimental results.

(2012) Physical Review Letters. 108, 21, Abstract
Synchronization in large laser networks with both homogeneous and heterogeneous coupling delay times is examined. The number of synchronized clusters of lasers is established to equal the greatest common divisor of network loops. We experimentally demonstrate up to 16 multicluster phase synchronization scenarios within unidirectional coupled laser networks, whereby synchronization in heterogeneous networks is deduced by mapping to an equivalent homogeneous network. The synchronization in large laser networks is controlled by means of tunable coupling and selfcoupling.

(2012) Journal Of The Optical Society Of America AOptics Image Science And Vision. 29, 4, p. 541544 Abstract
The dynamics of modes and their states of polarizations in multimode fibers as a function of time, space, and wavelength are experimentally and theoretically investigated. The results reveal that the states of polarizations are displaced in Poincare sphere representation when varying the angular orientations of the polarization at the incident light. Such displacements, which complicate the interpretation of the results, are overcome by resorting to modified Poincare sphere representation. With such modification it should be possible to predict the output modes and their state of polarization when the input mode and state of polarization are known. (C) 2012 Optical Society of America

(2012) Physical Review Letters. 108, 9, Abstract
We experimentally study anomalous diffusion of ultracold atoms in a one dimensional polarization optical lattice. The atomic spatial distribution is recorded at different times and its dynamics and shape are analyzed. We find that the width of the cloud exhibits a powerlaw time dependence with an exponent that depends on the lattice depth. Moreover, the distribution exhibits fractional selfsimilarity with the same characteristic exponent. The selfsimilar shape of the distribution is found to be well fitted by a Levy distribution, but with a characteristic exponent that differs from the temporal one. Numerical simulations suggest that this is due to long trapping times in the lattice and correlations between the atom's velocity and flight duration.

(2012) Physical Review E. 85, 2, Abstract
We determined the probability distribution of the combined output power from 25 coupled fiber lasers and show that it agrees well with the TracyWidom and MajumdarVergassola distributions of the largest eigenvalue of Wishart random matrices with no fitting parameters. This was achieved with 500 000 measurements of the combined output power from the fiber lasers, that continuously changes with variations of the fiber lasers lengths. We show experimentally that for small deviations of the combined output power over its mean value the TracyWidom distribution is correct, while for large deviations the MajumdarVergassola distribution is correct.

(2012) Laser Technology For Defense And Security Viii. 8381, Abstract
Selections from our recent developments in passive phase locking and coherent combining of lasers are presented. These include the principles of our approaches, lasers configurations, experimental procedures and results with solid state lasers and fiber lasers.
2011

(2011) JOURNAL OF PHYSICS BATOMIC MOLECULAR AND OPTICAL PHYSICS. 44, 15, 154006. Abstract
Decoherence is a major obstacle to any practical implementation of quantum information processing. One of the leading strategies to reduce decoherence is dynamical decouplingthe use of an external field to average out the effect of the environment. The decoherence rate under any control field can be calculated if the spectrum of the coupling to the environment is known. We present a direct measurement of the bathcoupling spectrum in an ensemble of optically trapped ultracold atoms, by applying a spectrally narrowband control field. The measured spectrum follows a Lorentzian shape at low frequencies but exhibits nonmonotonic features at higher frequencies due to the oscillatory motion of the atoms in the trap. These features agree with our analytical models and numerical Monte Carlo simulations of the collisional bath. From the inferred bathcoupling spectrum, we predict the performance of some wellknown dynamical decoupling sequences. We then apply these sequences in experiment and compare the results to predictions, finding good agreement in the weakcoupling limit. Thus, our work establishes experimentally the validity of the overlap integral formalism and is an important step towards the implementation of an optimal dynamical decoupling sequence for a given measured bath spectrum.

(2011) Physical Review Letters. 106, 22, 223901. Abstract
We experimentally investigate the phase dynamics of laser networks with homogenous timedelayed mutual coupling and establish the fundamental rules that govern their state of synchronization. We identified a specific substructure that imposes its synchronization state on the entire network and show that for any coupling configuration the network forms at most two synchronized clusters. Our results indicate that the synchronization state of the network is a nonlocal phenomenon and cannot be deduced by decomposing the network into smaller substructures, each with its individual synchronization state.

(2011) Applied Physics Letters. 98, 14, 141107. Abstract
A compact configuration for realtime achromatic measurements of spacevariant light polarization is presented. The experimental results reveal that the full state of polarization at each location within a light beam or at each wavelength can be obtained with accuracy of over pi/18. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3575567]

(2011) Physical Review A. 83, 4, 043821. Abstract
We show that the spectrum of an ensemble of twolevel systems can be broadened through "resetting" discrete fluctuations, in contrast to the wellknown motionalnarrowing effect. The broadening occurs if the ensemble frequency distribution has heavy tails with a diverging first moment. The asymptotic motional broadened line shape is then a Lorentzian. In case there is a physical upper cutoff in the frequency distribution, the broadening effect may still be observed, though only up to a certain fluctuation rate.

Enhanced coherence of weakly coupled lasers(2011) Optics Letters. 36, 8, p. 13201322 Abstract
We present the phaselocking and coherence properties between two weakly coupled lasers. We show how the degree of coherence between the two lasers can be enhanced by nearly 1 order of magnitude after taking into account the effects of coupling on both their phases as well as their amplitudes. Specifically, correlations between synchronized spikes in the amplitude dynamics and the phase dynamics of the lasers allow for an interference pattern with a fringe visibility of 90%, even when the coupling strength is far below the critical value and they are not phase locked. (c) 2011 Optical Society of America

(2011) Optics Letters. 36, 3, p. 388390 Abstract
The dynamics of the state of polarization in multimode fiber amplifiers is presented. The experimental results reveal that although the state of polarizations at the output can vary over a large range when changing the temperatures of the fiber amplifiers, the variations are significantly reduced when resorting to the principal states of polarization in singlemode fiber amplifiers and principal modes in multimode fiber amplifiers. (C) 2011 Optical Society of America
2010

(2010) IEEE Journal of Quantum Electronics. 46, 12, p. 18211826 Abstract
In order to achieve synchronization between chaotic lasers the coupling strength must typically exceed a certain threshold. Here, we show how this threshold can be significantly reduced by adding loss that is minimized when the coupled lasers are both phase locked and synchronized. A model is developed for this loss enhanced synchronization and the calculated results are in good agreement to those obtained experimentally with two coupled chaotic fiber lasers.

(2010) Physical Review Letters. 105, 18, 183602. Abstract
Selfsimilar solutions of the coherent diffusion equation are derived and measured. The set of real similarity solutions is generalized by the introduction of a nonuniform phase, based on the elegant Gaussian modes of optical diffraction. In a lightstorage experiment, the complex solutions are imprinted on a gas of diffusing atoms, and the selfsimilar evolution of both their amplitude and phase pattern is demonstrated. An algebraic decay depending on the mode order is measured. Notably, as opposed to the regular diffusion spreading, a subset of the solutions exhibits a selfsimilar contraction.

(2010) Physical Review Letters. 105, 9, 93001. Abstract
We study the spectral narrowing induced by collisions in a dense cold atomic ensemble. We report on experiments showing a prolongation of the coherence time of optically trapped (87)Rb atoms as the density increases, a phenomenon we call collisional narrowing in analogy to the motional narrowing effect in NMR. We derive an expression for the new dephasing time scale in terms of the collision rate and the inhomogeneous decay time. Remarkably, this time scale universally depends only on the atomic phase space density.

(2010) Physical Review Letters. 105, 5, 53201. Abstract
Atomic ensembles have many potential applications in quantum information science. Owing to collective enhancement, working with ensembles at high densities increases the efficiency of quantum operations, but at the same time also increases the collision rate and leads to decoherence. Here we report on experiments with optically trapped (87)Rb atoms demonstrating a 20fold increase of the coherence time when a dynamical decoupling sequence with more than 200 pi pulses is applied. Using quantum process tomography we demonstrate that a dense ensemble with an optical depth of 230 can be used as an atomic memory with coherence times exceeding 3 seconds.

(2010) Physical Review Letters. 104, 25, 253003. Abstract
We study, theoretically and experimentally, an ensemble of twolevel systems coupled to an environment which induces random jumps in their resonant frequency. We present a closedform formula for the spectrum in terms of the resonant frequency distribution and the Poisson rate constant. For a normal distribution the spectrum deviates from a generalized Gumbel function, a wellknown result for continuous stochastic Gaussian processes. We perform experiments with optically trapped cold (87)Rb atoms and show that the predictions of our theory for a 3D harmonic trap match the measured spectra without fitting parameters.

(2010) Optics Letters. 35, 9, p. 14341436 Abstract
Our experiments on passively phase locking twodimensional arrays of coupled fiber lasers reveal that the average phase locking level of 25 lasers is low (20%30%) but can exceed 90% in rare brief events. The average phase locking level was found to decrease for a larger number of lasers in the array and to increase with the connectivity of the array. (C) 2010 Optical Society of America

(2010) Optics Letters. 35, 9, p. 13321334 Abstract
The results of amplifying either radially or azimuthally polarized light with a fiber amplifier are presented. Experimental results reveal that more than 85% polarization purity can be retained at the output even with 40 dB amplification and that efficient conversion of the amplified light to linear polarization can be obtained. (C) 2010 Optical Society of America

(2010) Optics Express. 18, 10, p. 1080510812 Abstract
A novel configuration for realtime measurement of spacevariant polarizations is presented. The experimental results reveal that the full state of polarization at each location within the beam can be accurately obtained every 10msec, limited only by the detection camera frame rate. We also present a more compact configuration which can be modified to determine the realtime wavelength variant polarization measurements. (C) 2010 Optical Society of America

Phase locking of two coupled lasers with many longitudinal modes(2010) Optics Letters. 35, 4, p. 526528 Abstract
Detailed experimental and theoretical investigations of two coupled fiber lasers, each with many longitudinal modes, reveal that the behavior of the longitudinal modes depends on both the coupling strength and the detuning between them. For low to moderate coupling strength only longitudinal modes that are common for both lasers phase lock, while those that are not common gradually disappear. For larger coupling strengths, the longitudinal modes that are not common reappear and phase lock. When the coupling strength approaches unity the coupled lasers behave as a single long cavity with correspondingly denser longitudinal modes. Finally, we show that the gradual increase in phase locking as a function of the coupling strength results from competition between phaselocked and nonphaselocked longitudinal modes. (C) 2010 Optical Society of America

(2010) IEEE Journal of Quantum Electronics. 46, 1, p. 7679 Abstract
A new compact and robust slab laser configuration which involves passive combining of several laser channels is presented. Light from each laser channel is transferred into the adjacent laser channels by means of a polarizing beamdisplacer so as to obtain a combined output with high brightness. In this configuration it is possible to tune the coupling between the laser beams in order to control and optimize the combined output energy. The configuration is experimentally demonstrated by combining five laser channels with high efficiency, and the calculated results are in good agree to experimental results.
2009

(2009) Optics and Photonics News. 20, 12, p. 3232 Abstract
Images imprinted on a laser pulse can be dramatically slowed when traversing an alkali vapor medium via electromagnetically induced transparency.

(2009) Physical Review A. 80, 5, Abstract
We present an approach to dynamic decoherence control of finitetemperature BoseEinstein condensates in a doublewell potential. Due to the manybody interactions the standard "echo" control method becomes less effective. The approach described here takes advantage of the interactioninduced change of the spectrum, to obtain the optimal rate of pi flips of the relative phase between maximally distinguishable collective states. This method is particularly useful for probing and diagnosing the decoherence dynamics.

(2009) Nature Physics. 5, 9, p. 665668 Abstract
Any image, imprinted on a wave field and propagating in free space, undergoes a paraxial diffraction spreading. The reduction or manipulation of diffraction is desirable for many applications, such as imaging, waveguiding, microlithography and optical data processing. As was recently demonstrated, arbitrary images imprinted on light pulses are dramatically slowed(1,2) when traversing an atomic medium of electromagnetically induced transparency(3,4) and undergo diffusion due to the thermal atomic motion(5,6). Here we experimentally demonstrate a new technique to eliminate the paraxial diffraction and the diffusion of slow light, regardless of its position and shape(7). Unlike former suggestions for diffraction manipulation(812), our scheme is linear and operates in the wavevector space, eliminating the diffraction for arbitrary images throughout their propagation. By tuning the interaction, we further demonstrate acceleration of diffraction, biased diffraction and induced deflection, and reverse diffraction, implementing a negativediffraction lens(13). Alongside recent advances in slowlight amplification(14) and image entanglement(15), diffraction control opens various possibilities for classical and quantum image manipulation.

(2009) Physical Review Letters. 103, 3, Abstract
We experimentally demonstrate electromagnetically induced transparency and light storage with ultracold (87)Rb atoms in a Mott insulating state in a threedimensional optical lattice. We have observed light storage times of similar or equal to 240 ms, to our knowledge the longest ever achieved in ultracold atomic samples. Using the differential light shift caused by a spatially inhomogeneous far detuned light field we imprint a "phase gradient" across the atomic sample, resulting in controlled angular redirection of the retrieved light pulse.

(2009) Physical Review Letters. 103, 1, 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.

Phase locking of two fiber lasers with timedelayed coupling(2009) Optics Letters. 34, 12, p. 18641866 Abstract
Two coupled fiber laser arrangements demonstrating isochronal and achronal phase locking with longtimedelayed coupling are presented. Experimental results show that stable phase locking with coupling delay lines as long as 4 km can be obtained and that phase locking can be invariant to the time delay (C) 2009 Optical Society of America

(2009) Optics Communications. 282, 9, p. 18611866 Abstract
We show that by coherently combining several solid state lasers it is possible to obtain a single frequency output. This is experimentally demonstrated by coherently combining four Nd:YAG lasers channels, each with a properly chosen cavity length, in order to suppress unwanted longitudinal modes and obtain a single frequency output. We also present a model that accurately predicts and supports out experimental results. (C) 2009 Elsevier B.V. All rights reserved.

(2009) Physical Review Letters. 102, 15, Abstract
We report an experiment that directly measures the Laplace transform of the recurrence probability in one dimension using electromagnetically induced transparency (EIT) of coherent atoms diffusing in a vapor cell filled with buffer gas. We find a regime where the limiting form of the complex EIT spectrum is universal and only depends on the effective dimensionality in which the random recurrence takes place. In an effective onedimensional diffusion setting, the measured spectrum exhibits powerlaw dependence over two decades in the frequency domain with a critical exponent of 0.56 close to the expected value 0.5.

(2009) EPL. 86, 1, Abstract
The calculation of the heating rate of cold atoms in vibrating traps requires a theory that goes beyond the Kubo linear response formulation. If a strong "quantum chaos" assumption does not hold, the analysis of transitions shows similarities with a percolation problem in energy space. We show how the texture and the sparsity of the perturbation matrix, as determined by the geometry of the system, dictate the result. An improved sparse random matrix model is introduced: it captures the essential ingredients of the problem and leads to a generalized variable range hopping picture. Copyright (C) EPLA, 2009

(2009) IEEE Journal of Selected Topics in Quantum Electronics. 15, 2, p. 301311 Abstract
In this paper, we present an approach for passive laser beam combining using planeparallel intracavity interferometric combiners. With this approach, efficient coherent combining of Gaussian laser beams, transverse single highordermode laser beams, and even transverse multimode laser beams is possible. We experimentally obtained a combining efficiency of about 90% when coherently combining 16 solidstate laser channels, as well as when coherently combining four fiber laser channels. We also present an arrangement for simultaneous coherent combining and spectral combining, which could possibly overcome current upscaling limits. We present the basic coherent combining approach, review our past and recent investigations and results with both solidstate and fiber laser configurations, and discuss the possible upscaling and future prospects of this approach.

(2009) Physical Review Letters. 102, 11, Abstract
We study, both experimentally and theoretically, shorttime modifications of the decay of excitations in a BoseEinstein Condensate (BEC) embedded in an optical lattice. Strong enhancement of the decay is observed compared to the Golden Rule results. This enhancement of decay increases with the lattice depth. It indicates that the description of decay modifications of fewbody quantum systems also holds for decay of manybody excitations of a BEC.

(2009) Physical Review Letters. 102, 4, Abstract
We present a scheme for eliminating the optical diffraction of slow light in a thermal atomic medium of electromagnetically induced transparency. Nondiffraction is achieved for an arbitrary paraxial image by manipulating the susceptibility in momentum space, in contrast to the common approach, which employs guidance of specific modes by manipulating the susceptibility in real space. For negative twophoton detuning, the moving atoms drag the transverse momentum components unequally, resulting in a Doppler trapping of light by atoms in two dimensions.
2008

(2008) Optics Communications. 281, 24, p. 60916093 Abstract
Configurations for efficient free space coherent addition of four separate fiber lasers arranged in two dimensional array are presented. They include compact and robust interferometric combiners that can be inserted either inside or outside the cavity of the combined lasers system. The results reveal that over 85% combining efficiency can be obtained. (C) 2008 Elsevier B.V. All rights reserved.

(2008) Physical Review A. 78, 6, Abstract
Twophoton processes that involve different sublevels of the ground state of an atom, are highly sensitive to depopulation and decoherence within the ground state. For example, the spectral width of electromagnetically induced transparency resonances in a Lambdatype system, are strongly affected by the groundstate depopulation and decoherence rates. We present a direct measurement of decay rates between hyperfine and Zeeman sublevels in the ground state of (87)Rb vapor. Similar to the relaxationinthedark technique, pumping lasers are used to prealign the atomic vapor in a welldefined quantum state. The free propagation of the atomic state is monitored using a Ramseylike method. Coherence times in the range 110 ms were measured for room temperature atomic vapor. In the range of the experimental parameters used in this study, the dominant process inducing Zeeman decoherence is the spinexchange collisions between rubidium atoms.

(2008) Applied Physics Letters. 93, 19, Abstract
A simple, robust, and efficient method to produce either radially or azimuthally polarized output beam from a fiber laser is presented. Experimental results reveal that polarization purity of 90% or better can be obtained.

(2008) Optics Communications. 281, 22, p. 54995503 Abstract
Creating small bright or dark focal spots with good spherical symmetry requires 4 pi focusing, usually achieved by using two counterpropagating beams and two high NA lenses that need to be positioned with interferometric accuracy. We investigate a simple alternative way of focusing light to a small and uniform bright or dark focal spot. It only requires a single parabolic mirror illuminated with a single beam from one direction, resulting in a robust and compact setup. The focal spot distribution is tailored by optimizing the polarization, intensity and helicity of the incoming illumination. (c) 2008 Elsevier B.V. All rights reserved.

(2008) Optics Letters. 33, 20, p. 23052307 Abstract
New configurations for phase locking several laser beams with intracavity polarization elements are presented. With this configuration we demonstrated efficient phase lock of up to 24 ND:YAG laser beams with only two polarization beam displacers. (C) 2008 Optical Society of America

(2008) Optics Letters. 33, 18, p. 21342136 Abstract
Efficient freespace configurations for phase locking and coherent addition of two fiber lasers, each operating with a highorder mode, are presented. Experimental results reveal that when the two fiber lasers are coupled, the polarization and modal distribution of one are imposed on the other. Coherent addition with a combining efficiency larger than 90% was achieved. (c) 2008 Optical Society of America

(2008) Optical Materials. 30, 11, p. 17231730 Abstract
Radiallypolarized beams can be strongly amplified without significant birefringenceinduced aberrations. However, further improvement of the beam quality and beam brightness is desirable. We demonstrate two new efficient methods for transformation of the radiallypolarized LaguerreGaussian beam to a nearlyGaussian beam with much higher beam quality. The methods are based oil separation the radiallypolarized mode into two degenerate modes and their coherent addition after the phase flattening. We present detailed analysis and compare the two methods. With one of the methods, we transformed a highpower radiallypolarized (0, 1)* LaguerreGaussian beam with M(2) = 2.52 and power of 30 W, to a nearlyGaussian beam with M(2) = 1.3. As a result, the laser beam brightness increased by a factor of similar to 2.5. (c) 2007 Elsevier B.V. All rights reserved.

(2008) Physical Review Letters. 101, 1, Abstract
In a BoseEinstein condensate, the excitation of a Bogoliubov phonon with low momentum (e.g., by a twophoton Bragg process) is strongly suppressed due to destructive interference between two indistinguishable excitation pathways. Here we show that scattering of this sound excitation into a doublemomentum mode is strongly enhanced due to constructive interference. This enhancement yields an inherent amplification of secondorder sound excitations of the condensate, as we confirm experimentally. We further show that due to parity considerations, this effect is extended to higherorder excitations.

(2008) Physical Review Letters. 100, 22, Abstract
Reversible and coherent storage of light in an atomic medium is a promising method with possible applications in many fields. In this work, arbitrary twodimensional images are slowed and stored in warm atomic vapor for up to 30 mu s, utilizing electromagnetically induced transparency. Both the intensity and the phase patterns of the optical field are maintained. The main limitation on the storage resolution and duration is found to be the diffusion of atoms. A technique analogous to phaseshift lithography is employed to diminish the effect of diffusion on the visibility of the reconstructed image.

(2008) Physical Review A. 77, 6, Abstract
We present results on phaselocking dynamics of coupled lasers near threshold, and their dependence on spontaneous emission. Our experimental and analytical results clearly reveal that phase locking depends strongly on quantum noise when the coupled lasers oscillate near threshold.

(2008) Optics Letters. 33, 7, p. 648650 Abstract
A new approach for the simultaneous coherent and spectral addition of a twodimensional array of fiber lasers is presented. A combining efficiency of over 80% and a combined beam profile with M2 = 1.15 were experimentally obtained with an array of four fiber lasers. This approach can lead to significant upscaling in fiber laser additions. (C) 2008 Optical Society of America.

(2008) Physical Review A. 77, 4, Abstract
We present a theoretical model for electromagnetically induced transparency (EIT) in vapor that incorporates atomic motion and velocitychanging collisions into the dynamics of the densitymatrix distribution. Within a unified formalism, we demonstrate various motional effects, known for EIT in vapor: Doppler broadening of the absorption spectrum; Dicke narrowing and timeofflight broadening of the transmission window for a finitesized probe; diffusion of atomic coherence during storage of light and diffusion of the lightmatter excitation during slowlight propagation; and Ramsey narrowing of the spectrum for a probe and pump beams of finite size.

(2008) Physical Review A. 77, 3, Abstract
We measure the damping of excitations in cigar shaped Bose Einstein condensates of atomic vapor. By using postselection, excitation line shapes of the total population are compared with those of the undamped excitations. We find that the damping depends on the initial excitation energy of the decaying quasiparticle, as well as on the excitation momentum. We model the condensate as an infinite cylinder and calculate the damping rates of the different radial modes. The derived damping rates are in good agreement with the experimentally measured ones. The damping rates strongly depend on the destructive interference between pathways for damping, due to the quantum many body nature of both excitation and the vacuum modes.

(2008) Physical Review Letters. 100, 2, Abstract
Phase locking, which is achieved by transferring some energy from one oscillator to the others, strongly depends on the coupling strength between the oscillators. Typically, the coupling strength must be above a certain threshold in order to achieve phase locking. Here we show how this threshold can be significantly reduced when phasedependent losses are introduced into the oscillators. Specifically, the coupling strength can be reduced by at least an order of magnitude, thereby substantially decreasing the needed transfer of energy between oscillators. The resulting enhancement of phase locking does not only influence the laser research area, but also affects many other areas that involve coupled ensembles.
2007

(2007) Applied Optics. 46, 32, p. 78237828 Abstract
A new type of diffractive optical element for detecting and measuring the power distribution of transverse modes emanating from radially symmetric laser resonators is presented. It is based on a relatively simple straightforward design of a phaseonly diffractive optical element that serves as a matched filter, which correlates between specific prerecorded transverse modes with a certain azimuthal mode order and those in the incident laser light. Computer simulations supported by experimental results demonstrate how such elements can accurately detect modes with spiral phases and provide quantitative results on the modal power distribution. (C) 2007 Optical Society of America.

(2007) Optics Communications. 279, 2, p. 229234 Abstract
A new technique is proposed for generating a tight dark focal spot surrounded by uniform light intensity in all directions. It is based on a single focusing lens illuminated from one side, hence the alignment sensitivities associated with 4 pi methods are eliminated. Such a beam can be useful, e.g. as a dark atomic trap, and as the erase beam in three dimensional superresolution fluorescence microscopy. (c) 2007 Elsevier B.V. All rights reserved.

(2007) Applied Optics. 46, 30, p. 74267431 Abstract
A method for increasing the output energy from newly developed passively Qswitched Er:glass eyesafe lasers is presented. The increase of energy is achieved by incorporating binary phase elements inside the laser cavities. Experimental results reveal that the output energies can be increased by more than a factor of two. Moreover, by manipulating the output phase with the binary phase elements, the peak energy density in the far field is increased by more than a factor of 4.5. (c) 2007 Optical Society of America

(2007) Physical Review A. 76, 4, Abstract
We study experimentally superradiance in a BoseEinstein condensate using a twofrequency pump beam. By controlling the frequency difference between the beam components, we measure the spectrum of the backward (energymismatched) superradiant atomic modes. In addition, we show that the populations of these modes display coherent time dynamics. These results are compared to a semiclassical model based on coupled SchrodingerMaxwell equations.

(2007) Optics Communications. 276, 1, p. 139144 Abstract
An efficient approach to suppress thermal lensing, when coherently combining several laser distributions, is presented. It is based on incorporating a compensating lens inside the laser cavity. The results reveal that with compensation the overall efficiencies can be more than 80% when combining four laser distributions and more than 90% when combining two laser distributions even at relatively high pulse repetition rates. A model for analyzing coherent combining is developed, where predicted results are in good agreement with the experimental results. (C) 2007 Elsevier B.V. All rights reserved.

(2007) Physical Review A. 76, 2, Abstract
Dicke narrowing is a phenomenon that dramatically reduces the Doppler width of spectral lines, due to frequent velocitychanging collisions. A similar phenomenon occurs for electromagnetically induced transparency (EIT) resonances, and facilitates ultranarrow spectral features in roomtemperature vapor. We directly measure the Dickelike narrowing by studying EIT line shapes as a function of the angle between the pump and probe beams. The measurements are in good agreement with an analytic theory with no fit parameters. The results show that Dicke narrowing can increase substantially the tolerance of hotvapor EIT to angular deviations. We demonstrate the importance of this effect for applications such as imaging and spatial solitons using a singleshot imaging experiment, and discuss the implications for the feasibility of storing images in atomic vapor.

(2007) Physical Review A. 76, 1, Abstract
The Doppler effect is one of the dominant broadening mechanisms in thermal vapor spectroscopy. For twophoton transitions one would naively expect the Doppler effect to cause a residual broadening, proportional to the wavevector difference. In coherent population trapping (CPT), which is a twophoton narrowband phenomenon, such broadening was not observed experimentally. This has been commonly attributed to frequent velocitychanging collisions, known to narrow Dopplerbroadened onephoton absorption lines (Dicke narrowing). Here we show theoretically that such a narrowing mechanism indeed exists for CPT resonances. The narrowing factor is the ratio between the atom's mean free path and the wavelength associated with the wavevector difference of the two radiation fields. A possible experiment to verify the theory is suggested.

(2007) Optics Communications. 275, 2, p. 389393 Abstract
We investigate configurations for upscaling the number of laser distributions that can be coherently added by means of intracavity interferometric combiners. Experimental demonstrations of coherent addition of 5 and 25 laser distributions are presented. Calculated results indicate that with our configurations upscaling to a large number of coherently added distributions is possible but strongly depends on the tolerances of the interferometric combiners. (c) 2007 Elsevier B.V. All rights reserved.

(2007) Physical Review A. 76, 1, Abstract
Rapidly oscillating potentials with a vanishing time average have been used for a long time to trap charged particles in sourcefree regions. It has been argued that the motion of a particle inside such a potential can be approximately described by a time independent effective potential, which does not depend upon the initial phase of the oscillating potential. However, here we show that the motion of a particle and its trapping condition significantly depend upon this initial phase for arbitrarily high frequencies of the potential's oscillation. We explain this phenomenon by showing that the motion of a particle is determined by the effective potential stated in the literature only if its initial conditions are transformed according to a transformation which we show to significantly depend on the potential's initial phase for arbitrarily high frequencies. We confirm our theoretical findings by numerical simulations. Further, we demonstrate that the found phenomenon offers different ways to manipulate the dynamics of particles which are trapped by rapidly oscillating potentials. Finally, we propose a simple experiment to verify the theoretical findings of this work.

(2007) Applied Optics. 46, 16, p. 33043310 Abstract
We develop a roundtrip matrix diagonalization method for quantitative description of selection of radially or azimuthally polarized beams by birefringenceinduced bifocusing in a simple laser resonator. We employ different focusing between radially and tangentially polarized light in thermally stressed laser rods to obtain lowloss stable oscillation in a radially polarized LaguerreGaussian, LG(0, 1)*, mode. We derive a freespace propagator for the radially and azimuthally polarized LG(0, 1)* modes and explain basic principles of mode selection by use of a roundtrip matrix diagonalization method. Within this method we calculate roundtrip diffraction losses and intensity distributions for the lowestloss transverse modes. We show that, for the considered laser configuration, the roundtrip loss obtained for the radially polarized LG(0, 1)* mode is significantly smaller than that of the azimuthally polarized mode. Our experimental results, obtained with a diode sidepumped Nd:YAG rod in a flatconvex resonator, confirm the theoretical predictions. We achieved a pure radially polarized LG(0, 1)* beam with M2 = 2.5 and tens of watts of output power. (c) 2007 Optical Society of America.

(2007) Physical Review Letters. 98, 20, Abstract
We report an experiment in which an optical vortex is stored in a vapor of Rb atoms. Because of its 2 pi phase twist, this mode, also known as the LaguerreGauss mode, is topologically stable and cannot unwind even under conditions of strong diffusion. For comparison, we stored a Gaussian beam with a dark center and a uniform phase. Contrary to the optical vortex, which stays stable for over 100 mu s, the dark center in the retrieved flatphased image was filled with light after a storage time as short as 10 mu s. The experiment proves that higher electromagnetic modes can be converted into atomic coherences and that modes with phase singularities are robust to decoherence effects such as diffusion. This opens the possibility to more elaborate schemes for classical and quantum information storage in atomic vapors.

(2007) Optics Communications. 272, 1, p. 263268 Abstract
The dependence of fluorescence depletion on the relative polarizations of pump and erase beams is investigated for a sample of randomly oriented Rhodamine6G molecules. The significance of polarization effects is illustrated for two existing systems of fluorescence depletion superresolution microscopy: a circular polarization setup, and an azimuthlinear polarization setup. (c) 2006 Elsevier B.V. All rights reserved.

(2007) Optics Letters. 32, 8, p. 924926 Abstract
We demonstrate an efficient method for transformation of a radially polarized LaguerreGaussian beam to a nearly Gaussian beam with much higher beam quality. The method is based on separation of the radially polarized mode into two degenerate modes and coherent addition of the modes after phase flattening. We transformed a highpower Nd:YAG radially polarized (0, 1)*: LaguerreGaussian beam with M2 = 2.52 and power of 30 W into a nearly Gaussian beam with M2 = 1.3. As a result, the brightness increased by a factor of similar to 2.5. (c) 2007 Optical Society of America.

(2007) Optics Letters. 32, 7, p. 790792 Abstract
An efficient technique in which fiber lasers are coherently added in free space is presented. Since the high power of the combined output light propagates in free space rather than inside, fiber optical damage and deleterious nonlinear effects are substantially reduced. Two different configurations are investigated. One involves conventional intracavity coupling between the lasers. The other is a novel configuration where the coupling is done out of the combined cavities. The latter configuration requires much less coupling for obtaining the same output power, so the damage to the fiber is further reduced. (c) 2007 Optical Society of America.

(2007) Optics Communications. 270, 2, p. 145150 Abstract
The properties of tight dark focal spot created using a simple circular it phase plate are presented. For focusing elements with low numerical aperture, the focal plane intensity has r(4) dependence, while for focusing elements with high numerical aperture, vectorial diffraction effects become important, and the focal plane intensity surprisingly approaches r(2) dependence, indicating a much tighter dark spot. (c) 2006 Elsevier B.V. All rights reserved.
2006

(2006) Physical Review Letters. 97, 10, Abstract
We perform echo spectroscopy on ultracold atoms in atomoptics billiards to study their quantum dynamics. The detuning of the trapping laser is used to change the "perturbation", which causes a decay in the echo coherence. Two different regimes are observed: first, a perturbative regime in which the decay of echo coherence is nonmonotonic and partial revivals of coherence are observed in contrast with the predictions of random matrix theory. These revivals are more pronounced in traps with mixed dynamics as compared to traps where the dynamics is fully chaotic. Next, for stronger perturbations, the decay becomes monotonic and independent of the strength of the perturbation. In this regime no clear distinction can be made between chaotic traps and traps with mixed dynamics.

(2006) Optics Communications. 263, 1, p. 6064 Abstract
A new approach for stable intracavity phase locking of several laser channels is presented. In this approach, special interferometric couplers are incorporated inside a laser resonator to obtain efficient self phaselocking between separate laser channels. We analyze the approach and demonstrate experimentally phaselocking of two and of four laser channels, that are derived from Nd:YAG lasers, with 92% and 83% power efficiencies respectively. (c) 2006 Elsevier B.V. All rights reserved.

(2006) Optics Letters. 31, 3, p. 350352 Abstract
The efficient intracavity coherent addition of 16 separate laser Gaussian mode distributions is presented. The coherent addition is achieved in a multichannel pulsed Nd:YAG laser resonator by use of four intracavity interferometric beam combiners. The results reveal 88% combining efficiency with a combined output beam of nearly pure Gaussian distribution. (c) 2006 Optical Society of America.

(2006) Optics Letters. 31, 2, p. 149151 Abstract
The properties of the focal spot for 4 pi focusing with radially polarized firstorder LaguerreGaussian beams are calculated. It is shown that a focal spot that has an extremely sharp dark region at the center and an almostperfect spherical symmetry can be achieved. When such a hollow dark spherical spot is used in 4 pi fluorescence depletion microscopy, anaxial FWHM spot size of similar to 39 run and a transverse FWHM spot size of similar to 64 nm can be achieved simultaneously in a practical system. (c) 2006 Optical Society of America.

(2006) Applied Physics Letters. 88, 4, Abstract
A highly efficient intracavity coherent addition of nine individual laser distributions is presented. It is achieved with two passive interferometric combiners that are introduced into the combined laser cavity. The results reveal that the combined output power is greater by almost a factor of 9 compared to that of the single laser distributions, while the beam quality is the same. (c) 2006 American Institute of Physics.
2005

(2005) Review of Scientific Instruments. 76, 11, Abstract
In this article the design, optimization and characterization of diffractive optical elements formed on a curved surface are reviewed. For such curved diffractive optical elements not only the phase function, but also the surface shape are free parameters that can be used for optimization, yielding much better performances than both flat diffractive optical elements and reflective/refractive optical elements when operating with quasimonochromatic light. We present a new analytic design approach for the surface shape that ensures uniform collimation of a light source with any angular distribution. We demonstrate the usefulness of this design also for ideal (brightness conserving) collimation and concentration of diffuse light, aberrationfree imaging, and optical Fourier transform. We present experimental results that confirm our theoretical analysis.

(2005) Journal Of Optics BQuantum And Semiclassical Optics. 7, 8, p. R103R125 Abstract
We perform spectroscopy on the groundstate hyperfine splitting of Rb85 atoms trapped in faroffresonance optical traps. The existence of a spatially dependent shift in the energy levels is shown to induce an inherent dephasing effect, which causes a broadening of the spectroscopic line and hence an inhomogeneous loss of atomic coherence at a much faster rate than the homogeneous one caused by spontaneous photon scattering. We present here a number of approaches for reducing this inhomogeneous broadening, based on trap geometry, additional laser fields, and novel microwave pulse sequences. We then show how hyperfine spectroscopy can be used to study the quantum dynamics of optically trapped atoms.

(2005) Optics Letters. 30, 14, p. 17701772 Abstract
We report on efficient intracavity coherent addition of several single highorder mode distributions in a multichannel laser resonator. The phase locking and coherent addition is achieved by using an intracavity interferometric beam combiner. The principle, configuration, and experimental results with pulsed Nd:YAG LaguerreGaussian TEM01 and TEM02 laser beam distributions are presented. The results reveal more than 95% combining efficiency with a nearly pure highorder mode output beam distribution in both freerunning and Qswitched operation. (c) 2005 Optical Society of America.

(2005) Optics Express. 13, 13, p. 49524962 Abstract
An intracavity phase element, combined with a passive Qswitch saturable absorber and a suitable intracavity aperture, can provide extremely high mode discrimination, so as to obtain laser operation with single, pure, very high order LaguerreGaussian mode. With a Nd:YAG laser setup, well controlled and extremely stable Qswitched operation in the degenerate LaguerreGaussian TEM04, TEM14, TEM24, TEM34, and TEM44 modes was obtained. The measured output energy per pulse for each of these modes was 5.2mJ, 7.5mJ, 10mJ, 12.5mJ, and 13.7mJ respectively, compared to 2.5mJ for the Gaussian mode without the phase element ( more than a five fold increase in output energy). Correcting the phase for these modes, so that all transverse lobes have uniform phase, results in a very bright and narrow central lobe in the far field intensity distribution that can theoretically contain more than 90% of the output energy. (C) 2005 Optical Society of America

(2005) Physical Review Letters. 94, 19, Abstract
Nonresonant light scattering off atomic BoseEinstein condensates is predicted to give rise to hitherto unexplored composite quasiparticles: unstable polarons, i.e., local "impurities" dressed by virtual phonons. Optical monitoring of their spontaneous decay can display either Zeno or antiZeno deviations from the golden rule, and thereby probe the temporal correlations of elementary excitations in the condensates.

(2005) IEEE Journal of Quantum Electronics. 41, 5, p. 686693 Abstract
Intracavity coherent addition of several laser channel distributions where one is a Gaussian distribution and the others are multimode distributions is investigated. It is shown experimentally that the Gaussian distribution is inherently imposed on all the channels. A model for analyzing coherent addition of two or four beams is developed and used to support the experimental results.

(2005) Optics Express. 13, 7, p. 27222730 Abstract
Multimode laser operation is usually characterized by high output power, yet its beam quality is inferior to that of a laser with single TEM00 mode operation. Here we present an efficient approach for improving the beam quality of multimode laser resonators. The approach is based on splitting the intracavity multimode beam into an array of smaller beams, each with a high quality beam distribution, which are coherently added within the resonator. The coupling between the beams in the array and their coherent addition is achieved with planar interferometric beam combiners. Experimental verification, where the intracavity multimode beam in a pulsed Nd: YAG laser resonator is split into four Gaussian beams that are then coherently added, provides a total increase in brightness of one order of magnitude. Additional spectral measurements indicate that scaling to larger coherent arrays is possible. (C) 2005 Optical Society of America.


Atomoptics billiards  Nonlinear dynamics with cold atoms in optical traps(2005) Chaotic Dynamics And Transport In Classical And Quantum Systems. 182, p. 239267 Abstract
We developed a new experimental system (the "atomoptics billiard") and demonstrated chaotic and regular dynamics of cold, optically trapped atoms. We show that the softness of the walls and additional optical potentials can be used to manipulate the structure of phase space.
2004

(2004) New Journal of Physics. 6, Abstract
We study the linearized Bogoliubov excitation spectrum of infinitely long anharmonically trapped Bose  Einstein condensates, with the aim of overcoming inhomogeneous broadening. We compare the Bogoliubov spectrum of a harmonic trap with that of a theoretical flatbottom trap and find a significant reduction in the inhomogeneous broadening of the lineshape of Bogoliubov excitations. While the Bragg excitation spectrum for a condensate in a harmonic trap supports a number of radial modes, the flat trap is found to significantly support just one mode. We also study the excitation spectrum of realistic anharmonic traps with potentials of finite power dependence on the radial coordinate. We observe a correlation between the number of radial modes and the number of bound states in the effective potential of the quasiparticles. Finally, we compare a full numerical Gross  Pitaevskii simulation of a finitelength condensate to our model of infinite, linearized Gross  Pitaevskii excitations. We conclude that our model captures the essential physics.

(2004) Optics Express. 12, 20, p. 49294934 Abstract
We report on efficient coherent addition of spatially incoherent multimode laser beam distributions. Such addition is demonstrated within a multichannel laser resonator configuration, obtaining more than 90% combining efficiency while preserving the good beam quality. We explain the rather surprising physical phenomenon of coherently adding spatially incoherent light by selfphaselocking of each of the modal components within the multimode beams. Our approach could lead to significantly higher output powers concomitantly with good beam qualities than were hitherto possible in laser systems. (C) 2004 Optical Society of America.

(2004) Optics Letters. 29, 17, p. 19681970 Abstract
The properties of the focal spot for 4pi focusing with radially polarized light are presented for various apodization factors. With a focusing system satisfying the Herschel condition, sharp focal spots with almostperfect spherical symmetry (leading to equal axial and transverse resolution) and extremely low sidelobes are achieved. (C) 2004 Optical Society of America.

(2004) Optics Communications. 239, 3Jan, p. 147151 Abstract
We show that laser mode stability can be improved with an intracavity mode selecting phase element. Without the phase element, a subwavelength change of the distance between the mirrors of a CO2 laser leads to a substantial change of the transverse mode distribution, whereas with it the distribution remains essentially the same. (C) 2004 Elsevier B.V. All rights reserved.

(2004) Applied Physics Letters. 85, 12, p. 21872189 Abstract
We present a compact and practical combined laser resonator configuration in which several Gaussian beam distributions are efficiently combined. It is based on intracavity coherent addition of pairs of Gaussian beam distributions with a planar interferometric coupler. The principle, configuration, and experimental results using pulsed Nd:YAG laser beams are presented. The results reveal more than 92% combining efficiency with a nearly Gaussian output beam, in free running and Qswitched operation. (C) 2004 American Institute of Physics.

(2004) Physical Review A. 70, 1, Abstract
The decay in the hyperfine coherence of optically trapped atoms due to interactions with the environment, is only partly suppressed by "echo" spectroscopy [ M. F. Andersen Phys. Rev. Lett. 90, 023001 (2003) ], primarily due to dynamical (timedependent) dephasing mechanisms. We demonstrate here an improved pulse sequence, for which the decay of coherence is reduced by a factor of 2.5 beyond the reduction offered by the "echo" spectroscopy. This reduction occurs when each dark period inbetween pulses is shorter than the time scale over which substantial dephasing develops. The coherence time is then limited by mixing to other vibrational levels in the trap and, to a lesser extent, the lifetime of the internal states of the atoms.

(2004) Physical Review A. 69, 6, Abstract
We investigate the coherence properties of thermal atoms confined in optical dipole traps where the underlying classical dynamics is chaotic. A perturbative expression derived for the coherence of the echo scheme of Anderson [Phys. Rev. Lett. 90, 023001 (2003) ] shows that it is a function of the survival probability or fidelity of eigenstates of the motion of the atoms in the trap. The echo coherence and the survival probability display "system specific" features, even when the underlying classical dynamics is chaotic. In particular, partial revivals in the echo signal and the survival probability are found for a small shift of the potential. Next, a "semiclassical" expression for the averaged echo signal is presented and used to calculate the echo signal for atoms in a light sheet wedge billiard. Revivals in the echo coherence are found in this system, indicating that they may be a generic feature of dipole traps.

(2004) Optics Letters. 29, 12, p. 13181320 Abstract
Recently, Dorn et al. [Phys. Rev. Lett. 91, 233901 (2003)] demonstrated the significance of radially polarized doughnut beams in obtaining very small focal spots (with an area of similar to0.26lambda(2)) with highnumericalaperture (NA) aplanatic microscope objectives. We propose two simple alternative ways to focus such radially polarized beams: a parabolic mirror and a flat diffractive lens. Because of their large apodization factor for a high NA, a significant further reduction in spot area (up to a factor of 1.76 at a NA of 1) compared with the aplanatic system can be achieved. (C) 2004 Optical Society of America.

(2004) JOURNAL OF PHYSICS BATOMIC MOLECULAR AND OPTICAL PHYSICS. 37, 7, p. S155S164 Abstract
We study a gaseous atomic BoseEinstein condensate with laserinduced dipoledipole interactions using the HartreeFockBogoliubov theory within the Popov approximation. The dipolar interactions introduce longrange atomatom correlations which manifest themselves as increased depletion at momenta similar to that of the laser wavelength, as well as a 'roton' dip in the excitation spectrum. Surprisingly, the roton dip and the corresponding peak in the depletion are enhanced by raising the temperature above absolute zero.

(2004) Journal Of The Optical Society Of America AOptics Image Science And Vision. 21, 4, p. 656661 Abstract
A new type of diffractive optical instrument, the curved hologram, for which the spatial phase function and the hologram shape can be controlled independently, is investigated for finite distance concentration of diffuse (quasimonochromatic) light. We show how a simple analytic design for given light source and target geometries yields spatially uniform concentrations of diffuse light at the thermodynamic limit of brightness conservation. Such diffractive elements may provide a useful alternative to reflective cavities for efficient and uniform sidepumping of solidstate lasers. (C) 2004 Optical Society of America.

(2004) Applied Optics. 43, 12, p. 25612567 Abstract
We present an approach for efficient conversion of a singlehighordermode distribution from a laser to a Dearly Gaussian distribution and vice versa. It is based on dividing the highorder mode distribution into equal parts that are then combined together coherently. We implement our approach with several optical arrangements that include a combination of discrete elements and some with single interferometric elements. These arrangements are analyzed and experimentally evaluated for converting the TEM01 mode distribution with Mx(2) = 3 to a nearly Gaussian beam with Mx(2) = 1.045 or Mx(2) = 1.15. The basic principle, design, and experimental results obtained with several conversion arrangements are presented. The results reveal that conversion efficiency is typically greater than 90%, compared with theoretical ones. In addition, some arrangement is exploited for converting the fundamental Gaussianbeam distribution into the TEM01 mode distribution. (C) 2004 Optical Society of America

(2004) Physica DNonlinear Phenomena. 187, 4Jan, p. 136145 Abstract
We present numerical and experimental results for the development of islands of stability in atomoptics billiards with soft walls. As the walls are soften, stable regions appear near singular periodic trajectories in converging (focusing) and dispersing billiards, and are surrounded by areas of "stickiness" in phase space. The size of these islands depends on the softness of the potential in a very sensitive way. (C) 2003 Elsevier B.V. All nights reserved.

Laser configurations for highorder transverse mode selection and coherent beam combining(2004) Lfnm 2004: Proceedings Of The 6Th International Conference On Laser And FiberOptical Networks Modeling. p. 49 Abstract
Keywords: FIBER LASERS; QUALITY; POWER
2003

(2003) Optics Communications. 226, 6Jan, p. 8996 Abstract
Reflective lightguiding tubes having chaotic and integrable crosssectional shapes are investigated. The applicability of chaotic billiard shapes as crosssections versus integrable ones for specific beam shaping purposes is demonstrated. Three important cases are considered: homogenizing, anamorphic collimation and concentration of diffuse light. (C) 2003 Elsevier B.V. All rights reserved.

(2003) Optics Letters. 28, 7, p. 504506 Abstract
We present a new, compact, and practical optical mode converter that efficiently transforms a highorder HermiteGaussian (HG) laser beam into a nearly Gaussian beam. The mode converter is based on coherently adding different transverse parts of the highorder mode beam by use of a single planar interferometric element. The method, configuration, and experimental results obtained with a pulsed Nd:YAG HG TEM10 laser beam are presented. The results reveal that the efficiency of conversion of a HG beam to a nearly Gaussian beam can be as high as 90%. (C) 2003 Optical Society of America.

(2003) Optics For The Quality Of Life, Pts 1 And 2. 4829, p. 481482 Abstract
Dark optical traps use repulsive light forces to confine atoms mostly in the dark where their interaction with the trapping light is largely reduced, to enable long coherence times of the trapped atoms combined with tight confinement. We report on singlebeam dark optical traps for ultracold atoms. These traps were used to demonstrate new spectroscopic and dynamical effects of the trapped atoms, including the observation of regular and chaoticmotion in so called optical "billiards".

(2003) Alt'02 International Conference On Advanced Laser Technologies. 5147, p. 271275 Abstract
A simple method for obtaining a nearly Gaussian laser beam from a high order HermiteGaussian mode is presented. The method is based on separating the equal lobes of the high order mode and combining them together coherently. The method was experimentally verified with an arrangement of three mirrors, a 50% beam splitter and a phase tuning plate. The beam quality factor calculated in xdirection for the resulting output beam is 1.045, being very close to that of ideal Gaussian beam. The calculated power leakage is only 1.5%. The experimental nearfield and farfield intensity distributions of the output beam have nearly Gaussian cross sections in both the x and the y directions, with Mx(2) = 1.34 and My(2) = 1.32. With some modifications, it is possible to obtain an output beam with Mx(2) = 1.15 and no power leakage.


(2003) Physical Review Letters. 90, 2, Abstract
We investigate the dephasing of ultra cold Rb85 atoms trapped in an optical dipole trap and prepared in a coherent superposition of their two hyperfine ground states by interaction with a microwave pulse. We demonstrate that the dephasing, measured as the Ramsey fringe contrast, can be reversed by stimulating a coherence echo with a pi pulse between the two pi/2 pulses, in analogy to the photon echo. We also demonstrate that "echo spectroscopy" can be used to study the quantum dynamics in the trap even when more than 10(6) states are thermally populated and to study the crossover from quantum to classical dynamics.

(2003) IEEE Journal of Quantum Electronics. 39, 1, p. 7482 Abstract
Intracavity binary phase elements are incorporated into a Qswitched Nd: YAG laser resonator to obtain efficient highorder transverse mode selection. The resonator configuration is analyzed using the propagationmatrix diagonalization method and the FoxLi algorithm, and a simple model for predicting the relative output powers of the selected modes is developed. The predicted results are verified experimentally with binary phase elements for selecting the TEM01, TEM02 and TEM03, degenerate LaguerreGaussian modes. The output energy per pulse was 15 mJ for the TEM01, 16.5 mJ for TEM02 and 183 mJ for TEM03,, all higher than the 10mJ for the TEM00. The performance in Qswitched operation was found to be similar to that in freerunning operation. The numerical calculations, experimental procedure and experimental results are presented.
2002

(2002) Journal Of The Optical Society Of America AOptics Image Science And Vision. 19, 12, p. 24792483 Abstract
A new type of optical instrument, the curved hologram, is introduced that allows us the unique opportunity to independently control its spatial phase function and its shape (whereas in reflecting or refracting optical elements the shape uniquely determines the spatial phase function). We show how proper design of the hologram shape (using a simple analytic procedure) yields perfect uniform collimation and also collimation and concentration of diffuse (monochromatic) light at the thermodynamic limit of brightness conservation. The results of our experimental demonstration as well as those of our numerical raytracing simulations verify our design. (C) 2002 Optical Society of America.

(2002) Physical Review A. 66, 4, Abstract
We present a method for reducing the inhomogeneous frequency broadening in the hyperfine splitting of the ground state of optically trapped atoms. This reduction is achieved by the addition of a weak light field, spatially mode matched with the trapping field and whose frequency is tuned in between the two hyperfine levels. We experimentally demonstrate the new scheme with Rb85 atoms, and report a 50fold narrowing of the rf spectrum.

(2002) Optics Letters. 27, 17, p. 15011503 Abstract
We present a relatively simple method for efficiently transforming a single highorder mode into a nearly Gaussian beam of much higher quality. The method is based on dividing the mode into equal parts that are then combined coherently. We illustrate the method by transforming a HermiteGaussian (1, 0) mode with Mx(2) = 3 into a nearly Gaussian beam with Mx(2) = 1.045. Experimental results are presented and compared with theoretical results. (C) 2002 Optical Society of America.

(2002) Optics Communications. 209, 6Apr, p. 265271 Abstract
A method for efficiently converting a Gaussian beam into a helical LaguerreGaussian (LG) beam is presented. It is based on using a pair of axicons to produce a shiftedGaussian (doughnut) intensity distribution that is then passed through a spiral phase element. It is shown that the conversion efficiency can be as high as similar to98%, and the calculated farfield intensity distributions of the output beams are very close to those of corresponding pure LG intensity distributions. The principle of the method, the needed optical arrangement, and calculated and experimental results are presented. (C) 2002 Elsevier Science B.V. All rights reserved.

(2002) Journal Of The Optical Society Of America BOptical Physics. 19, 6, p. 12331238 Abstract
We propose a new scheme for constructing a singlebeam dark optical trap that minimizes lightinduced perturbations of the trapped atoms. The proposed scheme optimizes the trap depth for given trapping laser power and detuning by creating a light envelope with (a) an almost minimal surface area for a given volume and (b) the minimal wall thickness that is allowed by diffraction. The stiffness of the trap's walls, combined with the large detuning allowed by the efficient distribution of light intensity, yields a low spontaneous photon scattering rate for the trapped atoms. Our trap also optimizes the loading efficiency by maximizing the geometrical overlap between a magnetooptical trap and the dipole trap. We demonstrate this new scheme by generating the proposed light distribution of a singlebeam dark trap with a trap depth that is similar to33 times larger than that of existing bluedetuned traps and similar to13 times larger than that of a reddetuned trap with the same diameter, detuning, and laser power. Trapped atoms are predicted to have a decoherence rate that is >200 times smaller than in existing singlebeam dark traps and similar to1800 times smaller than in a reddetuned trap with the same diameter, depth, and laser power. (C) 2002 Optical Society of America.

(2002) Applied Optics. 41, 18, p. 36343637 Abstract
Laser resonator configurations, which enable laser operation with two orthogonally polarized transverse modes, are presented. The intensity distributions of these two modes can be chosen to be complementary, so the gain medium can be exploited more efficiently than with a single mode, leading to improved output power. Moreover, the two modes can be combined and efficiently transformed into a single highquality beam. Basic principles and experimental results with Nd:YAG lasers are presented. (C) 2002 Optical Society of America.

(2002) JOURNAL OF PHYSICS BATOMIC MOLECULAR AND OPTICAL PHYSICS. 35, 9, p. 21832190 Abstract
We investigate the effects of curving trajectories by applying external force fields on a particle in a billiard. We investigate two special cases: a constant force field and a parabolic potential. These perturbations change the stability conditions and can lead to formation of elliptical orbits in otherwise hyperbolic billiards. We demonstrate these effects experimentally with ultracold atoms in atomoptic billiards.

(2002) Optics Communications. 201, 6Apr, p. 243249 Abstract
A concept of anamorphic, adiabatic shaping of diffuse, spatially incoherent light beams is presented. The beam shaping is done with a long tube with reflective walls. The concept is numerically investigated and experimentally illustrated for the specific example of onedimensional diffusebeam collimation. Depending on the exact geometry of the tube, two cases are investigated: (1) when the onedimensional brightness is conserved in both transverse directions; (2) when there is coupling between the two transverse directions in terms of brightness, which leads to an output beam with a much more uniform angular distribution. (C) 2002 Elsevier Science B.V. All rights reserved.

2001

(2001) Physical Review Letters. 87, 27, Abstract
We report on numerical and experimental observations of islands of stability induced in a Bunimovich stadium atomoptics billiard by a soft wall repulsive potential. A deviation from exponential decay of the survival probability of atoms in an open billiard is observed, and explained by the presence of these stable islands and a sticky region surrounding them. We also investigate islands in dispersing billiards with soft walls, and predict a new mechanism for their formation.

(2001) Applied Optics. 40, 32, p. 58255829 Abstract
We demonstrate an imaging diffractive optical element (DOE) that is free of coma and other firstorder aberrations. The DOE was holographically recorded on a properly curved surface and thereby satisfies the Abbe sine (aplanatic) condition. Experimental results as well as numerical raytracing simulations indicate that the off axis aberrations of the curved DOE are much smaller than those of a flat DOE. Because the recording of the DOE involves a single step and only readily available: spherical and plane waves, it is extremely simple. (C) 2001 Optical Society of America.

(2001) Optics Communications. 196, 6Jan, p. 916 Abstract
A concept of adiabatic shaping of diffuse, spatially incoherent light beams is presented. The beam shaping is done with a gradientindex rod where the refractive index profile is changed adiabatically along the rod axis. The concept is numerically investigated and illustrated with a specific example of a Gaussiantouniform beam transformation. The condition for brightness conservation and robustness are determined. (C) 2001 Elsevier Science B.V. All rights reserved.

(2001) Optics Letters. 26, 14, p. 10781080 Abstract
We propose and experimentally demonstrate an acoustooptic cylindrical lens with a very fast (400kHz) focal scanning. The lens is realized by use of two adjacent acoustooptic scanners with counterpropagating acoustic waves that have the same frequency modulation but a pi phase difference. This scheme completely suppresses the lateral scan but adds the linear chirp of the two waves and thus functions as a fast focalscan lens. We also demonstrate the use of this scanning lens in a very fast confocal profilometer. (C) 2001 Optical Society of America.

(2001) Optics Communications. 193, 6Jan, p. 227232 Abstract
Despite the fact that the phase distribution across a beam emerging from a laser operating with several transverse modes is random, it is possible to improve the beam quality. This is achieved by manipulating the Wigner distribution function of the emerging beam, utilizing phase elements and a beam converter. The theoretical analysis along with experimental results with a Nd:YAG laser operating with two LaguerreGaussian modes are presented. (C) 2001 Elsevier Science B.V. All rights reserved.

(2001) Applied Optics. 40, 13, p. 21322137 Abstract
A technique for diffuse beam shaping is presented. The beam shaping is achieved by a single reflection on an element, which consists of many displaced parallel planar reflecting facets. The reflecting facets approximate a designed curved surface. We demonstrate the validity of the method for the conversion of a diffuse Gaussian beam into a uniform one in one of the spatial dimensions. (C) 2001 Optical Society of America.

(2001) Applied Optics. 40, 13, p. 20762080 Abstract
We discuss some properties of dielectric gratings with period comparable with the illuminating wavelength for slanted illumination (this illumination geometry is often referred to as concical mounting). We demonstrate the usefulness of such an illuminating geometry. We show that the threshold period (under which only the zeroth transmission and reflection orders are nonevanescent) can be significantly higher, thereby easing fabrication constraints, and that this illumination setup makes it possible to design achromatic phase retarders. Such a design, for an achromatic quarterwave plate with lambda /60 uniformity of the retardation phase in the 0.470.63mum wavelength interval, is demonstrated. (C) 2001 Optical Society of America.

(2001) Optics Communications. 191, 6Mar, p. 141147 Abstract
A novel configuration for concentrating diffuse, based on a reflective curved diffractive element, is proposed and demonstrated. Such an aplanatic element satisfies the Abbe sine condition, and hence can achieve diffuse light concentration close to the thermodynamic limit. In our experiments, a thin. cylindrically shaped diffractive element with a numerical aperture of 0.86 yielded a onedimensional concentration ratio that was 80%, of the thermodynamic limit. (C) 2001 Elsevier Science B.V. All rights reserved.

(2001) Physical Review Letters. 86, 8, p. 15181521 Abstract
We report on experimental observations of chaotic and regular motion of ultracold atoms confined by a billiardshaped optical dipole potential induced by a rapidly scanning laser beam. To investigate the dynamics of the atoms confined by such an "atomoptics" billiard we measure the decay of thr number of trapped atoms through a hole on the boundary. A fast and purely exponential decay, the clear signature of chaotic motion, is found fur a stadium billiard, but not fur a circular or an elliptical billiard, in agreement with theory. We also investigated the effects of decoherence, velocity spread, and gravity on regular and chaotic motion.

2000


(2000) Optics Letters. 25, 24, p. 17621764 Abstract
A very fast (>100 kHz) acoustooptic scanning system, which relies on two counterpropagating acoustic waves with the same frequency modulation, is proposed and experimentally demonstrated. This scheme completely suppresses linear frequency chirp and thus permits very fast nonlinear scans and nonconstant linear scans. By changing the phase between the modulating signals, this scheme also provides very fast longitudinal scans of the focal point. (C) 2000 Optical Society of America.

(2000) Applied Physics Letters. 77, 21, p. 33223324 Abstract
Laser resonator configurations for obtaining pure azimuthal and radial polarized beams are presented. They involve the coherent summation, inside the laser resonator, of two orthogonally polarized TEM01 modes. Basic principles and experimental results with a Nd:YAG laser are presented. The results include a full space variant polarization measurement and show efficient formation of highquality azimuthal and radial polarized beams. (C) 2000 American Institute of Physics. [S00036951(00)013474].

(2000) Applied Optics. 39, 27, p. 49704974 Abstract
We describe frequency locking of a diode laser to a twophoton transition of rubidium using the Zeeman modulation technique. We locked and tuned the laser frequency by modulating and shifting the twophoton transition frequency with ac and de magnetic fields. We achieved a linewidth of 500 kHz and continuous tunability over 280 MHz with no laser frequency modulation. (C) 2000 Optical Society of America OCIS codes: 020.7490, 140.2020, 300.6260.

(2000) Optics Communications. 182, 3Jan, p. 205208 Abstract
A novel method for forming pure helical laser beams of predetermined helicity is presented. It is mainly based on replacing one of the laser mirrors with a spiral phase element. The basic principles along with experimental results using a CO(2) laser are described. (C) 2000 Elsevier Science B.V. All rights reserved.

(2000) Applied Optics. 39, 22, p. 39633967 Abstract
We propose and demonstrate a new scheme for anamorphic concentration of a big (40 cm x 40 cm) diffuse light source to achieve an extremely high concentration in one lateral direction at the expense of that in the other direction, to preserve the total (twodimensional) optical brightness. Such anamorphic concentration is achieved by a combination of two conventional twodimensional concentrators and a properly designed retroreflector array. Our experiments in search of a diffuse whitelight source with properties comparable with those of solar radiation have yielded 28fold improvement of the onedimensional concentration ratio compared with those of conventional concentrators and 14fold improvement compared with the onedimensional thermodynamic limit. (C) 2000 Optical Society of America OCIS codes: 080.2740, 220.1770, 230.6080, 350.6050.

(2000) Optics Letters. 25, 13, p. 939941 Abstract
Siegman [Opt. Lett. 18, 675 (1993)] showed that binaryphase plates cannot improve laser beam quality. We demonstrate that continuous spiral phase elements can improve the quality of beams that originate from a laser operating with a pure highorder transverse mode. A theoretical analysis is presented, along with experimental results obtained with a CO2 laser. The results reveal that a nearly optimal Gaussian output beam can be obtained with only a small decrease in the output power. (C) 2000 Optical Society of America.

(2000) Applied Physics BLasers And Optics. 70, 5, p. 683688 Abstract
Adiabatic focusing of cold atoms in a bluedetuned laser standing wave is analyzed. It is shown that using repulsive light forces that push atoms towards dark regions and thus minimizes heating, cold atoms can be adiabatically compressed by more than an order of magnitude to yield backgroundfree sub10nm (rms) spots. The optimal parameters for the atomic lens and the maximal compression ratio ale predicted using an analytic model and found to be in agreement with the exact results of our Monte Carlo simulations. A combined adiabaticcoherent scheme is proposed and shown to yield 8.8 nm spot size even for a thermal atomic beam.

(2000) Europhysics Letters. 50, 4, p. 454459 Abstract
We propose a new spectroscopic method fur measuring weak transitions in cold and trapped atoms, which exploits the long spin relaxation times and tight spatial confinement offered by dark optical traps to achieve extremely high sensitivity. We demonstrate our scheme by measuring a 5S(1/2) > 5D(5/2) twophoton transition in cold Rb atoms trapped in a new singlebeam dark optical trap, using an extremely weak probe laser power of 25 mu W. We mere able to measure transitions with as small excitation rate as <0.1 s(1), using 10(7) fold quantum amplification of the transition rate due to spin shelving anti normalized detection with a strong cycling transition.

(2000) Optics Letters. 25, 7, p. 439441 Abstract
We show an anomalous behavior in a diffractive lens in which the spot size at the focus reaches a minimum at a numerical aperture of similar to 0.5 and then increases significantly at higher values. Theoretical and experimental results are presented, along with a comparison with refractive aplanatic lenses, in which the anomaly does not appear to exist. (C) 2000 Optical Society of America OCIS codes; 050.1970, 050.1380, 050.1960.

Intracavity phase elements for specific laser mode selection(2000) ElectroOptics And Microelectronics, Proceedings. 14, p. 161164 Abstract
We present novel phase elements that can be incorporated into the laser resonator, so as to select a specific highorder transverse mode. Such elements are essentially lossless for the desired selected mode, but introduce high losses to other modes. Thus, the laser operates with a single high order mode that is stable and has a better output beam quality than with multimode operation. The design of the phase elements, as well as experimental results with Nd:YAG and CO2 lasers, that typically operate with many transverse modes, are presented. The results reveal good mode selectivity, and a possibility for improving the laser output power with respect to the Fundamental mode operation by more than 50 percent.
1999

(1999) Optics Letters. 24, 24, p. 18351837 Abstract
A new scheme to improve the spectral resolution of gratingbased spectrometers for diffuse light is proposed and demonstrated. It exploits an anamorphic transformation that reduces the beam divergence in the direction of the grating grooves while increasing the divergence in the orthogonal direction to improve the spectral resolution without any loss of light. Up to 12fold improvement in the spectral resolution was obtained. (C) 1999 Optical Society of America. OCIS codes: 070.4790, 080.2740, 220.1770, 230.6080.

(1999) European Physical Journal D. 7, 3, p. 467473 Abstract
We present an experimental and theoretical investigation of the weak probe amplification in a cold and optically thick atomic cloud that is highly driven by a strong pump laser. We find that for high optical densities the probe amplification is strongly saturated. We compare our saturation measurements with a model based on dressedatom population equalization due to rescattering of spontaneous emission. Good agreement between theory and experiment is obtained only when corrections due to multiple scattering are included.

(1999) Optics Communications. 169, 6Jan, p. 115121 Abstract
A novel method for discriminating and selecting a specific high order mode is presented. It is based on introducing spiral phase elements into a laser resonator, so as to improve the output beam quality over that when the laser operates with multimodes, and yet maintain a relatively high output power. The theoretical analysis, along with experimental results with CO2 and Nd:YAG lasers are presented. The results reveal that a 50% increase of laser output power over that operating with TEM00 mode is possible. (C) 1999 Elsevier Science B.V. All rights reserved.

(1999) Applied Optics. 38, 16, p. 35933597 Abstract
A new technique for geometric transformation of a diffuse linear Light source into a twodimensional symmetric virtual source is proposed and demonstrated. It involves two cylindrical lenses and a single holographic element that is optically recorded with a simple and single step. The results for a specific arrangement that transforms a 40 mm x 0.4 mm source, with diffusive angle of 0.1 rad, into a 1 mm x 1.2 mm nearly symmetric spot are presented. The lack of sharp boundaries in either the beam or the optical elements suppresses any diffraction losses, relaxes alignment tolerances, and improves stability. (C) 1999 Optical Society of America.

(1999) Journal Of The Optical Society Of America BOptical Physics. 16, 5, p. 702709 Abstract
We analyze the lightinduced atomatom interactions in optically thick atomic clouds and show that, when the laser frequency is onresonance with the atomic transition, they become attractive. On the basis of this analysis we propose and demonstrate a novel scheme to compress a cold and dense atomic cloud with a short onresonance laser pulse. The compression force arises from attenuation of the laser light by the atomic cloud. The following free propagation of the atoms shows a lenslike behavior that yields a transient density increase at the focal time, where neither laser nor magnetic field perturbations exist. A cooling pulse, which is applied at the focal Lime of this lens, restores the initial temperature of atoms, and hence the phase space density is increased. Finally, we adopt our compression scheme to a quasisteadystate mode by temporally chopping it with the cooling and trapping beams of a magnetoptical trap. (C) 1999 Optical Society of America [S07403224(99)019050].

(1999) Applied Physics Letters. 74, 10, p. 13731375 Abstract
Discontinuous phase elements can be inserted into laser resonators so that the lasers will operate with only one desired high order transverse mode. These elements introduce sharp discontinuous phase changes so as to result in minimal losses for a desired transverse mode but high losses to others. The basic principles, along with experimental results with Nd:yttriumaluminumgarnet and CO2 lasers, illustrating improved output powers with a high beam quality of low divergence, are presented. (C) 1999 American Institute of Physics. [S00036951(99)026108].
1997

(1997) Fiber and Integrated Optics. 16, 1, p. 125 Abstract
This paper presents methods for designing and recording optimal computergenerated diffractive optical elements, The design method is based on an analytic raytracing procedure for minimizing aberrations, The recording involves computergenerated mask and multiple lithographic processes in order to form reflective and transmissive multilevel, surface reliefphase, diffractive elements. As a result, the elements can have high diffraction efficiencies over a broad range of incidence angles. Even generalized diffractive elements that operate with highly uniform diffraction efficiency and polychromatic radiation can be designed and recorded by optimizing the shape and height of the relief gratings. To illustrate the effectiveness of the diffractive optical elements, they have been incorporated into a number of applications, involving visible as well as infrared radiation. Some that deal with coordinate transformation beam shaping, and polarization control are briefly reviewed.
1994

REFLECTIVE AND REFRACTIVE SYSTEMS FOR GENERAL 2DIMENSIONAL BEAM TRANSFORMATIONS(1994) Applied Optics. 33, 5, p. 815820 Abstract
A method for designing reflective and refractive surfaces that perform general transformations on twodimensional beams is presented. In some cases the shape of the surfaces is represented by a simple integral of an analytic expression, whereas in other specific cases it is represented as a solution of a Poissonlike equation. Finally, the possible use of noncontinuous surfaces (facets) is discussed and evaluated quantitatively. Some of the novel techniques developed are also applicable for beam transformations that are realized with diffractive systems.
1993

(1993) Applied Optics. 32, 25, p. 47704774 Abstract
An analytic design of hybrid achromats that combine refractive and diffractive elements is presented. The design procedure does not rely on paraxial approximations and involves two separate stages. In the first stage the chromatic aberrations are corrected for the paraxial rays, and in the second stage the spherical aberrations are corrected by addition of an aspherical phase function to the diffractive element. The residual spherochromatic aberrations of the achromat are evaluated both analytically and numerically, with good agreement between the results. Finally, we illustrate the design procedure by designing a planoconvex achromat for IR radiation with little chromatic dispersion.

(1993) Journal Of The Optical Society Of America AOptics Image Science And Vision. 10, 8, p. 17251728 Abstract
A formalism is presented for determining the amount of symmetry of optical interconnects in terms of their required number of degrees of freedom. The formalism is based on matrix representation of optical interconnects and shows that the relevant property of the interconnect matrix is its rank. On the basis of this formalism, an optical arrangement is proposed for the implementation of general interconnects. This arrangement exploits the symmetry of the interconnects to increase the spacebandwidthproduct capabilities, up to the limit imposed by information theory. Some examples that illustrate the effectiveness of our approach are also given.

(1993) Optics Communications. 99, 4Mar, p. 162166 Abstract
An approach is presented for anamorphic concentration of diffuse light to achieve extremely high concentration in one direction at the expense of the other direction. The approach is based on freespace optics and involves two arrays of prisms, or holographic optical elements that perform the necessary coupling between the two spatial directions.

(1993) Applied Physics Letters. 62, 4, p. 334336 Abstract
We present an approach for concentration and collimation of a diffuse linear light source to achieve extremely high concentration in the long direction at the expense of the short direction. The approach is based on freespace optics and involves either arrays of prisms or holographic optical elements that couple between the two spatial directions. The results for a specific arrangement that concentrates a 40 mm by 0.4 mm source, with diffusive angle of about 0.1 rad, into a 1 mm by 1 mm isotropic spot are presented.
1992

(1992) Optics Letters. 17, 21, p. 15411543 Abstract
A method for exploiting nonuniform ultrahigh spatialfrequency relief gratings as spacevariant polarization elements is presented. In this method the local direction of the gratings determines the polarization angles, while the period of the gratings is controlled to ensure continuity of the grating function for any desired polarization operation. We illustrate the method with a specific spacevariant halfwave plate for laser radiation of 10.6 mum.

ON THE LIMITS OF OPTICAL INTERCONNECTS(1992) Applied Optics. 31, 26, p. 54265430 Abstract
The density capabilities of freespace optical interconnects are analyzed by applying Gabor's theory of information. It is shown that it is possible to increase the spacebandwidth product capabilities of spacevariant interconnect schemes if they have symmetry properties. Several examples of such symmetries (locality, separability and smoothness) are discussed in detail, together with some experimental results.

(1992) Journal Of The Optical Society Of America AOptics Image Science And Vision. 9, 7, p. 11961199 Abstract
A new approach for optimizing the groove depth of blazed holographic gratings that are illuminated by light with a wide range of wavelengths or incidence angles is presented. The approach is based on choosing the groove depth that maximizes the overall diffraction efficiency over the entire range of incidence angles and wavelengths. The scalar approximation is used for the diffraction efficiency calculations, with some results verified by rigorous vectorial calculations. Analytic solutions are given for some simple examples, together with experimental results.

(1992) Applied Physics Letters. 61, 4, p. 381383 Abstract
A diffractive optical system for the transformation of an annular laser beam to a uniform circular beam is presented. It is composed of two diffractive elements which are recorded as surface relief transmission gratings with multilevel discrete binary steps. Our experiments with CO2 laser radiation show that high diffraction efficiencies and good uniformity of the output beam can be achieved.

(1992) Optics Communications. 89, 4Feb, p. 306315 Abstract
A novel method for designing and recording aspheric computer generated holographic optical elements for far infrared radiation has been developed. The design method is based on analytic raytracing procedure that exploits the minimization of the meansquared difference of the propagation vectors between the actual output wavefronts and the desired output wavefronts. This minimization yields a solution for the aspheric grating vector. The design method is illustrated by recording and testing aspheric reflective and transmissive offaxis focussing elements for 10.6mum wavelength having diffractionlimited performance over a broad range of incidence angles.

(1992) Optics Communications. 88, 6Apr, p. 326330 Abstract
A technique for forming nondiffracting beams having essentially constant intensity along the propagation direction is presented. In this technique two phaseonly holographic elements are exploited to form the nondiffractive beam as well as to obtain a high diffraction efficiency. The elements were designed and formed as computer generated holograms, and the combination was tested experimentally, showing a good agreement with the predicted behavior.

REALIZATION OF PERFECT SHUFFLE AND INVERSE PERFECT SHUFFLE TRANSFORMS WITH HOLOGRAPHIC ELEMENTS(1992) Applied Optics. 31, 11, p. 18101812 Abstract
Techniques for implementing perfect shuffle and inverse perfect shuffle operations with the aid of a single holographic optical element are presented. The element is composed of subholographic lenses which operate on a different input area. For the inverse perfect shuffle operation, polarization coding is added in order to separate the input into distinct groups. Experimental results illustrating the effectiveness of the proposed techniques are given.

OPTICAL COORDINATE TRANSFORMATIONS(1992) Applied Optics. 31, 8, p. 10671073 Abstract
A novel technique for designing holographic optical elements that can perform general types of coordinate transformation is presented. The design is based on analytic raytracing techniques for finding the grating vector of the element, from which the holographic grating function is obtained as a solution of a Poissonlike equation. The grating function can be formed either as a computergenerated or as a computeroriginated hologram. The design and realization procedure are illustrated for a specific holographic element that performs a logarithmic coordinate transformation on twodimensional patterns.

(1992) Optical Engineering. 31, 2, p. 363368 Abstract
A novel single multifunctional holographic optical element is incorporated into a surface measurement system. As a result the system is lightweight, compact, and simpler than conventional ones. Numerical calculation reveals that submicron resolutions are possible both in the horizontal and vertical directions. Finally, experimental results demonstrate the feasibility of the approach.
1991

(1991) Optics Letters. 16, 19, p. 14601462 Abstract
A method for forming multilevel diffractive elements (kinoforms) that have highly accurate level heights so as to obtain high diffraction efficiencies is presented. The method, which leads to heterostructure multilevel binary optical elements, relies on conventional deposition technology, selective etching, and multimask lithography. As an illustration, a reflection multilevel element for 10.6mum radiation is designed, recorded, and tested.

(1991) Optics Letters. 16, 18, p. 14301432 Abstract
A new method for performing optical coordinate transformation is presented. It is based on a curved holographic element on which the interference pattern of two perpendicular plane waves is recorded. The design procedures and results for two curved elements that perform a onedimensional logarithmic coordinate transformation and a Gaussiantouniformbeam conversion are given.

(1991) Optics Letters. 16, 7, p. 523525 Abstract
We report a novel aspheric holographic optical element, the holographic axilens, for achieving extended focal depth while keeping high lateral resolution. The element is designed according to special optimization techniques and recorded as a computergenerated hologram. The results for a specific element, which has a depth of focus of 30 mm, a lateral resolution of 80mum, a focal length of 1250 mm, and a diameter of 12.5 mm at a wavelength of 633 nm, are presented.

(1991) Optics Letters. 16, 6, p. 423425 Abstract
Multilevel phase holograms for monochromatic radiation at a wavelength of 10.6mum are recorded as surface relief gratings with multilevel discrete binary steps. Our experiments show that diffraction efficiencies close to 90% can be achieved both for transmissive and reflective elements. The reduction of efficiency due to errors in the depth and the width of the step levels is considered.
1990
1988

OPTIMALDESIGN FOR HOLOGRAPHIC FOCUSING ELEMENTS(1988) Romanian Journal of Physics. 33, 6Apr, p. 643651 Abstract