Oleg V. Kobozev

Light-induced absorption in a Bi12TiO20 crystal

Evolution of the light-induced absorption in a Bi12TiO20 crystal is investigated at a wavelength of 633 nm. It is found that the absorption coefficient evolution consists of fast and slow components with different characteristic response time. The light absorption coefficient decreases from 0.9 to 0.16 cm-1 after keeping the sample in darkness for six months. We propose a new charge-transport model that includes the deep donors in three valence states and the shallow traps. It is shown that the theory developed provides an explanation for the observed features of the increase and dark decay of the light-induced absorption. The material parameters of the Bi12TiO20 crystal are estimated from the best fit of the theoretical model with the experimental data.

Fast photorefractive response in B(12)SiO(20) in the near infrared.

The polarization self-modulation effect was applied for effective measurement of the characteristic response time of nominally pure Bi(12)SiO(20) (BSO) at wavelengths of 810 and 980 nm. Owing to oxygen deficiency in the crystal lattice, the BSO crystals showed unusual photorefractive sensitivity and remarkable operation speed in the near-infrared spectral region. A response time of 130 ms was measured at 810 nm, and a response time of 540 ms was measured at 980 nm, with incident intensities of 110 and 200mW/cm(2), respectively. To our knowledge, this is the first experimental evidence of a subsecond response in the infrared for a nonsemiconductor photorefractive material.

Two-beam coupling in sillenite crystals

We present the results of a study of two-beam mixing and light-induced absorption in sillenite crystals at wavelength (lambda) equals 633 nm. We have employed the single-carrier model of the photorefractive effect with shallow and deep traps to the explanation of the experimental results.

Photorefractive properties of doped sillenite crystals

We present a technology of a growing of pure and doped sillenite crystals and the results of a study of two-beam mixing in these crystals at wavelength (lambda) equals 633 nm. We have employed the single-carrier model of the photorefractive effect with shallow and deep traps to the explanation of the experimental results.

Origin of transient effects in two-wave mixing experiments in BSO crystals

The transient dynamics of the internal space charge field was observed in two-wave mixing experiments in BSO crystals carried out at the wavelength of 633 nm under the external ac electric field. The effects involve practically complete disappearance of the initially strong space-charge field during continuous illumination of a sample by an interference pattern and the retrieval of the significant part of the initial field after the signal beam is switched off. Moreover, the compensated grating can be retrieved even after homogeneous illumination of whole sample without applying external field during many hours. The observations are explained in the frames of the monopolar free-carriers- transport model using the idea of spatial modulation of a sample photoconductivity. The change of the photoconductivity is due to non-equilibrium population of deep traps participating in photoexcitation of free electrons. The model is in accordance with the data of photoconductivity measurements indicating to the transient behavior of the photocurrent in studied BSO samples at the wavelength of 633 nm.

Two-wave mixing of phase-modulated beams in GaP under a dc electric field.

Two-wave mixing of phase-modulated light beams in crystals of cubic symmetry is analyzed on the basis of the vectorial theory of light diffraction. We derive an analytical expression for phase-to-intensity transformation in crystals of the 43m point group of symmetry, which differs from the previously obtained solution based on the scalar approach. The most effective transformation is achieved when the amplitude of the space-charge-field grating is equal to the quarter-wave field. It is shown that the space-charge-field grating created in GaP semi-insulating crystal at the wavelength of 632 nm is much smaller than can be predicted from the one-level band-transport model.

Fast adaptive interferometer in a GaP crystal using a near-infrared laser diode

We describe an adaptive interferometer for linear detection of small out-of-plane vibrations of optically rough surfaces. The interferometer is based on the polarization self-modulation effect in a GaP crystal and it uses a laser diode at 807 nm as a light source. The achieved response time of 1.9 ms is the fastest among known adaptive interferometers in the near infrared.

Photorefractive effect in sillenite crystals with shallow traps in a sign-alternating electric field

The results of a theoretical analysis of the photorefractive response in crystals with shallow traps to in a sign-alternating, square-wave electric field are presented. The numerical analysis method developed imposes no restrictions on the frequency of the external field and the period of the photorefractive grating. The parameters characterizing deep donor and shallow trap centers are estimated on the basis of investigations of two-beam interaction in a Bi12SiO20:Cd crystal with the application of a sign-alternating, square-wave electric field.

Manifestation of long-lived photosensitivity gratings in two-wave mixing experiments with sillenite crystals

Transient recording dynamics and hologram fixing and retrieval were observed in photorefractive Bi12SiO20 crystal in two-wave mixing experiments under an ac electric field. These effects were explained by the light- and space-charge-induced spatial modulation of sample photoconductivity originating from the nonequilibrium initial population of relevant photoactive levels. Such a photosensitivity grating exhibits a relaxation time of several hours even when the crystal is illuminated by a spatially uniform light beam. Thus we have shown for what is the first time to our knowledge that optical information can be stored in a photoconductive crystal as a spatially modulated photosensitivity rather than in the form of a space charge.

Optical characterization of sillenite crystals using polarization self-modulation effect

Abstract We propose to use the polarization self-modulation effect for the optical characterization of sillenite crystals. The proposed method allows the characterization of crystals in a broader spectral range compared to conventional wave mixing techniques. The nominally pure Bi 12 SiO 20 bismuth silicon oxide (BSO) crystals are characterized at wavelengths λ =633, 810, and 980 nm demonstrating the unusually fast operation speed in the infrared part of the spectrum. We show that the intensity dependence of the materials response is sublinear and can be approximated as ∝ I −0.7 . The mobility-lifetime product μτ is determined for BSO by fitting the experimentally measured data with the above function.