Polarization-Dependent Optical Nonlinearities in GaAs/AlGaAs Fractional Layer Superlattices

Summary

(AlAs) _m (GaAs) _n sub-monolayer superlattices (m,n < 1), also called fractional layer superlattices (FLS), exhibit nanometer-scale compositional corrugation parallel to the grown surface. This attractive feature makes it possible to obtain structures exhibiting quantum confinement effects in the direction perpendicular to, as well as along, the growth direction. One important consequence of lateral confinement is the appearance of anisotropies in the linear and nonlinear optical properties. The study of the optical anisotropies not only reveals the underlying physical properties but also may offer new optical device functionalities. We measure the room temperature polarization-dependent nonlinear absorption and refractive index spectra of a (Al_0.5Ga_0.5As)_1/2(GaAs)_1/2 fractional-layer superlattice (FLS) structure grown by metalorganic chemical vapor deposition. The size of the room temperature optical nonlinearity expressed by σ _eh (= Δα/N) is 6 × 10⁻¹⁵cm². The microscopic nonlinear mechanism in this FLS is band-filling and the switch-on time of the nonlinearity we measure to be less than 4 ps. We directly measure the anisotropic nonlinear effects between the directions parallel and perpendicular to the superlattice steps that give rise to a nonlinear optical birefringence in the plane of the growth surface. The nonlinear birefringence makes the FLS attractive as a nonlinear material for arrays of surface-normal, polarization-based semiconductor optical switching devices.