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We present systematic studies of the transient dynamics of GaAs by ultrafasttime-resolved optical reflectivity. In photo excited non-equilibrium states, wefound 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 thelatter is attributed to an exciton insulating state, in which the transientelectronic properties is mostly dominated by excitons, resulting in a transientmetal-insulator transition (MIT). Two transition temperatures (T$_1$ and T$_2$)are well identified by analysing the intensity change of the time-resolvedoptical spectra. We found that photoexcited MIT starts emerging at T$_1$ ashigh as $\sim$ 230 K, in terms of a negative dip feature at 0.4 ps, and becomesstabilized below T$_2$ associated with a negative constant after 40 ps inspectra. Our results address a phase diagram that provides a framework for MITthrough temperature and photoexcitation, and shed light on the understanding oflight-semiconductor interaction and exciton physics.

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