The Photorefractive Effect for Optical Processing

Abstract

Since the discovery of the photorefractive effect in LiNbO₃ two decades ago, considerable effort has been devoted to understanding and optimizing the phenomenon for a variety of applications. It is now recognized that this effect offers a very versatile nonlinear optical mechanism for parallel optical processing. The studies and application to date have made use of oxide materials such as LiNbO₃, K(Ta_xNb_l−x)O₃, BaTiO₃ and Bi₁₂SiO₂₀. Most of these materials exhibit relatively slow relaxation times which limit the manner in which optical processing can be performed. However, we have recently found that high mobility electro-optic semiconductors including InP, GaAs and CdTe offer attractive alternatives for photorefractive image processing: (a) their recording sensitivities are comparable to oxide photorefractors, (b) they offer near infrared responsivity in the spectral region of importance for semiconductor lasers, (c) the high mobility allows fast photorefractive response ranging from subnanoseconds to milliseconds and, (d) the optical and electrical properties are readily tailored by doping to suit specific applications. The basic mechanisms giving rise to the photorefractive effect in electro-optic semiconductors will be described,and the sensitivity and speed limitations will be contrasted with those of ferroelectric crystals. Four-wave mixing experiments demonstrating submicrosecond writing and erasure with microjoule pulses of infrared radiation will be reported. This corresponds to information processing at 10¹² bits/sec with energies of 1 picojoule/bit for high contrast images. Considerably lower energies are required for reduced contrast, and considerably faster response can be achieved with a degree of parallelism.