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Two-wave mixing adaptive interferometer based on a liquid crystal light valve with a semiconductor GaAs substrate is realized and studied at 1064 nm wavelength. The local response of the dynamic hologram recorded in the liquid crystal layer of the light valve allows for detection of small phase modulations of the object beam. The characteristics of the interferometer are estimated experimentally. The temporal adaptability lies in the subsecond range. The large optical nonlinearity of the cell is favorable for measurements of small displacements with high sensitivity.

Recording of dynamic holograms in the near infrared spectral range is reported for a liquid crystal light valve with a GaAs semi-insulating substrate. The interaction of weak signal and strong pump waves are studied. It is demonstrated that an out-of-phase local dynamic grating is recorded. Optimal amplitude and frequency of the applied voltage are found for maximum amplification of the weak beam. Step-like external phase modulation of the signal wave with amplitude π/2 is used to change the type of response from local to nonlocal and to achieve in such a way additional transient signal beam amplification. Study of the cells with different thicknesses of the liquid crystal layer at different grating spacings allows a seventeen-fold gain of the weak beam in the device with the thickest liquid crystal layer of 16 μm at a grating period of 1000 μm (the largest possible in our set-up). The amplitude of the refractive index modulation and nonlinear coupling constant n2 are estimated from the experimental results.

Two-wave mixing adaptive interferometer based on photorefractive crystal allows for compensation of temporal disturbances in ambient environment and operation with speckled beams. The crystal should exhibit large effective trap density, low dark conductivity and large photoconductivity. Deliberately doped semiconductor may meet these requirements. In the present work the photorefractive, spectroscopic and magneto-optical study of CdTe:Sn is performed aiming to estimate these characteristics and to describe the space-charge formation. The photon energies for optical ionization/neutralization of the tin ions are estimated. The crystal is characterized as a medium for two-wave mixing adaptive interferometer with excellent performance.

Liquid crystal light valve with GaAs substrate operating in the transmission mode in the infrared is studied. The nonlinear phase shift of the transmitted light wave is measured as a function of applied voltage. The dynamic grating recording is achieved. A fourfold amplification of the weak signal beam is reached. The gain is increased by means of proper tilting of the cell that increases an effective pretilt of the liquid crystal molecules. The amplitude of the refractive index modulation and nonlinear coupling constant are estimated from the experimental results.

The nonlinear phase shift per unit length acquired by the transmitted and phase conjugate waves in backward-wave four-wave mixing may be considered as an effective refractive index. Consequently, a narrow spectrum of the nonlinear phase shift in non-degenerate frequency interaction corresponds to natural dispersion and such interaction may be used for temporal manipulation of light pulses. Analysis of the narrow spectra of the nonlinear phase shift shows high potential of the backward-wave four-wave for slowing down of light pulses. The slowing down is achieved using backward-wave four-wave mixing in media with local and nonlocal response. The delay and shape transformation of output pulses are studied and compared for the transmitted and phase conjugate channels. It is shown that the phase conjugate pulse achieves a longer delay under typical experimental conditions.

A liquid crystal light valve (LCLV) is an optically controlled spatial light modulator that allows recording of dynamic holograms. Almost all known LCLVs operate in the visible range of the spectrum. In the present work we demonstrate a LCLV operating in the infrared. The interaction of signal and pump waves is studied for different applied voltages, grating spacings, and intensities of the recording beams. A fourfold amplification of the weak signal beam is achieved. The amplitude of the refractive index modulation Δn=0.007 and nonlinear coupling constant n2=−1 cm2/W are estimated from the experimental results. External phase modulation of one of the recording beams is used for a further transient increase of the signal beam gain.

Photorefractive properties of tin-doped CdTe crystals are studied. The material demonstrates sensitivity for low-intensity recording. A one-center model of the space-charge formation perfectly describes the experimental results. The tin impurity center is characterized using the data of optical absorption and light-induced absorption spectroscopy, pseudo-3D mapping of the photoabsorption, magnetic circular dichroism, and optical detection of magnetic resonance. Spectroscopic studies confirm appropriateness of the one-center model for describing the space-charge formation with tin as a main photorefractive center. Optical ionization (neutralization) energies are estimated for phototransitions from Sn+ (1.14 eV) and Sn0 (1.09 eV). Possible ways for improvement of the photorefractive performances are discussed.

We demonstrate experimentally the operation of an adaptive two-wave mixing interferometer with no field applied to a photorefractive crystal in which the quadrature condition is introduced by an external π/2 phase step. To achieve high signal to noise ratio the counter-propagating geometry for grating recording is used with a CdTe:Ge crystal, ensuring effective recording of reflection holograms. It is demonstrated that the interferometer with a photorefractive semiconductor crystal provides good sensitivity without the adverse effects that are normally associated with an applied field.

We investigate the physics of a novel nonlinear system, feedback controlled photorefractive beam coupling. Inertia of the electronic feedback is found to be an element crucial for permanent operation of this system. Theoretically and experimentally we have found a wealth of periodic and quasi-periodic regimes for observable characteristics of the feedback controlled wave coupling. A good qualitative agreement between theory and experiment is obtained for LiNbO 3 crystals.

We report on the photoabsorption, magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) of Ge-doped CdTe. Strong photoinduced absorption changes have been detected for two samples at ambient and low temperatures (down to 4.3 K). They reveal the existence of four absorption bands around 0.94, 1.10, 1.22 and 1.36 eV at 4.3 K. MCD experiments at 1.4 K allowed the tagging of different paramagnetic defects to the two bands at lower energies. EPR demonstrated that one of them is Ge+Cd. We propose that the second one is an isolated or Ge-related A-centre. A diagram showing the various transport processes is given.

A photorefractive CdTe:Ge single crystal was investigated via photo-absorption at room temperature, 98 K and 4.3 K, and magnetic circular dichroism at 1.4 K. At all wavelengths, except near 1064 nm, absorption spectra suffer relatively large changes under various illuminations. Magnetic circular dichroism demonstrates unambiguously the presence of two paramagnetic defects, one of them being the ionised donor Ge + at the Cd site. Our results further reveal the existence of four absorption components around 0.94 eV, 1.10 eV, 1.22 eV and 1.36 eV. The two extreme ones are assigned to the photoneutralisation of Ge Cd + and to the photoionisation of Ge Cd 0 respectively.

A liquid crystal light valve is an optically controlled spatial light modulator made as a cell with liquid crystal on a photoconductive substrate. Dynamic holograms recorded in such a hybrid device may give rise to the amplification of one of the recording beams at the expense of the other. In the present work, liquid crystal light valves with different thicknesses of the liquid crystal layer are studied at various conditions of the dynamic grating recording aiming to maximize the signal beam amplification in the infrared. A seventeen-fold gain is achieved in the improved cell for optimal recording conditions.

Recording of efficient reflection holograms is achieved in CdTe:Ge at lambda=1.064 microm and lambda=1.55 microm. The gain factor measured at both wavelengths for counterpropagating two-beam coupling considerably exceeds the absorption constant and transcends all values previously reported for semiconductors with no external field. The dependences of the gain factor on intensity and grating spacing are studied. Some crystal characteristics are estimated in the frame of a single band, one mobile species approximation of space-charge formation. The homogeneity of photorefractive properties in the crystal volume is demonstrated.

The slowing down of light pulses is achieved using backward-wave four-wave mixing in a medium with local response. A Bi12TiO20 crystal with an external dc field is used in the experiment as a proof-of-concept material. The delay and shape transformation of output pulses are studied and compared for the transmitted and phase conjugate channels. It is shown that the phase conjugate pulse achieves a longer delay under typical experimental conditions with equal intensities of the pump beams. This advantage of the phase conjugate beam is especially pronounced for short pulses with half-widths smaller than the response time of the medium. The agreement of the experimental results with numerical calculations confirms the validity of the previously developed theoretical model. Analytical solutions are derived for input pulses with triangular shapes.

Vanadium and Germanium are known to induce photorefraction in cadmium telluride [1,2]. Although the understanding of V-doping is well advanced at present, relatively little is known about Ge-doping apart from the fact that a GeCd3+ (ionic notation) defect has been identified by electron paramagnetic resonance [3,4]. We have measured the photoinduced changes of the absorption as well as the Magnetic Circular Dichroism (MCD) of two CdTe: Ge crystals with the hope to further identify the photorefractive centers and processes.

A considerable deceleration of light pulses in crystals with nearly compensated space-charge field proves that the strong dispersion of dynamic photorefractive gratings in the vicinity of Bragg resonance is of primary importance for light slowing down.

Summary form only given. CdTe:Ge ensures the largest two-beam coupling gain factor among all photorefractive semiconductors with no electric field. At the same time a large scatter of data is observed even for the samples cut from the same ingot and the ultimate values of the diffraction efficiency and gain factor are still smaller than the predicted by the theory.

Tin is shown to be a suitable dopant that makes it possible to grow cadmium telluride crystals with considerably reduced conductivity (semi-insulating material) and ensures a well pronounced photorefractive response both for cw and Q- switched Nd3+:YAG laser radiation.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We investigate theoretically and experimentally the operation of a nonlinear system, feedback controlled photorefractive beam coupling. It is shown that the feedback equations used earlier ( 6p/2 phase shift between the diffracted and transmitted beam components! are satisfactory only during a relatively short initial stage of development. We have generalized these equations by taking into account the inertia inherent in any real feedback loop. The inertial feedback equations ensure the permanent operation of the nonlinear system and lead to a variety of periodic and quasiperiodic regimes. We investigate the distinctive features of these regimes and transitions between them for moderately thick crystals. Good qualitative agreement between theory and experimental data is obtained for LiNbO3 crystals.

Two qualitatively different modes of operation are possible when the active feedback is used to optimize the grating recording in photorefractive crystals with local response. For light-induced changes of the sample optical thickness much less than optical wavelength, the feedback introduces a linear in time phase shift into one of the recording beams, i.e., leads to a non-degeneracy of two-wave mixing. For large light-induced changes the periodic in time phase variation is superimposed on the linear growth. The transition from one mode of operation to the other is shown to be similar to phase transitions, with a critical slowing down of the periodic phase variations.

The charge transfer processes in photorefractive CdTe:Ge were modeled using the data of optical absorption, magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopies. Within the developed model the variations in the photorefractive properties of different CdTe:Ge samples are explained by differences in the relative concentrations of donor and trap centers. The existence of two different centers of comparable concentrations, each in two charge states, allows charge redistribution between them and gives rise to optical sensitization of some CdTe:Ge samples for photorefractive recording under an auxiliary illumination. In the present article we follow the proposal of pseudo-3D presentation of light-induced absorption to distinguish the main charge transfer processes at different excitation energies and explain the sensitization of CdTe:Ge for photorefractive recording at 1.06, 1.32 and 1.55 μm by light with appropriate wavelength.

Analytically and numerically we analyze a novel nonlinear system, feedback-controlled photorefractive beam coupling. This system, being experimentally realized, keeps diffractivity of the recorded photorefractive grating near 1 or 0 within a wide range of crystal thickness and pump ratio. Inertia of the electronic feedback loop is shown to be a crucial element of the permanent operation. Typically, the feedback system has no steady state. We have found a wealth of periodic and quasi-periodic regimes (and transitions between them) for observable characteristics of the feedback-controlled beam coupling. In many respects the found behaviour is new for nonlinear systems. A good qualitative agreement between theory and experiment is obtained for LiNbO3 crystals.

Two-beam coupling gain spectra with the deep at zero detuning frequency are observed in germanium doped cadmium telluride crystal. Experimental data manifest the strong electron-hole competition in grating recording process. Two out-of-phase gratings are induced by photoexcited and thermally excited carriers.

Charge redistribution between two different photorefractive centers in CdTe:Ge allows for enhancement of two-beam coupling at 1.064 µm and 1.32 µm by the additional illumination of the sample. The gain factor reached at 1.064 µm without external electric field is only 20% smaller than the theoretical limit for CdTe. The "net" gain is obtained at 1.064 µm as well as at 1.32 µm and 1.3 times amplification of the signal wave is achieved at 1.06 µm with no applied field.

We have characterized a two-wave mixing adaptive interferometer based on dc-biased photorefractive CdTe:Ge crystal at 1.06 and 1.55 μm. Excellent performance is shown at both wavelengths by demonstration of high sensitivity for measurement of small displacements and high cutoff frequency at low intensity. We have achieved a considerable reduction of the undesired low-frequency response using controlled heating of the crystal, which ensures depopulation of corresponding traps. The experimental data are used for measurement of the real and imaginary parts of the coupling constant, as well as the dielectric relaxation time of the crystal and the mobility-lifetime product of the free charge carriers.

We study experimentally the photorefractive gratings decay in semi-insulating germanium doped cadmium telluride crystals. The estimates for the diffusion length of free carriers LD and for Debye screening length ls, for dark and photoconductivities of the sample are done from the angular and intensity dependences of the time constant.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recording of efficient reflection holograms is achieved in CdTe:Ge at λ=1.064 μm and λ=1.55 μm. The gain factor measured at both wavelengths for counterpropagating two-beam coupling considerably exceeds the absorption constant and transcends all values previously reported for semiconductors with no external field. The dependences of the gain factor on intensity and grating spacing are studied. Some crystal characteristics are estimated in the frame of a single band, one mobile species approximation of space-charge formation. The homogeneity of photorefractive properties in the crystal volume is demonstrated.

Cadmium fluoride codoped with Ga and Y is characterized as a medium for phase grating recording. The dependence of the three-dimensional grating diffraction efficiency on the intensity of the recording light, on the polarization of the recording light, on the fringe contrast, and on the spatial frequency of the fringe was measured together with the dynamic characteristics (grating decay time and grating buildup time). The crystal behaves as a χ(3) medium with its refractive-index change depending on linear intensity and with nearly exponential buildup–decay temporal characteristics for moderate intensities of the green recording light at room temperature.

The photorefractive recording dynamics of two-beam coupling in semi-insulating semiconductors by beams with slightly different frequencies are studied theoretically and experimentally. The influences of bulk absorption, Gaussian beam profiles, and experimental geometry on the temporal response are analyzed. These effects act to narrow the bandwidth. Measurement of the material photorefractive time constant is discussed.

An enhancement of the beam coupling in the near infrared is observed in Ba12TiO20 preexposed to visible light. The experiments indicate that photorefractive nonlinearity in this spectral range is determined by the diffusion charge transport. To explain the oscillations in the transmitted beam intensities we consider a model with two types of moving species (electrons and holes) and several charge traps. It is shown that the optical excitation energy for deep centers is roughly 2 eV.

Because of the light-induced bleaching of GaAs at λ = 1.064 µm, the amplitude gratings strongly affect the photorefractive recording in this material. We extract two contributions that are due to photorefraction and nonlinear absorption and use the results of measurements to evaluate the material constants. It is shown that both the phase and the amplitude gratings arise from the same process of space-charge formation.