Bandgap and Defect Engineering for Semiconductor Holographic Materials: Photorefractive Quantum Wells and Thin Films

Abstract

Bandgap engineering of thin semiconductor layers and defect engineering combine to form photorefractive (PR) quantum well structures. PR quantum wells are semi-insulating thin films useful for dynamic holography and other coherent and incoherent optical applications. As materials for thin-film dynamic holography, they have high nonlinear-optical sensitivity and high speed.The PR effect translates a spatially varying irradiance, from the interference of two or more coherent light beams, into a refractive index grating. The multiple-step PR process begins with photoexcitation of charge carriers, followed by transport and trapping of charge at deep defects. The trapped space-charge generates electric fields that alter the refractive index of the material through the electrooptic effect. The same laser beams that generate the gratings diffract from the gratings, leading to a rich variety of multiple-beam effects, such as two-wave and four-wave mixing.Because the PR process involves several distinct physical parameters, such as carrier mobility and electrooptic coefficients, optimized performance requires a coincidence of favorable properties in a single material. Rather than relying on coincidence, bandgap engineering of multiple layers of semiconductors provides a way to individually tune the separate material pa rameters. Likewise, defect engineering in semiconductors provides flexibility in the choice of defects, their concentrations, and degree of compensation. Bandgap and defect engineering combined make custom designed PR materials possible.