S. Odoulov

Parametric four-wave processes in photorefractive crystals

Abstract A theory of polarization-dependent four-wave processes is developed for photorefractive nonlinear media. It includes the classification of possible elementary nonlinear processes, an analysis of their polarization and geometric properties, and a unified technique for the calculation of wave amplitudes. The theory is applied to the description of different types of conical scattering in BaTiO 3 and LiNbO 3 crystals. Effects of crystal anisotropy, angular and frequency detunings for interacting waves are taken into account. A good agreement between theory and experiment is demonstrated. Applications of the theory to treatment of a number of important photorefractive effects are considered.

Photorefractive recording in BTO in the near infrared

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.

Nonlinear scattering in BaTiO3 induced by two orthogonally polarized waves

A new nonlinear scattering effect is described which is induced by the simultaneous action of ordinary and extraordinary waves impigning on a BaTiO3 crystal in the plane perpendicular to the c-axis. The scattered light of ordinary polarization propagates along a circular cone and the light intensity on this cone is nonuniform. We present a model that describes the experimental observations rather well, including phase matching conditions and the unusual intensity distribution of the scattered light.

Beam-coupling, four-wave mixing and optical oscillation due to spatially-oscillating photovoltaic currents in lithium niobate crystals

Frequency degenerate four-wave mixing of orthogonally polarized waves (ordinary and extraordinary) in 3 m point group crystal with photovoltaic nonlinearity is studied theoretically and experimentally in LiNbO3:Fe.

Photorefractive beam coupling in tin hypothiodiphosphate in the near infrared

Infrared recording of phase gratings is investigated in Sn(2)P(2)S(6) crystals by radiation of a Nd(3+):YAG laser (lambda = 1.06 microm) Space-charge formation and hologram recording are dominated by diffusion charge transport. Without an applied electric field, but with preexposure of the sample to incoherent white light, the gain factor exceeds 6 cm(-1) at laser intensities above 50 W/cm(2).

Studies of light-induced charge transfer in Sn2P2S6 by combined EPR/optical absorption spectroscopy

The light-induced charge transfer in ferroelectric tin hypothiodiphosphate Sn 2 P 2 S 6 is investigated by means of optical absorption and EPR spectroscopy and their combination. Light-induced metastability at 298 K, known to affect the holographic sensitivity, is observed via optical absorption. EPR measurements support the recent identification of holes as the dominating charge carriers. For excitation energies exceeding the band gap of 2.5 eV at 10 K, EPR reveals that the following processes are likely to occur: a hole is captured at one of two different Sn 2+ sites, creating Sn 3+ . At an energy of 1.5 eV the hole is first transferred to the other Sn 2+ and for excitation of 2.0 eV to a further center. Since these defects are intrinsic and therefore not limited in quantity, the light-induced sensitisation is a very effective way to improve the holographic performance.

Coupling of orthogonally polarized waves in BaTiO3

The first observation of anisotropic diffraction (with a 90|Mo change of the polarization of the diffracted wave) in BaTiO3 from a grating recorded by two orthogonally polarized waves, an ordinary and an extraordinary wave, is reported. Three possible origins of this effect are considered: (i) direct one-step diffraction from a grating recorded anisotropically by spatially oscillating photovoltaic currents, (ii) indirect, sequential diffraction from two gratings, each recorded by one incident wave and an additional wave due to light-induced scattering, (iii) direct one-step diffraction from a grating with the grating vector equal to the difference of the grating vectors of two conjugated “noisy” gratings. The experimental evidence indicates that the process (iii) contributes most, and process (i) more than 10% to the overall diffraction efficiency, whereas process (ii) seems to be of minor importance.

Modeling of the photorefractive nonlinear response in Sn 2 P 2 S 6 crystals

We develop a theory of the photorefractive nonlinear response for Sn2P2S6 crystals. The theory incorporates two types of charge carrier (optically active and passive), provides explicit expressions for the characteristic buildup-relaxation rates and gain factors, explains naturally a big variety of accumulated experimental data, and facilitates characterization-optimization of this important nonlinear material.

Polarization-degenerate parametric light scattering in photorefractive crystals

Parametric scattering of two copropagating coherent light waves into cones of light with the same polarization is considered both experimentally (in photo-refractive LiNbO3:Fe) and theoretically.

Oscillation spectra of semilinear photorefractive coherent oscillator with two pump waves

The transition of the single-frequency oscillation of a semilinear photorefractive coherent oscillator for sufficiently large coupling strengths into two-frequency oscillation is predicted and is observed experimentally. The critical value of coupling strength at which the bifurcation occurs is a function of pump-intensity ratio and cavity losses. For certain combinations of these parameters, the critical coupling strength for spectrum bifurcation becomes smaller than the threshold coupling strength: in these cases double-frequency oscillation appears at the threshold. The supercritical bifurcation in the oscillation spectrum is analogous to the second-order phase transition.