Effect In Photorefractive Crystals Research Articles

Optical properties of lithium niobate crystals

According to optical absorption spectra the width of the forbidden band of nominally pure congruent and stoichiometric lithium niobate crystals, as well as a series of congruent crystals doped with cations of Mg2+(0.35wt.%), Zn2+(2.05), B3+(0.12), Gd3+(0.26, 0.44, 0.51), Y3+(0.46), Gd3+(0.23):Mg2+(0.75), Mg2+(0.86):Fe3+(0.0036), Ta5+(1.13):Mg2+(0.01), Y3+(0.24):Mg2+(0.63), Er3+(3.1) is determined. The photorefractive effect in crystals, their structural and optical homogeneity was studied by photo induced and Raman scattering methods, laser conoscopy.

Switching effects in photorefractive crystals belonging to 3m point group of symmetry

Nonlinear optical mixing of optical fields in birefringent photorefractive crystals is considered within the framework of a model where pump depletion is regarded. A simple phase dependent all-optical switching based on the use of the crystals is suggested. Use of the crystals is shown to allow to obtain intensity-nondependent switching unlike usual nonlinear crystals with their intensity-dependent refractive index.

Dynamic effects in photorefractive crystals at simultaneous read–write of holographic gratings

A two-level model of a simultaneous read–write of holographic gratings in photorefractive crystals has been developed for the nonstationary case at the applied electric field and unequality of recording beam intensities. The recurrent relationships both for the internal field amplitudes and for the amplitudes of the optical waves involved in interactions have been obtained. The use of a truncated power series expansion has allowed to obtain the expressions for diffraction efficiency and intensities of optical waves in nonstationary case. The obtained results were used for the analysis of simultaneous read–write dynamics in Bi12SiO20 crystals.

Grating oscillations and nonlinear effects in photorefractive crystals

The theory of grating oscillations in photorefractive materials is developed in the linear and nonlinear approximations for the drift mechanism of holographic recording. The resonance dependence of diffraction efficiency on the phase-modulation frequency is predicted for the linear regime in which holograms are recorded by two beams, one of which is phase modulated. For long drift lengths (KgL0≫1) the resonance frequency is shown to be Ωr≈(τ1KgL0)-1. A more pronounced resonance peak is expected for the non-Bragg diffraction orders. In the nonlinear regime of recording, additional resonance maxima at Ωr/p (p is an integer) are found. Grating oscillations are experimentally studied in thin holograms of Bi12TiO20. A sharp resonance for the non-Bragg order in the interval 100–2000 Hz is detected. The position of the resonance is shown to depend on the experimental conditions. The experiment is in excellent agreement with the theory. At a high contrast ratio and in a high external electric field, distortions in the resonance dependence at Ω<Ωr and even a chaotic frequency dependence are found, which points to the nonlinear character of grating oscillations.

Formation of spatial solitons and spatial shock waves in photorefractive crystals

An analytical model, which describes the drift and diffusion mechanisms for the formation of the nonlinear response (local and nonlocal nonlinearities) of photorefractive crystals on the microscopic level, is constructed. New types of stable self-consistent distributions of the light field intensity, i.e., spatial solitons, are found. The trajectories of their motion (self-bending) are calculated, and the possibility of observing a new nonlinear-optical effect in photorefractive crystals, viz., the formation of spatial shock waves, is demonstrated. The modulation instability appearing when plane waves propagate in photorefractive crystals is analyzed, and the characteristic spatial scales of the light field distribution formed as a result of self-interaction (fanning) are determined. The results of the analysis are confirmed by computer simulation data.

Theoretical and experimental studies of fanning effects in photorefractive crystals

A model to describe fanning effects in photorefractive crystals is presented. The intensity distribution of the fanning beam for one incident beam and two incident beams in 45°-cut BaTiO3 crystals has been studied theoretically and experimentally, with good agreement. The theoretical results suggest that, when there is a fanning effect, only at intermediate effective interaction lengths does the signal beam have a high gain in two-wave mixing. The transient behavior of the fanning beam in a 45°-cut BaTiO3 crystal has also been studied theoretically and experimentally.

Fanning effects in photorefractive crystals

A model to describe the fanning effect based on the beam-coupling mechanism in photorefractive crystals is presented. The intensity distribution of the fanning beam in a 45 degrees -cut BaTiO(3) crystal is measured experimentally. The results show good quantitative agreement with theory.

Photorefractive effect in crystals with a nonlinear recombination of charge carriers: theory and observation in KNbO_3

Photorefractive and photoconductivity steady-state as well as time-resolved measurements have been performed in strongly reduced KNbO3. The recorded photorefractive gratings show a nonexponential dark decay and an intensity-dependent two-wave mixing gain. The measured photoconductivity depends sublinearly on light intensity. The observed behavior is interpreted in terms of a band transport model that uses a single donor level with only electrons as free charge carriers, taking the nonlinear recombination of free charge carriers (owing to variation of the trap density) and the time dependence of the free-charge-carrier density into account. An analytical solution for the nonexponential dark decay of the photorefractive grating is derived. As an application of this model the results of the photorefractive and the stationary and transient photoconductivity experiments are used to determine the model parameters (thermal excitation rate, photoexcitation cross section, recombination coefficient, acceptor and donor densities, and electron mobility).

Dynamic holography effects in ferroelectrics induced by spatially oscillating photovoltaic currents

The principal theoretical ideas concerning optical effects in photorefractive crystals that are relevant to polarization-dependent photovoltaic currents are presented. Data resulting from experimental investigations of these effects are summarized. Features of electron transport are considered for crystals with a large photovoltaic nonlinearity. The two-component model of electron transport is supported for these crystals. New dynamic-holography effects that are predicted by the model are discussed.