Publications

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

Coherent anti-Stokes Raman scattering (CARS) has found wide applications in biomedical research. Compared with alternatives, single-beam CARS is especially attractive at low frequencies. Yet, currently existing schemes necessitate a relatively complicated setup to perform high-resolution spectroscopy. Here we show that the spectral sharp edge formed by an ultra-steep long-pass filter is sufficient for performing CARS spectroscopy, simplifying the system significantly. We compare the sensitivity of the presented methodology with available counterparts both theoretically and experimentally. Importantly, we show that this method, to the best of our knowledge, is the simplest and most suitable for vibrational imaging and spectroscopy in the very low-frequency regime (

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

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

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

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

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

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

We investigate a quasi-one-dimensional periodic array of coupled waveguides, with one extended and one bound dimension, incorporating both first-and second-order coupling. We study the evolution of optical fields in this system, and measure quantum correlations when path-entangled photon pairs are launched into them. We observe a surprisingly large and nontrivial effect of second-order coupling on these correlations-while quantum correlations are symmetric when only first-order coupling is present, the introduction of next-nearest-neighbor coupling often breaks the symmetry to reflections.

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

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

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

We experimentally demonstrate a method for creating broad bandwidth photon pairs in the visible spectral region, centered at a frequency that is higher than that of the initial pump source. Spontaneous down conversion of a narrowband 1053 nm pulsed Nd:YLF laser is followed by highly efficient upconversion in adiabatic nonlinear frequency-conversion process. Photon pairs are generated from 693 to 708 nm, and the complete conversion process occurs within a single monolithic 5-cm-long stoichiometric lithium tantalate nonlinear crystal. We have characterized the dependence of this structure with respect to pump intensity and crystal temperature.

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

We measure ensemble-averaged quantum correlations of path-entangled photons, propagating in a disordered lattice and undergoing Anderson localization. These result in intriguing patterns, which show that quantum interference leads to unexpected dependencies of the location of one particle on the location of the other. These correlations are shared between localized and nonlocalized components of the two-photon wave function, and, moreover, yield information regarding the nature of the disorder itself. Such effects cannot be reproduced with classical waves, and are undetectable without ensemble averaging.

Vibrational modes are often localized in certain regions of a molecule, and so the coupling between these modes is sensitive to the molecular structure. Two-dimensional vibrational spectroscopy can probe the strength of this coupling in a manner analogous to two-dimensional NMR spectroscopy, but on ultrafast timescales. Here, we demonstrate how two-dimensional Raman spectroscopy, based on fifth-order optical nonlinearity, can be performed with a single beam of shaped femtosecond optical pulses. Our spectroscopy scheme offers not only a major simplification of the conventional set-up, but also an inherent elimination of a competing nonlinear signal, which overwhelms the desired signal in other schemes and carries no coupling information.

The electric and magnetic components of an electromagnetic wave in free space are believed by many to be perpendicular to each other. We outline a procedure by which electromagnetic potentials are constructed, and we derive free-space nonperpendicular electric-magnetic fields from these potentials. We show, for example, that in free-space Bessel-related fields, at a small region near the origin, the angle between these components spans a range of 7 degrees-173 degrees,that is, they are far from being perpendicular. This can be contrasted with plane waves, where, following the same procedure, we verify that the electric field strength (E(x, y, z, t)) and the magnetic flux density (B(x, y, z, t)) are indeed perpendicular to each other and to the direction of propagation. (C) 2015 Optical Society of America

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

Diffraction-limited imaging through complex scattering media is a long-sought-after goal with important applications in biomedical research. In recent years, high-resolution wavefront shaping has emerged as a powerful approach to generate a sharp focus through highly scattering, visually opaque samples. However, it requires a localized feedback signal from the target point of interest, which necessitates an invasive procedure in all-optical techniques. Here, we show that by exploiting optical nonlinearities, a diffraction-limited focus can be formed inside or through a complex sample, even when the feedback signal is not localized. We prove our approach theoretically and numerically, and experimentally demonstrate it with a two-photon fluorescence signal through highly scattering biological samples. We use the formed focus to perform two-photon microscopy through highly scattering, visually opaque layers. (C) 2014 Optical Society of America

A new coherent anti-Stokes Raman spectroscopy (CARS) technique is reported for real-time detection and classification of several chemical constituents, utilizing a single detector and a single beam of shaped femtosecond pulses. The technique is based on rapidly switching between differently shaped pulses that either maximize or minimize the targeted vibrational lines excitation, thus creating temporally modulated 'bright' and 'dark' profiles in the total CARS signal that are measured by a single photomultiplier tube and demodulated by a multi-channel lock-in amplifier. Using a two-dimensional spatial light modulator displaying 24 different pulse shapes, we demonstrate pulse shaping at 80 kHz and chemically specific microscopy with pixel dwell times of less than 0.5 ms.

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

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

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

We present a method to control photodissociation by manipulating the bond-softening mechanism occurring in strong shaped laser fields, namely by varying the chirp sign and magnitude of an ultrashort laser pulse. Manipulation of bond softening is experimentally demonstrated for strong-field (1012-1013 W/cm2) photodissociation of H2+, exhibiting a substantial increase of dissociation by positively chirped pulses with respect to both negatively chirped and transform-limited pulses. The measured kinetic energy release and angular distributions are used to quantify the degree of dissociation control. The control mechanism is attributed to the interplay of dynamic alignment and chirped light induced potential curves.

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

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

We experimentally show that two-photon path-entangled states can be coherently manipulated by multimode interference in multimode waveguides. By measuring the output two-photon spatial correlation function versus the phase of the input state, we show that multimode waveguides perform as nearly ideal multiport beam splitters at the quantum level, creating a large variety of entangled and separable multipath two-photon states.

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

A new concept for focusing light through a randomly disordered media is demonstrated. Results show how by placing the randomly scattering media directly into a laser cavity tight focusing is accomplished in less than 600ns.

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

We report the experimental findings of a systematic study of the effect of linear chirp on strong field photodissociation of H₂⁺. For vibrational levels around or above the one photon crossing, the effect manifests itself in terms of a shift in the kinetic energy release (KER) peaks. The peaks shift up for negative chirp whereas they shift down for positive chirp. The measurements are carried out by varying two of the three laser pulse characteristics, energy, pulse peak intensity and linear chirp, while keeping the third constant. The shifts in the KER peaks are found to be intensity dependent for a given value of chirp. However, in the last two cases (i.e., fixed pulsed energy and fixed pulse peak intensity), they are found to be independent of the chirp magnitude. The results are understood on the basis of saturation of photodissociation probabilities for these levels.

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

We demonstrate the acquisition of stimulated Raman scattering spectra with the use of a single femtosecond pulse. High-resolution vibrational spectra are obtained by shifting the phase of a narrow band of frequencies within the input pulse spectrum, using spectral shaping. The vibrational lines are resolved via amplitude features formed in the spectrum after interaction with the sample. Using this technique, low-frequency Raman lines (

We show that a periodic two-dimensional (2D) photonic lattice with Kerr nonlinearity exhibits a Berezinskii-Kosterlitz-Thouless (BKT) crossover associated with a vortex-unbinding transition. We find that averaging over random initial conditions is equivalent to Boltzmann thermal averaging with the discrete nonlinear Schrodinger Hamiltonian. By controlling the initial randomness we can continuously vary the effective temperature. Since this Hamiltonian is in the 2D XY universality class, a BKT transition ensues. We verify this prediction using experimentally accessible observables and find good agreement between theory and simulations. This opens the possibility of experimental access to interesting phase transitions known in condensed matter using nonlinear optics.

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

We present a simple and easily implementable scheme for multiplexed Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy and microscopy using a single femtosecond pulse, shaped with a narrow spectral notch. We show that a tunable spectral notch, shaped by a resonant photonic crystal slab, can serve as a narrowband, optimally time-delayed probe, resolving a broad vibrational spectrum with high spectral resolution in a single-shot measurement. Our single-source, single-beam scheme allows the simple transformation of any multiphoton microscope with adequate bandwidth into a nearly alignment-free CARS microscope. (C) 2010 Optical Society of America

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

Precision measurements can be brought to their ultimate limit by harnessing the principles of quantum mechanics. In optics, multiphoton entangled states, known as NOON states, can be used to obtain high-precision phase measurements, becoming more and more advantageous as the number of photons grows. We generated "high-NOON" states (N = 5) by multiphoton interference of quantum down-converted light with a classical coherent state in an approach that is inherently scalable. Super-resolving phase measurements with up to five entangled photons were produced with a visibility higher than that obtainable using classical light only.

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

A method for efficient frequency conversion of ultrafast pulses is demonstrated using an adiabatic aperiodically poled KTP crystal. We produce broadband blue pulses centered at 450 nm by upconverting 30 fs pulses in the near-IR.

We introduce an illumination configuration which is a spatiotemporal analog of a non-diffracting X-wave. By interfering multiple ultrashort converging plane waves, we generate a tight central spot at which a transform limited ultrashort pulse is formed. Outside this tight focus a spatiotemporal speckle field with longer duration and reduced peak power is created. We investigate this spatiotemporal X-wave configuration analytically, numerically, and experimentally demonstrate the effect using two photon excitation fluorescence.

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

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

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

We present a novel technique to achieve both high efficiency and broad bandwidth in SFG process using adiabatic conversion scheme, adapted from NMR and light-matter interaction. The robustness and tunability of the scheme are discussed.

Multiphoton excitation in Rubidium can be effectively controlled using simple pulse shaping parameters. Interplay between ionization and dynamic Stark shifts is revealed by mapping onto 2D landscapes using a recently developed spatio-temporal coherent control technique.

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

We provide an overview of recent experimental and theoretical developments in the area of optical discrete solitons. By nature, discrete solitons represent self-trapped wavepackets in nonlinear periodic structures and result from the interplay between lattice diffraction (or dispersion) and material nonlinearity. In optics, this class of self-localized states has been successfully observed in both one- and two-dimensional nonlinear waveguide arrays. In recent years such photonic lattices have been implemented or induced in a variety of material systems, including those with cubic (Kerr), quadratic, photorefractive, and liquid-crystal nonlinearities. In all cases the underlying periodicity or discreteness leads to altogether new families of optical solitons that have no counterpart whatsoever in continuous systems. We first review the linear properties of photonic lattices that are key in the understanding of discrete solitons. The physics and dynamics of the fundamental discrete and gap solitons are then analyzed along with those of many other exotic classes - e.g. twisted, vector and multi-band, cavity, spatio-temporal, random-phase, vortex, and non-local lattice solitons, just to mention a few. The possibility of all-optically routing optical discrete solitons in 2D and 3D periodic environments using soliton collisions is also presented. Finally, soliton formation in optical quasi-crystals and at the boundaries of waveguide array structures are discussed. (C) 2008 Elsevier B.V. All rights reserved.

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

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

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

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

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

The spectral polarization of an ultrashort pulse is fully controlled by a novel pulse shaper design, including three liquid-crystal spatial light modulator arrays at three different orientations. The added degree of controllability permits generation of previously unattainable pulse shapes, with possible applications in multi-dimensional spectroscopy, coherent control, and ultrafast polarization gating.

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

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

Using a single shaped femtosecond pulse, a molecular wavepacket is both tailored and monitored. Several toluene vibrational levels are excited in a controlled manner, both in amplitude and phase, through a quantum coherently controlled non-resonant Raman process. The entire coherent anti-Stokes Raman spectroscopy (CARS) process, that includes excitation and probing of the Raman-induced wavepacket, is enabled by polarization and phase shaping. We achieve a wavepacket detection sensitive to the modes relative phase and further extend the capabilities of the single-pulse CARS technique.

We report high-rate, computer-controlled femtosecond pulse shaping by use of an electro-optical gallium arsenide optical phased-array modulator with 2304 controlled waveguides. It provides fast modulation speed of both spectral phases and amplitudes. Limited by the driving electronics of our current setup, we were able to update a pulse shape in similar to 30 ns. This technique paves the way toward individual shaping of every single pulse of typical femtosecond mode-locked oscillators. (c) 2005 Optical Society of America

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

We investigate experimentally and numerically the interaction of a highly localized, single-channel discrete soliton (blocker) with a wide, tilted beam in a one-dimensional AlGaAs array. In agreement with theory the blocker is observed to discretely shift its position by multiple channels, depending on the intensity and relative phase of the tilted beam. (c) 2005 Optical Society of America.

Coherent-control schemes to manipulate weak-field interactions are generally invalid at stronger fields, since strong-field interactions are accompanied by level power broadenings and level shifts that usually elude simple analytical treatments. Here we show that a broad subgroup of weak-field solutions (those with real fields, i.e., fields with only one quadrature in the complex plane) can be extended to the strong-field regime while retaining their properties. The salient feature of these fields is a symmetry that cancels out power broadening effects. Such fields can be generated from ultrashort coherent pulses or from incoherent broadband down-converted light. Weak-field coherent-control approaches based on these solutions can therefore be extended to the strong-field regime as we demonstrate in a two-photon absorption experiment in atomic cesium.

The ability to perform optical sectioning is one of the great advantages of laser-scanning microscopy. This introduces, however, a number of difficulties due to the scanning process, such as lower frame rates due to the serial acquisition process. Here we show that by introducing spatiotemporal pulse shaping techniques to multiphoton microscopy it is possible to obtain full-frame depth resolved imaging completely without scanning. Our method relies on temporal focusing of the illumination pulse. The pulsed excitation field is compressed as it propagates through the sample, reaching its shortest duration at the focal plane, before stretching again beyond it. This method is applied to obtain depth-resolved two-photon excitation fluorescence (TPEF) images of drosophila egg-chambers with nearly 10⁵ effective pixels using a standard Ti:Sapphire laser oscillator.

We experimentally demonstrate sum-frequency generation with entangled photon pairs, generating as many as 40000 photons per second, visible even to the naked eye. The nonclassical nature of the interaction is exhibited by a linear intensity dependence of the nonlinear process. The key element in our scheme is the generation of an ultrahigh flux of entangled photons while maintaining their nonclassical properties. This is made possible by generating the down-converted photons as broadband as possible, orders of magnitude wider than the pump. This approach can be applied to other nonlinear interactions, and may become useful for various quantum-measurement tasks.

The ability to perform optical sectioning is one of the great advantages of laser-scanning microscopy. This introduces, however, a number of difficulties due to the scanning process, such as lower frame rates due to the serial acquisition process. Here we show that by introducing spatiotemporal pulse shaping techniques to multiphoton microscopy it is possible to obtain full-frame depth resolved imaging completely without scanning. Our method relies on temporal focusing of the illumination pulse. The pulsed excitation field is compressed as it propagates through the sample, reaching its shortest duration at the focal plane, before stretching again beyond it. Combining temporal focusing with line-scanning microscopy results in an enhanced depth resolution, equivalent to that achieved by point scanning. Both the scanningless and the line-scanning techniques are applied to obtain depth-resolved two-photon excitation fluorescence (TPEF) images of drosophila egg-chambers.

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

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

Third harmonic generation microscopy is shown to be a robust method for obtaining structural information on a variety of biological specimens. Its nature allows depth-resolved imaging of inhomogeneities with virtually no background from surrounding homogeneous media. With an appropriate illumination geometry, third harmonic generation microscopy is shown to be particularly suitable for imaging of biogenic crystals, enabling extraction of the crystal orientation.

A novel approach for an optical direct-sequence spread spectrum is presented. It is based on the complementary processes of broad-band parametric down-conversion and up-conversion. With parametric down-conversion, a narrow-band continuous-wave (CW) optical field is transformed into two CW broad-band white-noise fields that are complex conjugates of each other. These noise fields are exploited as the key and conjugate key in optical direct-sequence spread spectrum. The inverse process of parametric up-conversion is then used for multiplying the key by the conjugate key at the receiver in order to extract the transmitted data. A complete scheme for optical code-division multiple access (OCDMA) based on this approach is presented. The salient feature of the approach presented in this paper is that an ideal white-noise key is automatically generated, leading to high-capacity versatile code-division multiple-access configurations.

We demonstrate a new scanning femtosecond pulse-shaping technique that allows pulse shapes to be modulated at kilohertz rates. This technique is particularly useful for lock-in measurements in which the signal is synchronized with the alternating pulse shapes. The pulse-shape lock-in technique is demonstrated in resonant coherent anti-Stokes Raman scattering, where it is shown to significantly improve the ratio of the resonant signal to both the nonresonant background and to noise.

The polarization pulse shaping was applied to the control of two-photon absorption in atomic rubidium. It was shown that the technique enables the manipulation of the transient vector properties of a light matter interaction. It was established that control can be exerted on the angular distribution of the final state. The ability to control the final state population of a nearly degenerate system and to perform M-state resolved spectroscopy were demonstrated.

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

Light propagating in linear and nonlinear waveguide lattices exhibits behaviour characteristic of that encountered in discrete systems. The diffraction properties of these systems can be engineered, which opens up new possibilities for controlling the flow of light that would have been otherwise impossible in the bulk: these effects can be exploited to achieve diffraction- free propagation and minimize the power requirements for nonlinear processes. In two-dimensional networks of waveguides, self-localized states-or discrete solitons-can travel along 'wire-like' paths and can be routed to any destination port. Such possibilities may be useful for photonic switching architectures.

A novel method for detection of noble-metal nanoparticles by their nonlinear optical properties is presented and applied for specific labeling of cellular organelles. When illuminated by laser light in resonance with their plasmon frequency these nanoparticles generate an enhanced multiphoton signal. This enhanced signal is measured to obtain a depth-resolved image in a laser scanning microscope setup. Plasmon-resonance images of both live and fixed cells, showing specific labeling of cellular organelles and membranes, either by two-photon autofluorescence or by third-harmonic generation, are presented.

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

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

Quantum control of coherent anti-Stokes Raman spectroscopy (CARS) was demonstrated. Raman spectra was measured with a resolution of the order of 5 cm^-1 using shaped pulses with a bandwidth of about 120 cm^-1. Nearly complete suppression of the nonresonant background signal was also demonstrated. Selective population of only one of the two Raman levels of pyridine, lying within the bandwidth of excitation pulses, was achieved. Simple modeling was used to derive the required spectral phases and the measured signals were found in agreement with the model predictions.

A narrow-band coherent anti-Stokes Raman spectroscopy (CARS) signal was generated from a broad-band probe pulse through simple spectral phase manipulation of the probe pulse. The spectral resolution increased by an order of magnitude. The signal at a given wavelength was enhanced by almost a factor of 2 relative to the maximal signal obtained at this frequency by a transform limited pulse.

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

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

Adaptive self-learning optical systems for ultrashort pulse compression and shaping are presented. In these systems, the input pulses are modified iteratively by updatable femtosecond pulse shapers, according to feedback signals derived from measurements of the output pulses, to approach desired waveforms. These adaptive techniques remove many of the difficulties associated with femtosecond pulse manipulations, such as the need for characterization of the input pulses, and optimization of the spectral filters of ultrashort pulse shapers in the presence of noise and uncertainty resulting from experimental constraints. The potential of the adaptive self-learning approach is demonstrated experimentally by compressing uncharacterized 80 fs long pulses of a mode-locked Ti:Sapphire laser down to 11 fs, and by adaptively shaping uncharacterized input pulses into desired temporal intensity distributions. These adaptive techniques for ultrashort pulse manipulations can be extended towards quantum coherent control, where quantum systems can be adaptively steered towards desired final quantum states.

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

Multiphoton transitions can be reached by many routes through a continuum of virtual levels. We show that the transition probability of two-photon and multiphoton processes can be controlled by tailoring the shape of an ultrashort excitation pulse, so that the many paths leading to the final state interfere to give a desired probability amplitude. We analyze the effect of pulse shapes on N-photon absorption as well as on Raman transitions. We show theoretically that certain tailored dark pulses do not excite the system at all, while other shaped pulses induce transitions as effectively as transform limited pulses, even when their peak amplitudes are greatly reduced. These results are confirmed experimentally for two-photon transitions in atomic cesium. [S1050-2947(99)00808-2].

Third-harmonic microscopy is shown to be a powerful tool for the study of liquid crystal structures. The third-harmonic probe enables the investigation of molecular order well inside a liquid crystal cell. The observation of phase transition in various nematic liquid crystal samples is presented as a demonstration for the power of this technique. For example, nematic and isotropic regions are shown to coexist across the depth of the cell. Thermal fluctuations are observed in the nematic regions close to the isotropic-nematic transition temperature. (C) 1999 American Institute of Physics. [S0003-6951(99)04321-1].

Noiselike generation in lasers can be controlled by changing the cavity in order to obtain pulses with unique properties: Intense noiselike pulses, as narrow as a few picoseconds, were obtained, These are two orders of magnitude narrower than pulses obtained in previous work, In long cavities, coherent and incoherent two-color noiselike generation were demonstrated. The wavelength difference between the generated pulses could be tuned in a wide wavelength range, which is much broader than the amplifier bandwidth. High-energy approximate to 16-nJ noiselike pulses with a broad-band spectrum and narrow intensity autocorrelation trace were also demonstrated.

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

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

Real-time linear spectral interference measurements of ultrashort pulses are shown experimentally. The technique involves measurements of the two-dimensional interference pattern of the spectral interference between a reference and a signal pulse propagating at an angle with respect to each other. No postprocessing is needed to extract the spectral phase difference between the two pulses. Quadratic spectral phase distortions as well as spectral phase discontinuities are measured. The method is applicable to single-shot measurements of ultraweak pulses and is useful for identification of the critical adjustments of ultrashort pulse shapers and compressors. (C) 1997 Optical Society of America.

A practical adaptive approach for ultrashort optical pulse compression and shaping is nummerically investigated. A simple single valued feedback measurement was suggested and proven useful for compression. The method is robust, and capable of handling the general case of pulse compression and shaping, where the input pulses are completely uncharacterized or partially characterized.

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

The characteristics and limits of parallel transmission of Two-Dimensional imagery through single optical fibers are reviewed, and methods for overcoming the inherent smearing that occurs at the output are discussed. These solutions indude: a) control of the fiber's refractive index profile; b) incorporation of complex filters; and c) encoding the information at the input and decoding the output after transmission.

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