Light-induced scattering in photorefractive crystals

Light-induced scattering features in LiNbO₃- and BaTiO₃-crytals are compared with theories on holographic writing in photorefractive crystals. It is shown that they describe the experimental facts concerning the expected main scattering directions for a given incident polarization, the time development, the thickness and the wavelength dependence. Time records of the transmission offer a useful alternative for the determination of the photoconductivity. Furthermore, a new method for birefringence measurements is established. The high accuracy of this method is based on the automatic fulfillment of a phase matching condition by the anisotropically scattered radiation.     

Temporal behavior of two-wave-mixing in photorefractive InP:Fe versus temperature

The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc electric field at different temperatures has been studied. In particular, the temperature dependence of the characteristic time constant has been studied both theoretically and experimentally, showing a strongly decreasing time constant with increasing temperature.     

The role of the longitudinal component of the electric field vector in two-wave mixing in photorefractive BaTiO3

It is shown that for waves incident with a polarisation in the plane of the grating the component of the electric field in the direction of propagation must be included in the calculations. Coupled differential equations for both the transverse and longitudinal components are derived for wave interaction in photorefractive BaTiO₃. Analytic solutions for the electric field of the signal wave are obtained under the assumption that the pump wave is undepleted. For the parameters considered the longitudinal component is shown to be negligible.     

Subharmonic generation in photorefractive crystals: Application of theory to experiment

The main results of the theory of spatial subharmonics in photorefractive crystals are summarized. The theory shows that Kukhtarev's equations for the photorefractive response are not valid for sufficiently large values of the mobility-lifetime product and for certain parameters (external field, frequency detuning, grating vector, pump intensity) of a two-beam experiment. The solution for the space-charge field including only the fundamental and the higher harmonics is not stable against the generation of new spatial frequencies. The condition of subharmonic generation and the main characteristics of the subharmonics are expressed by standard experimental and crystal parameters. Good quantitative agreement between theory and experiment is obtained. The prospects of further investigations of subharmonics are discussed.     

Detection of small phase modulation using two-wave mixing in photorefractive crystals

Two-wave mixing of phase-modulated beams in photorefractive crystals under a dc electric field is studied both theoretically and experimentally. Based on the vectorial theory of light diffraction in crystals of cubic symmetry, we derived an analytical expression that describes the phase demodulation in crystals with an arbitrarily oriented electric field. The phase-demodulation technique was used for estimation of the space-charge field created in photorefractive crystals. It is shown that the space-charge-field grating created in GaP and Bi₁₂TiO₂₀ crystals is much smaller than that predicted from the one-level band-transport model of the photorefractive effect. Received: 12 December 2000 / Revised version: 8 February 2001 / Published online: 27 April 2001     

Holographic scattering lines observed with photorefractive BaTiO3

We investigate holographic scattering processes in BaTiO₃ pumped by two coherent copropagating waves. Four lines are observed on a screen behind the sample. The experimental results are well described by a model taking into account parametric mixing of four copropagating waves and limitations of optically induced space charge fields because of the finite trap density.     

Longitudinal parametric oscillation in photorefractive sillenites: comparison between theory and experiment

By using an alternative setup for photorefractive parametric oscillation in which wave mixing between the recording beams is avoided it has become possible to make more detailed comparisons with the space-charge wave theory. In the present paper we compare the experimental features of longitudinal parametric oscillation observed in a crystal of Bi₁₂SiO₂₀ with the theoretical predictions. Received: 24 November 1998 / Revised version: 13 January 1999 / Published online: 7 April 1999     

Anisotropic scattering generated by photorefractive four-wave mixing in BaTiO3 crystal

A new type of anisotropic scattering with ring-and-line structure is observed for the first time from a BaTiO₃ crystal illuminated by a linearly polarized laser beam. The analysis presented herein is based on the photorefractive four-wave interaction of the incident, reflected, and scattered beams. The experimental results agree well with the theoretical analysis.     

Electrooptic effect for cold neutrons in photorefractive materials

First experimental evidence for the existence of an electrooptic effect for cold neutrons in ⁷LiNbO₃ is reported and discussed. The effect is based on the fact that in certain photorefractive crystals a light-induced space-charge field can be generated which causes a modulation of the density via the inverse piezoelectric effect. From such density structures, induced by the internal electric field, cold neutrons can be diffracted. Received: 13 November 1998 / Revised version: 21 December 1998 / Published online: 12 April 1999     

Space charge field limitations in photorefractive LiNbO3:Fe crystals

We use holographic techniques for the investigation of strongly oxidized LiNbO₃:Fe crystals with small Fe²⁺ concentrations and compare the results with theoretical predictions. Experimental evidence is presented for enhanced phase shifts between light intensity pattern and refractive index grating and for limitations of optically induced space charge fields in photovoltaic crystals due to the low concentration of filled traps. Our findings do not support the model of a nonlocal photovoltaic effect in LiNbO₃.     

Influence of the optical activity on hologram formation in photorefractive crystals

It is shown that dynamical hologram formation in photorefractive, optically active crystals is accompanied by polarization and energy exchange. These effects may be used like new effective holographic methods for determinations of crystal parameters and optical signal processing in real time. In self-diffraction a polarization rotation the order of degrees/cm for Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ is predicted.     

On the multiple-root problem when solving directly the nonlinear coupled-wave equations for two-wave mixing in a reflection geometry

A detailed analysis is given of the properties of the algebraic equations obtained when solving directly the nonlinear coupled-wave equations for degenerate two-wave mixing in a reflection geometry. An efficient method is developed for computing the appropriate root only amongst the multiple roots of these algebraic equations.     

Degenerate four-wave mixing based on excited-state absorption in azo-dye-doped polymer films

Degenerate Four-Wave Mixing (DFWM) based on excited-state absorption in azo-dye doped polymer films is firstly reported. Under a pre-exciting argon laser at 514.5 nm, DFWM is performed using a 632.8 nm HeNe laser which is absorbed only slightly when the molecules are in the ground state. The dynamic behavior of the DFWM signal due to excited-state absorption is also treated theoretically.     

Steady-state vectorial self-diffraction on a non-local photorefractive grating in a crystal of symmetry \overline{4}3m at symmetrical transmitting geometry

The steady-state two-wave interaction in a cubic crystal of the symmetry group 3m with the non-local photorefractive response in the absence of an external electric field is considered for the case of arbitrary interaction orientation with respect to the crystallographic coordinate system and for arbitrary intensities and polarization states of incident light waves. The self-diffraction problem is described on the basis of four coupled-wave equations in terms of the complex scalar amplitudes of components of the light waves with orthogonal linear polarization. The derived conservation laws are valid for the non-linear dependency of the photorefractive-grating amplitude on the modulation coefficient of the interference light pattern. It follows from these laws that the two non-unidirectional energy fluxes can form the total energy exchange between the two interacting light waves. A set of independent conservation laws allows us to decouple the coupled-wave equations and to obtain their analytical solution, at least, in the form of quadrature formulae. For example, such a solution is derived for the case of linearly polarized incident light waves and for the linearized dependency of the photorefractive-grating amplitude on the modulation coefficient. The explicit analytical expressions for the scalar amplitudes are obtained for the transversal electro-optic configuration of interaction. The possibility of polarization-state transformation of light waves without energy exchange between them is shown. Received: 30 July 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +7-3822/414321, E-mail: litvinov@ed.rk.tusur.ru     

Characterization of the KNSBN: Cu crystal with two-wave coupling at visible and infrared wavelengths

We have optically characterized the newly-developed photorefractive crystal KNSBN: Cu by two-wave coupling at visible and near-infrared wavelengths. The effective electrooptic coefficients(k _g)r ₁₃ and(k _g)r ₃₃, the effective charge carrier densityN _eff, the dark conductivity _d, the product of mobility and electron-trap recombination time _R, the formation and decay rate of index grating and the absorption coefficient of the crystal were determined from visible to near-infrared wavelengths. Their wavelength dependences were also studied. It is interesting to notice that the KNSBN: Cu crystal has a very largeN _eff and large gain coefficient for ordinary ray (o-ray) in the visible region, and the photorefractive properties of the crystal are very different in the visible and near-infrared spectral regions.     

Manifestation of circular photogalvanic current by dynamic holography in BaTiO3

We report the first measurement of the photo-galvanic circular current antisymmetric tensor component in BaTiO₃: Co. The measurement gives a value of this coefficient, for extraordinary beam amplification, of 4×10^–9 A/W using a nonstationary measurement technique at a wavelength of 0.632 m.Professor N. Kukhtarev is on leave from the Institute of Physics in the Ukraine and is supported by the NSFDr. G. Dovgalenko is on leave from the IRPA Rotor-Control research laboratory of the Ukraine and is supported by the University of Arkansas     

Approach to wavelet multiresolution analysis using Coiflets and a two-wave mixing arrangement

An approach to the wavelet multiresolution analysis is realized using Coiflet wavelets and a two-wave mixing arrangement via transmission gratings in a bismuth silicon oxide (BSO) crystal. A Coiflet filter acts not only as a band-pass filter but as a high-pass or low-pass filter. 2-D wavelet bases are generated by a tensorial product of the 1-D Coiflet bases and are separable in x and y. Therefore, the 1-D band-pass property produces direction ability in the 2-D tensorial products. In this way, unlike the Mexican hat-based filters, the Coiflet-based filters allow selecting horizontal, vertical or diagonals details of an original image. The decomposition of a signal in two levels of multiresolution is experimentally demonstrated in this scheme. The original image is discomposed in an average image and several detail images depending on the level of multiresolution. Coif12 wavelets are used to achieve the multiresolution analysis. The photorefractive implementation of the wavelet analysis is numerically simulated and experimentally performed. An agreement between numerical and experimental results is obtained.     

Instability of moving gratings in photorefractive crystals

It is shown that a moving intensity grating in a photorefractive crystal with a sufficiently large lifetimemobility product can give rise to an instability against excitation of weakly damped space-charge waves. Increments and threshold of the instability are investigated for the threedimensional case. Subharmonics and other nonlinear scenarios of saturation are discussed.