  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.
