Transient transition from free carrier metallic state to exciton insulating state in GaAs by ultrafast photoexcitation

We present systematic studies of the transient dynamics of GaAs by ultrafast time-resolved reflectivity. In photoexcited non-equilibrium states, we found a sign reverse in reflectivity change Delta R/R, from positive around room temperature to negative at cryogenic temperatures. The former corresponds to a free carrier metallic state, while the latter is attributed to an exciton insulating state, in which the transient electronic properties is mostly dominated by excitons, resulting in a transient metal-insulator transition (MIT). Two transition temperatures (T-1 and T-2) are well identified by analyzing the intensity change of the transient reflectivity. We found that photoexcited MIT starts emerging at T-1 as high as similar to 230 K, in terms of a dip feature at 0.4 ps, and becomes stabilized below T-2 that is up to similar to 180 K, associated with a negative constant after 40 ps. Our results address a phase diagram that provides a framework for the inducing of MIT through temperature and photoexcitation, and may shed light on the understanding of light-semiconductor interaction and exciton physics.