The Photorefractive Properties of Quantum-Confined Excitons

Multiple-quantum-well structures in semiconductors have strong spatial inhomogeneity. Bandgaps in these multilayer samples vary by several tenths of eV over monolayer distances. These dramatic spatial changes in bandstructure have significant consequences for the photorefractive effect, creating new effects that have no analog in bulk photorefractive materials. The ability to design new materials and devices, and control desired photorefractive properties, has few limitations. Several processes give photorefractive quantum well structures unique advantages. Two charge separation processes, in particular, contribute to the novel effects: 1) bandgap energies can be selectively tuned to isolate optical absorption to some layers, but not others; 2) carriers in quantum wells tunnel into barrier regions with larger bandgaps, generating metastable defect occupancies with associated electric fields. These processes couple with one of the strongest advantages of quantum well structures: quantum-confined excitons. Quantum-confined excitons in semiconductors exhibit large quadratic electro-optic effects. The quadratic electro-optic effect combines with the charge separation processes to yield ultra-high sensitivity photorefractive effects with large diffraction efficiencies[1] and beam coupling gains. In this paper, we present the theory of photorefractive effects in quantum well structures, concentrating on the role of spatial inhomogeneity in the nonlinear optical behavior.