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

Abstract

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 ΔR/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 (T1 and T2) are well identified by analyzing the intensity change of the transient reflectivity. We found that photoexcited MIT starts emerging at T1 as high as ∼ 230 K, in terms of a dip feature at 0.4 ps, and becomes stabilized below T2 that is up 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.