PROPERTIES AND APPLICATIONS OF PHOTOREFRACTIVE CRYSTALSGerald ROOSEN and J.P. HUIGNARD*Institut d'Optique, U.A. CNRS, Bât. 503, Centre Universitaire d'Orsay, B.P. 4391406 ORSAY Cedex, FRANCELab. Central de Recherche, Thomson CSF, D. de Corbeville, B.P. 10, 91401 ORSAY Cedex, FRANCECoherent optics offers very unique advantages for information processing and transmission with a highparallelism and a strong data rate. The implementation of these idea requires non linear materialspermitting efficient interactions between optical beams.Electro-optic and photoconductive crystals are of considerable importance as they allow such non linearinteractions with low power laser beams through the photorefractive effect. This effect has been foundin a large variety of crystals (LiNb03, BaTiO3, KNb03, Bi12Si(Ge)020, Ga As, InP, ...)1 -4 resulting ina broad range of spectral sentivities, spatial resolutions, efficiencies and speeds. Therefore numerousoperations for optical signal processing have been demonstrated using these photorefractive crystals596.However no material displays all the desired features for the realisation of truly operating devices.Extensive research work is still required in order to determine the limits of the effect, to understandhow structural changes and material compositions affect the photorefractive properties and to optimizethe performances according to the considered application.As most of the basic photorefractive properties, optical methods of characterisation and developpedapplications have similar features for any photorefractive material, we will illustrate these pointsconsidering crystals of the sillenite family, i.e. Bi12Si020 (BSO) or Bi12Ge020 (BGO). However theimportant specificities of other classes of crystals will be underlined.The photorefractive effect results from charge excitation by the light, followed by their migration andretrapping in another location.This charge redistribution induces a strong electrostatic field inside the crystalline material. Throughthe linear electro -optic effect (Pockels' effect) this spatially varying space charge field is transfor-med into a refractive index modulation, thus a phase grating. The effect is mainly sensitive to theabsorbed optical energy. Under low power continuous excitations, the time constant T of thephotorefractive effect is controlled by Idi, the dielectric relaxation time, which is much longer thanTR and Td, respectively the recombination and diffusion times of the excited charges. I also depends ontwo other parameters, L the diffusion length of the excited carriers and N the density of trap sites7.Thus, studying the grating decay rate under uniform illumination permits to determine Idi, L and N. Thiswas performed for various BSO and BGO samples by either varying the grating period$ or the magnitude ofan externally applied electric field9. Strong differences are evidenced pointing out the importance ofsuch basic researches. Indeed the speed and magnitude of the space charge grating induced by quasicontinuous illumination is governed by these only three material parameters that gives the effectivenessof the photorefractive effect for device applications.Physical mechanisms in photorefractive crystals lead to the recording of an index grating spatiallyshifted relatively to the incident light pattern1,5. This may result in an energy redistribution betweenthe interfering beams which direction is directly related to the sign of the charge carriers. Eitherhole or electron conduction can be dominant according to samples,10 the origin of which being not fullyunderstood yet.In spite of the little reliability of the materials,many interesting applications have been developpedbased on either two or four wave mixing configurations.Two wave mixing permits either the amplification of a low intensity signal beam containing spatial infor-mationt1 or the realisation of self starting optical resonators when an optical feedback is added 12-14.In both cases one takes advantage of the energy transfer phenomenon that can exhibit very high gains. Insillenite crystals such gains are obtained in the drift mode (applied electric field) by moving thefringe pattern at a constant velocityl5 -17. The speed is adjusted so that at any time the recorded indexmodulation has a 7/2 spatial phase shift relatively to the incident fringe pattern.

Gerald Roosen

Papers

Properties And Applications Of Photorefractive Crystals