Photoemission and high-order harmonic generation from solid surfaces in intense laser fields

We give a Floquet analysis of photoemission of electrons and high-order harmonic generation from a solid surface interacting with intense laser fields. A general theoretical algorithm, within a three-dimensional quasi-free-electron model, is presented. It takes account of the effective masses of the electron both within and outside the solid, as well as treating both reflected and refracted laser fields. The steady-state photocurrent is obtained from the diagonal photon components of the expectation value of the time-dependent current operator outside the surface, while the off-diagonal terms of the expectation value of the current operator from both inside and outside the surface provide the probability of coherent emission of high-order harmonic radiation. It is found, among other things, that electrons can be emitted from the surface requiring no net absorption of photons or even accompanied by emission of extra photons. The origin of the effect lies in the difference in the ponderomotive energy inside and outside the surface that can arise from the small ratio of the effective mass of the electron inside the solid (e.g., GaAs) and the mass of the free electron. Another result of qualitative significance is the formation of a plateau, well known in the atomic case, in both the kinetic energy distribution of the photocurrent and the spectrum of the high-order harmonic radiation. They are shown to arise in the present case from the evanescent part of the electron wave function at the surface.