This chapter is an overview of the photorefractive effect, the important photorefractive nonlinear processes and common photorefractive materials. In Section 2, the formation of steadystate spatial solitons by laser beam propagation through photorefractive crystals (PRC) with optical activity and absorption is theoretically and experimentally described. The spatial soliton features, which characterize the dynamic waveguide built in PRC, are analyzed. The soliton polarisation dynamics reaches a stable behavior under high external electric fields. Two-wave mixing and self-diffraction in dynamic harmonic and inharmonic gratings, induced by lasers in PRC, are presented in Section 3. The diffraction efficiency and beam amplification can reach high values, with promising potential in optical interconnections and storage. Dynamic and adaptive gratings by double phase conjugation (DPC) in PRC are discussed in Section 4. High phase conjugated reflectivity, high coupling transmission efficiency and robust interconnections of mutually incoherent lasers are achieved in DPC using Rh:BaTiO 3 crystals.
Overview of the photorefractive effect and materials
Photoexcited processes in photorefractive crystals (PRC)The photorefractive effect is a process, in which the refractive index of an optical material is changed, when illuminated by light. The photorefractive effect is commonly understood by the band transport model [1][2][3][4][5][6][7][8]. According to this model, a photorefractive material contains deep and shallow levels in the forbidden bandgap due to impurities or defects in that material. In Figure 1, this model is illustrated for the case of a deep level associated with a donor and the shallow level-trap, to an acceptor. It is assumed that the donor concentration is greater than the acceptor, such as when the shallow acceptors are completely filled by electrons.It is also assumed that the donor level is deep in the bandgap and consequently, the thermal excitation is neglected.When the photorefractive material is illuminated by light with a high photon energy, the deep donors are photoionized and free carriers are raised into the conduction band. If additionally, nonuniform light distributions, such as focused laser beams or intensity interference patterns are used, the free electrons move into the conduction band by diffusion, drift and photovoltaic transport processes. Another important process in these materials is the recombination of the free carriers with CHAPTER 3