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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.

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.

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.

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.

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.

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.

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.

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.