Above-Threshold Ionization of Quasiperiodic Structures by Low-Frequency Laser Fields

We investigate the theoretical problem of the photoelectron cutoff change in periodical structures induced by an infrared laser field. We use a one-dimensional Kronig-Penney potential including a finite number of wells, and the analysis is fulfilled by resolving the time-dependent Schrödinger equation. The electron spectra, calculated for an increasing number of wells, clearly show that a plateau quickly appears as the periodic nature of the potential builds up, even at a moderate intensity (10  TW/cm(2)). Varying the intensity from 10 to 30  TW/cm(2) we observe a net increase of both the yield and accessible energy range of the ionization spectrum. In order to gain insight into the dynamics of the system at these intensities, we use an analytical approach, based on exact solutions of the full Hamiltonian in a periodic potential. We show that the population transfers efficiently from lower to upper bands when the Bloch and laser frequencies become comparable. The model leads to a quantitative prediction of the intensity range where ionization enters the nonperturbative regime. Moreover, it reveals the physics underlying the increase of the photoelectron energy cutoff at moderate intensities, as observed experimentally.