Abstract

The photorefractive effect (optically induced index change) has been extensively studied in electrooptic insulators because of its potential for all-optical processing, i.e., convolution and correlation image amplification, phase conjugate resonators, holographic interferometry, and matrix algebra. All the materials to date have exhibited slow response because of the low carrier mobilities and are only sensitive in the visible spectrum. We report for the first time the observation of the photorefractive effect in semi-insulating GaAs:Cr and lnP:Fe by four-wave mixing. In both of these materials the dopants give rise to optical absorption and photoconduction at photon energies below the band edge, to ~1.3 μm In lnP:Fe (~10¹⁷ cm⁻³), and to 1.8 μm in GaAs:Cr (~10¹⁶ cm⁻³). The observed material response times are ~10⁻⁴ sec, determined by the dielectric relaxation time, and can be decreased to subnanosecond time scales by increasing the crystal conductivity (i.e., by decreasing the dopant concentration). This lower limit is determined by the drift velocity of photoexcited carriers during space charge separation and relaxation. Thus the material response time can be varied to suit the application. Despite the small electrooptic coefficients of InP and GaAs the photorefractive recording sensitivities (the index change per absorbed photon) are comparable to the best nonlinear recording media, ~1 pJ/bit. Diffraction efficiencies of 10⁻³ have been achieved with only 50 mW/cm² of cw YAG:Nd laser irradiation at 1.06 μm. Large high-optical-quality crystals are now available which allow the long interaction length (~10 cm) necessary to obtain diffraction efficiencies close to unity. The electrical and optical properties of the crystals can be tailored to optimize the material parameters for specific applications.¹

© 1984 Optical Society of America