Femtosecond gain dynamics due to initial thermalization of hot carriers injected at 2 eV in GaAs

The transient absorption changes near the band edge of a thin GaAs film are measured at room temperature with ∼100 fs time resolution for various carrier densities photoexcited at 2 eV. For N≳3×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, gain is observed on a subpicosecond time scale. By comparing the delay time at which gain is observed at different excitation levels and wavelengths with a simple kinetic model, this ultrafast onset of gain is attributed to (i) nearly instantaneous equilibration of the carrier distributions near the Γ point of the Brillouin zone, together with (ii) a very efficient cooling mechanism for Γ-valley electrons due to preferential scattering of high-energy electrons to the X and L valleys. Evidence for a heated hole distribution on a subpicosecond time scale is presented, which points to the importance of electron-hole interactions.