S. Stepanov

SPATIAL SOLITONS IN PHOTOREFRACTIVE BI12TIO20 WITH DRIFT MECHANISM OF NONLINEARITY

The analysis performed shows that in cubic photorefractive crystals (of Bi12SiO20 and GaAs type in particular) with drift (‘‘quasilocal’’) mechanism of optical nonlinearity spatial soliton propagation can be observed for reasonable external dc electric fields 10–15 kV/cm. These photorefractive solitons which can be excited at very low cw laser power are stable in a broad region of signal(soliton)/incoherent(spatially uniform) intensity ratio, and allow easy switching from bright to dark type. Original experimental results of self‐channeling of submicrowatt HeNe laser beam in a cubic Bi12TiO20 sample under external dc field are presented.

(1 + 1)-Dimension dark spatial solitons in photorefractive Bi12TiO20 crystal

Abstract The generation of (1 + 1)-dimension photorefractive spatial dark solitons by 180° phase or amplitude jumps is reported. Experiments are performed in a cubic photorefractive Bi 12 TiO 20 crystal under an external dc electric field at the wavelength of the cw HeNe laser (633 nm). The waveguiding properties of the generated dark solitons are investigated using another HeNe (or semiconductor) cw laser probe beam.

Interferometric measurements of the photoinduced refractive index profiles in photorefractive Bi12TiO20 crystal

Abstract By direct interferometric measurement in Bi 12 TiO 20 under external DC electric field, we demonstrate that the steady-state photorefractive lens produced by the laser beam with the initial circular cross-section is essentially anisotropic: its focusing strength along the direction of the applied electric field is remarkably higher. Experimental data are supported by computer simulations and analytical results.

Z-scan experiments with cubic photorefractive crystal Bi12TiO20

Abstract We present original data on experiments with cubic photorefractive crystal Bi12TiO20 under an external dc electric field in the configuration of Z-scan characterization technique. In particular, behavior of the Z-scan curves versus the external dc field, light polarization, and the (focused beam)/(uniform sample illumination) intensity ratio is reported. The data obtained are interpreted in the framework of an original theoretical analysis based on the standard drift model of the photorefractive effect and assumption of a Gaussian profile of the propagating laser beam.

Anisotropy of steady-state two-dimensional lenses in photorefractive crystals with drift nonlinearity

Abstract As a result of the original theoretical analysis in different approximations we show that the steady-state photoinduced photorefractive lens formed under an external transverse d.c. electric field by the light beam with cylindrical symmetry is essentially astigmatic. This behaviour is demonstrated in self-focusing-defocusing experiments with cubic Bi12 TiO20 photorefractive crystals at λ=633 nm. The result implies in particular that the steady-state (2+1)-dimensional spatial solitons generated in photorefractive crystals under similar conditions are to be characterized by an elliptic cross-section.

Guided modes in a dielectric slab with diffusion-type photorefractive nonlinearity

We report the results of electromagnetic mode analysis of a dielectric slab with a diffusion-type photorefractive nonlinearity. The influence of dark conductivity on the optical field distribution as well as photorefractive mode excitation by the spatially limited laser beams are discussed in detail.

Formation of steady-state cylindrical thermal lenses in dark stripes.

Characteristic features of the formation of cylindrical laser-induced thermal steady-state lenses in the dark stripes associated with spatial dark-soliton propagation are analyzed and verified experimentally in an ethanol-dye solution. Positive focusing power perpendicular to the dark stripe is ensured by defocusing along the axis of this stripe and depends strongly on the shape of the light-beam profile and the orientation of the dark stripe within it.

Dynamics of photoinduced lens formation in a photorefractive Bi 12 TiO 20 crystal under an external dc electric field

We report experimental results on the dynamics of formation of a photoinduced lens produced by a two-dimensional cw He–Ne Gaussian laser beam (633 nm) in a cubic Bi12TiO20 photorefractive crystal under an external dc electric field. The main features of the effect, namely, astigmatism of the dynamic lens formed, the pulse shape of the far-field response after switching on of the focused beam, inverse linear dependence of time of its growth on the beam intensity, and approximate constancy of its peak level, are explained in the framework of the analytical model for a one-dimensional Gaussian beam and numerical calculations for a two-dimensional one.

Running space charge wave instability in photorefractive crystals

Abstract It is shown that in materials with a special type of photoconductivity temporal response, the running space charge wave is generated under uniform illumination when the external electric field exceeds the threshold value. The experimental evidence of the effect existence in semi-insulating GaAs is presented. The effect can possibly be used to obtain high two-wave mixing gain in photorefractive crystals.

Modulation Z-scan technique for characterization of photorefractive crystals

We propose a simple single-beam configuration for characterization of the amplitude, speed of growth, and polarization properties of the photoinduced refractive-index change that is due to a drift photorefractive mechanism of nonlinearity in crystals, namely, the modulation Z-scan technique, based on the modulation of an externally applied electric field. The results of a simple theoretical model developed for one-dimensional parabolic photorefractive lens formation in this configuration are illustrated by original experiments with a semi-insulating GaAs crystal at lambda -1.06 microm.