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 optical reflectivity. In photo excited non-equilibrium states, we found a sign reverse in transient reflectivity spectra $\Delta R/R$ (t $>$ 0), from positive around room temperature to negative at cryogenic temperatures. The former corresponds to a transient 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 analysing the intensity change of the time-resolved optical spectra. We found that photoexcited MIT starts emerging at T$_1$ as high as $\sim$ 230 K, in terms of a negative dip feature at 0.4 ps, and becomes stabilized below T$_2$ associated with a negative constant after 40 ps in spectra. Our results address a phase diagram that provides a framework for MIT through temperature and photoexcitation, and shed light on the understanding of light-semiconductor interaction and exciton physics.