Ultrafast carrier-induced changes of absorption and refractive index in GaAs

Abstract

We report room-temperature time-resolved pump-probe transmission (T) and reflectivity (R) measurements of a GaAs thin film with 80 fs time resolution by means of our copper-vapor laser-amplified colliding-pulse mode-locking dye laser. Ultrafast carrier-induced changes of the absorption coefficient and refractive index are then calculated from measured T and R values. Photon energies at the band edge (1.38-1.46 eV) and near the initial excited states (1.90-2.18 eV) are used, and photoexcited carrier densities are varied from 1017 to 1019 cm-3. Our results reveal the dynamics of band-gap renormalization and the importance of carrier-carrier and intervalley scattering. Mechanisms that cause an ultrafast change in refractive index are presented. The carrierdensity dependence of these hot-carrier processes are also discussed.