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.
Two-wave mixing adaptive interferometers based on photorefractive crystals allow for precise remote detection of small displacements. Using dynamic holograms, they compensate for ambient disturbances in factory environments and can process speckled beams with complicated wavefronts. Linear phase-to-intensity conversion with maximum sensitivity is achieved when the response becomes local when a dc-field is applied to the photorefractive crystal. In the present work we study experimentally the change of the shape of the amplification spectrum induced by a dc field in the two-wave mixing geometry. The shape of the spectrum is used for identification of the type of response (local or nonlocal). High sensitivity for detection of surface displacements is demonstrated for a two-wave mixing interferometer with a dc-biased CdTe:Ge crystal.
The phase shift occurred by optical waves in nonlinear interaction may be considered as phase shift resulted from a nonlinear refractive index. Consequently, a spectrum of the nonlinear phase shift in non-degenerate in frequency interactions represents nonlinear dispersion. Thus the nonlinear interaction with such dispersion may be used to achieve slow and fast light. The phase-conjugate reflectivity in photorefractive four-wave mixing may reveal spectrum with two maxima, which are located symmetrically with respect to zero frequency detuning. A complicated nonlinear dispersion corresponds to such reflectivity spectrum. Qualitative analysis of the nonlinear dispersion suggests unusual behavior when the nonlinear effect, i.e., delay of the light pulse, may decrease with increase of nonlinear coupling strength. The numerical calculations confirm such a nontrivial behavior. The experimental conditions are found, for which the delay of the phase-conjugate pulse decreases when the coupling strength increases. The conclusions of the theoretical analysis are confirmed experimentally for photorefractive four-wave mixing in barium titanate.
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.
We report the effective excitation of space-charge waves (SCW) in photorefractive ac-biased CdTe:Ge crystal illuminated by a laser beam (λ = 1150 nm) and an auxiliary incoherent light with different wavelength. We show that laser beam scattered from SCW has resonant dependence on the auxiliary light wavelength. The effect of auxiliary illumination on photorefractive two-wave coupling and subharmonic generation is presented.
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 Nd<SUP>3+</SUP>:YAG laser radiation.
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.
We study experimentally the photorefractive gratings decay in semi-insulating germanium doped cadmium telluride crystals. The estimates for the diffusion length of free carriers L<SUB>D</SUB> and for Debye screening length l<SUB>s</SUB>, for dark and photoconductivities of the sample are done from the angular and intensity dependences of the time constant.