Showing papers by "Amnon Yariv published in 1994"

Steady-state spatial screening solitons in photorefractive materials with external applied field.

Abstract: Steady-state dark (bright) planar spatial solitons are predicted for photorefractive materials when the diffraction of an optical beam is exactly compensated by nonlinear self-defocusing (focusing), due to the screening field set up around a dark notch (or a bright beam) in a photorefractive material to which an external field is applied. These screening solitons appear in steady state and differ from previously observed spatial solitons in their properties and physical origin.

Dark and bright photovoltaic spatial solitons

Abstract: Dark (bright) planar spatial solitons are predicted for photovoltaic photorefractive materials when the diffraction of an optical beam is exactly compensated by nonlinear self-defocusing (focusing) due to the photovoltaic field and electro-optic effect. These solitons may have steady-state irradiances of microwatts to milliwatts per square centimeter and widths as small as 10 [mu]m in lithium niobate. Optical control is provided by incoherent illumination, and the nonlinear index of a dark soliton may be used to trap a bright soliton by rotating the plane of polarization of the soliton field.

Dimensionality and size of photorefractive spatial solitons

Abstract: We study experimentally self-trapping of optical beams in photorefractive media and show that the trapping is inherently asymmetric with respect to the two (transverse) trapping dimensions. We also present experimental results that show how the sizes of the resultant photorefractive spatial solitons are independent (within their range of existence) of the amplitude of the externally applied electric field used to generate them.

Synthesis of an arbitrary axial field profile by computer-generated holograms.

Abstract: Computer-generated holograms are employed to design any desired intensity distribution along the propagation axis for a finite specified distance

Stability of photorefractive spatial solitons.

Abstract: We present a theoretical analysis of the stability of photorefractive spatial solitons along with experimental results that show that the solitons are stable for small-scale perturbations but break down when the perturbations exhibit a transverse scale comparable with the soliton size (cross section).

Tunable quasi‐phase matching using dynamic ferroelectric domain gratings induced by photorefractive space‐charge fields

Abstract: We demonstrate a method of dynamic, tunable quasi‐phase matched second‐harmonic generation using optically induced polarization gratings with periods equal to twice the coherence length. These gratings increase the peak second‐harmonic conversion efficiency by a factor of 17 above a poled strontium barium niobate crystal, to 0.01% for fundamental beam intensities of 0.8 MW cm−2. We generate quasi‐phase matching spectral response peaks as narrow as 0.175 nm and tailor the response by writing an ensemble of gratings in the same volume, each of which enhances the second‐harmonic generation at a predetermined wavelength.

Amplitude noise reduction in semiconductor lasers with weak, dispersive optical feedback.

Abstract: We present the theory and measurements of the amplitude noise spectrum from a semiconductor laser with weak optical feedback (Pfb/Pout ~10^-6) from an external cavity containing an element of dispersive loss. The laser noise is found to be reduced over most of the low-frequency spectrum, although an increase in the noise is observed at frequencies corresponding to multiples of the external-cavity free spectral range. The low-frequency noise reduction closely follows theoretical predictions, and a reduction of as much as 7 dB is measured at an injection current of 1.5 times the threshold current. The potential of this method for contributing to the production of amplitude-squeezed light is discussed.

Light-induced absorption in photorefractive strontium barium niobate.

Abstract: Sr(0.59)Ba(0.41)Nb(2)O(6) and Sr(0.75)Ba(0.25)Nb(2)O(6) exhibit an intensity-dependent absorption in the visible spectral range. We perform pump (green)-probe (red) measurements and find variation as high as 0.45 cm(-1) in the absorption coefficient, which depends significantly on the polarization of the probe light beam. Our results indicate a nonexponential dark decay time of the induced absorption and suggest that the trapping potential of the shallow traps varies significantly from one trap to another (multiple isolated shallow traps).

Conjugation fidelity and reflectivity in photorefractive double phase-conjugate mirrors.

Abstract: We present an experimental study of the behavior of photorefractive double phase-conjugate mirrors that illustrates recent theoretical predictions. We observe a sharp fidelity threshold that significantly depends on the specific feature size in the input beams. Furthermore, we find that if the two input beams have unequal intensities the conjugation process is asymmetric and the steady-state fidelity is better on the side of the crystal on which the more-intense beam enters.

Double phase conjugation

Abstract: We model the double phase-conjugate mirror (DPCM) as a function of time, the average direction of propagation of the two beams forming the DPCM, and one transverse coordinate. Calculations show that the conjugation fidelity and reflectivity have different dependencies on the photorefractive coupling coefficient times length; the fidelity turns on abruptly with a threshold, whereas the reflectivity increases smoothly. The DPCM behaves as an oscillator at and above threshold: the time required for the reflectivity to reach the steady state dramatically slows down near threshold (like critical slowing down in lasers); above threshold the DPCM is self-sustaining even if the random noise terms used to start the process are set to zero. A decrease in the noise level improves the fidelity but increases the response time. The use of unbalanced input beam ratios results in asymmetric conjugation such that the fidelity obtained on the side of the weaker input beam is significantly reduced. The slowing down diminishes with increasing noise level or unbalanced input intensities.

Spectral response of fixed photorefractive grating interference filters

Abstract: We report a theoretical investigation of the frequency response of optical interference filters written in photorefractive materials. Counterpropagating coherent beams interact in the volume of a photorefractive crystal through two-beam coupling. The resulting hologram is fixed. The reflectivity of the hologram is calculated as a function of frequency. An analytic solution is obtained for arbitrary grating phase φ in the lossless case, α = 0. Numerical solutions are performed for α > 0. Experimental results are compared favorably with the theory.

Second-harmonic generation in absorptive media.

Abstract: The solution of the coupled-wave equations for second-harmonic generation in a near-resonant three-level system is extended to include absorption. It is shown, within second-order perturbation theory, that double resonance is the optimal conversion condition, despite absorption enhancement. We extend the solution numerically, using nonperturbative susceptibilities derived within the rotating-wave approximation, to saturating intensities and discuss the modifications to the perturbative conclusions as well as the regimes of validity for the various approximations.

Intersubband-transition-induced phase matching.

Abstract: We suggest the use of the refractive-index changes associated with the intersubband transitions in quantum wells for phase matching in nonlinear materials. An improvement in the conversion efficiency of mid-IR second-harmonic generation by almost 2 orders of magnitude over non-phase-matched bulk GaAs is predicted. We also show that the linear phase contributions of intersubband transitions used for resonant enhancement of second-harmonic generation must be considered, as they could limit the conversion efficiency by increasing the phase mismatch on one hand or offset the bulk's dispersion and lead to phase matching on the other.

Imaging of coherent fields through lenslike systems.

Abstract: I derive the imaging condition for the complex amplitude of a monochromatic field by a sequence of lenslike elements.

Symmetry-breaking effects induced by intense laser fields.

Abstract: We consider the interaction of an intense laser field with an ensemble of electrons confined by a symmetric potential well, and show that, for times shorter than the dephasing time, the laser field can break the symmetry of the system regardless of its initial conditions, resulting in optical rectification and even-harmonic generation. In particular, we show that relatively moderate fields can give rise to significant even-harmonic generation, even if the system is initially in thermal equilibrium, with no need for pulse shaping, suggesting that the detection of symmetry-breaking effects may be experimentally accessible.

Two-section gain-levered tunable distributed feedback laser with active tuning section

Abstract: A composite cavity semiconductor laser structure that consists of two active (gain) sections is analyzed. One of the sections has a grating corrugation, while the other is flat. The operation of the device is enhanced by utilizing the gain-lever effect. A round-trip analysis is combined with the rate equations pertinent for this structure to study the characteristics of the device. The performance of the device is compared to that of a similar case without the gain lever. It is shown that, utilizing the gain-lever effect, improved operation in terms of tuning range, modulation efficiency, and output power control can be obtained. >

Light-induced absorption in photorefractive strontium-barium niobate

Abstract: Photorefractive (PR) strontium barium niobate (SBN) has been the subject of extensive research in recent years. Large PR nonlinearities and relatively fast response time at low power levels make this material a preferred candidate for a variety of applications such as optical data storage. Intensity dependent absorption and related phenomena were discovered and investigated extensively in different PR crystals and at different optical light intensities spanning the range of 1 mW/cm/sup 2/ to 10/sup 4/ W/cm/sup 2/. In this study we investigate the light-induced absorption through pump-probe measurement at relatively low optical power densities in photorefractive SBN:60 and SBN:75. >

Optically induced ferroelectric domain gratings in SBN: theory and applications to quasi-phase matching and optical data storage

Abstract: Illuminating a photoconducting strontium barium niobate crystal with a low intensity optical standing wave generates dynamic as well as remnant polarization gratings. We study the kinetics of these gratings and present a thermodynamic model for the grating formation. Fundamentally, light induced space charge fields dynamically modify the positions of ions within the unit cell and tailor the electronic, optical, and acoustic properties of the material.

Photorefractive spatial solitons—theory and experiments

Abstract: The existence of photorefractive (PR) spatial solitons has been predicted by us some two years ago.1,2 The self-trapping effects occur when diffraction is exactly balanced by self-scattering (two-wave mixing) of the spatial (plane wave) components of the soliton beam. These photorefractive spatial solitons were observed by us recently.3

Photorefractive self-focusing and defocusing as an optical limiter

Abstract: Focusing and defocusing of laser light has been observed for many years. Kerr type materials exhibit this effect but only for high intensities. We show experimental evidence that photorefractive materials can also produce dramatic focusing and defocusing. Whereas Kerr materials produce this effect for high intensities, photorefractive materials produce these effects independent of intensity indicating that this effect would be ideal for an optical limiter. We compare the characteristics of Kerr and photorefractive materials, discuss the physical models for both materials and present experimental evidence for photorefractive defocusing. Self-focusing and defocusing was observed for any incident polarization although the effect was more pronounced using extraordinary polarized light. In addition, self-focusing or defocusing could be observed depending on the direction of the applied electric field. When the applied field was in the same direction as the crystal spontaneous polarization, focusing was observed. When the applied field was opposite the material spontaneous polarization, the incident laser light was dramatically defocused.

Effects of optical feedback with dispersive loss on the amplitude noise and relaxation resonance of semiconductor lasers

Abstract: Optical feedback from an external cavity containing an element of dispersive loss was used to reduce the amplitude noise of a semiconductor laser. At feedback levels of P_(fb)/P_(out) ≈ 10^(-2)), a maximum amplitude noise reduction of 16 decibel was measured close to threshold but the potiential for reduction was reduced considerably at higher injection currents as the laser noise approached the shot noise limit. In addition, the threshold current decreased and the linewidth was reduced to 10 kHz. The relaxation oscillation peak in the amplitude noise spectrum was also found to be dramatically suppressed and we find evidence that the relaxation resonance can be moved to much higher frequencies using optical feedback techniques.

Optically induced dynamic polarization gratings for tunable, quasi-phase matched second harmonic generation

Abstract: Light interference patterns can photoexcite mobile charge and generate periodic space charge fields with periods of typically 01 to 100 microns in photorefractive crystals The space charge field attains values of 01 to 1 kV cm/sup -1/ in ferroelectrics such as Sr/sub 075/Ba/sub 025/Nb/sub 2/O/sub 6/ (SBN:75) A fundamental question is whether these periodic space charge fields can spatially modulate the ferroelectric polarization in materials which possess a coercive field of the order of the space charge field We present experimental evidence demonstrating that this is indeed the case in SBN:75 We observe dynamic domain gratings, which form simultaneously with the build up of the space charge field, and remnant domain gratings, which persist long after the space charge field disappears >

Vertical Cavity Surface Emitting Lasers with Dielectric Mirrors

Abstract: We have developed new low threshold surface emitting laser designs with dielectric high reflectivity top mirrors. Here, we describe the characteristics of these surface emitting vertical cavity lasers (VCSELs) which exhibit stable mode patterns and low threshold currents. The new device fabrication sequence which we employ is able to adjust the emission wavelength of the lasers during the final fabrication step and allow the development of stable multi-wavelength laser arrays. These quantum-well based laser diodes are demonstrated at 0.72 μm with threshold currents of 20 mA, at 0.85 μm with threshold currents of 3 mA, at 0.98 μm with threshold currents of 4 mA, and at 1.55 μm with threshold currents of 17 mA. Our VCSELs also display remarkably low threshold voltages, thus minimizing the laser power dissipation and improving the wallplug efficiency. The flexibility resulting from depositing one or both of the mirrors after the fabrication of the laser diodes opens the way to the development of new and more versatile laser structures.

Fidelity-threshold and critical slowing down in photorefractive double phase conjugate mirrors

Abstract: Summary form only given. In conclusion, we present theoretical and experimental results that analyze the double phase conjugation process. We establish the existence of gain-threshold and point out the differences between conjugation fidelity and conjugation reflectivity. We study the oscillatory nature of the DPCM and predict the existence of critical slowing down near the gain-threshold. >

Photorefractive spatial solitons-theory and experiments

Abstract: The existence of photorefractive (PR) spatial solitons has been predicted by us some two years ago/sup (1,2)/. The self-trapping effects occur when diffraction is exactly balanced by self-scattering (two-wave mixing) of the spatial (plane wave) components of the soliton beam. >

Multi-Quantum Well Integrated Stacks for Detection in the Mid-Infrared

Abstract: The development and improvement of advanced epitaxial crystal growth techniques such as molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) during the last two decades, has opened the door for the realization of devices in the quantum size regime. Quantum size phenomena can be observed when the experimental dimensions approach the order of magnitude of the DeBroglie wavelength associated with the system under investigation. The tools needed to understand and design artificial semiconductor structures, are known under the label “bandgap engineering.”

Multi λ Controlled Operation of Quantum Well IR Detectors Using Electric Field Switching and Rearrangement

Abstract: We describe a new mode of operating quantum well infrared photodetectors which is based on electric field domain formation and readjustment. The domain readjustment and electric field spatial redistribution results from an interplay between the fixed applied voltage and the quantum tunneling physics. The paper will describe the relevant theoretical background and present supporting data. Also described will be the application of domain switching to attain multi λ response of such detectors.