Photoexcitation

Abstract

The evolution of the system excited by a high-frequency electromagnetic field can be investigated by averaging the statistical operator over the period of perturbation. Applying this approach to electrons, one can introduce the rate of photogeneration, which is proportional to the intensity of the radiation and describes electron redistribution between the ground and excited states. Below, as examples of such approach, we discuss the photon drag current under the intersubband transitions and the response to ultrafast photoexcitation. The distribution of photoexcited electrons is essentially non-equilibrium, and this property causes some peculiar features of the linear response to an additional probe field. For example, the negative transient conductivity appears. If the electromagnetic field is strong enough, the radiation cannot be treated as a perturbation. Beyond the second-order response, the non-Markovian photogeneration leads to the Rabi oscillations. A more careful approach is applied for studying the mixing of electron and hole states by the field of the radiation, when one should introduce new quasiparticles. We also consider the coherent response of the phonon system to the radiation and describe the relaxation of coherent phonon oscillations due to phonon-phonon interaction.