Coherence of Electron Photoexcitation by Extremely Short Strong Light Pulses

For a semiconductor alloy with a predominant disorder scattering, we show that, under appropriate conditions, electrons photoexcited by a short strong light pulse form a coherent transient at first, while the incoherently backscattered electrons take over only gradually, with a time lag comparable with the pulse duration. The time evolution of the electron distribution is obtained by a direct evaluation of the non-equilibrium Green function.