Time-resolved far-infrared reflectance ofn-type GaAs

We present here our results of far-infrared room-temperature time-resolved reflectivity experiments on bulk n-type GaAs with a high carrier density $(\sim 2\times {10}^{18}{\mathrm{cm}}^{\mathrm{-}3})$ over a wavelength range from 15 to 50 μm. In our experiments, in which no additional carriers are optically created by the laser, we observe that the plasma frequency shifts towards smaller frequencies upon excitation with an intense far-infrared laser pulse, and that the magnitude of this shift increases with higher pump-pulse intensities. The decay of these pump-induced reflectivity changes is on the order of 3 ps at 300 K, and is caused by the change in the effective carrier density due to the scattering of electrons between the Γ valley and the L valley.