Subpicosecond plasma dynamics and absorption saturation in GaAs

We develop a model calculation to describe the athermal dynamics of an electron-hole plasma in a direct-gap semiconductor under subpicosecond optical excitation. The computations are especially focused on the problem of optical absorption saturation in such a regime. The model fits perfectly recent experimental results on GaAs. Carrier dampings (due to LO-phonon–carrier and carrier-carrier scatterings) are calculated as a function of the electron states distribution. We show that the carrier dampings first increase quickly when the plasma density is increased under laser excitation (ρ<5×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$) but thereafter decay due to the plasma degeneracy and the screening effect which reduces the amplitude of the renormalized interactions. The occurrence of the absorption saturation in subpicosecond pulse experiments is considered in GaAs for excitation photons, the energy of which ranges from 1.538 to 1.61 eV.