Classical and quantum-mechanical treatments of nonsequential double ionization with few-cycle laser pulses

We address nonsequential double ionization induced by strong, linearly polarized laser fields of only a few cycles, considering a physical mechanism in which the second electron is dislodged by the inelastic collision of the first electron with its parent ion. The problem is treated classically, using an ensemble model, and quantum-mechanically, within the strong-field and uniform saddle-point approximations. In the latter case, the results are interpreted in terms of "quantum orbits", which can be related to the trajectories of a classical electron in an electric field. We obtain highly asymmetric electron momentum distributions, which strongly depend on the absolute phase, i.e., on the phase difference between the pulse envelope and its carrier frequency. Around a particular value of this parameter, the distributions shift from the region of positive to that of negative momenta, or vice-versa, in a radical fashion. This behavior is investigated in detail for several driving-field parameters, and provides a very efficient method for measuring the absolute phase. Both models yield very similar distributions, which share the same physical explanation. There exist, however, minor discrepancies due to the fact that, beyond the region for which electron-impact ionization is classically allowed, the yields from the quantum mechanical computation decay exponentially, whereas their classical counterparts vanish.Comment: 12 pages revtex, 12 figures (eps files

Effect of radiation on transport in graphene

We study transport properties of graphene-based p-n junctions irradiated by an electromagnetic field (EF). The resonant interaction of propagating quasiparticles with an external monochromatic radiation opens dynamical gaps in their spectrum, resulting in a strong modification of current-voltage characteristics of the junctions. The values of the gaps are proportional to the amplitude of EF. We find that the transmission of the quasiparticles in the junctions is determined by the tunneling through the gaps, and can be fully suppressed when applying a sufficiently large radiation power. However, EF can not only suppress the current but also generate it. We demonstrate that if the height of the potential barrier exceeds a half of the photon energy, the directed current (photocurrent) flows through the junction without any dc bias voltage applied. Such a photocurrent arises as a result of inelastic quasiparticle tunneling assisted by one- or two-photon absorption. We calculate current-voltage characteristics of diverse graphene based junctions and estimate their parameters necessary for the experimental observation of the photocurrent and transmission suppression.Comment: 21 pages, 15 figure

High-order above-threshold ionization: The uniform approximation and the effect of the binding potential .

A versatile semiclassical approximation for intense laser-atom processes is presented. This uniform approximation is no more complicated than the frequently used multidimensional saddle-point approximation and far superior, since it applies for all energies, both close to as well as away from the classical cutoffs. In the latter case, it reduces to the standard saddle-point approximation. The uniform approximation agrees accurately with numerical evaluations for potentials, for which these are feasible, and constitutes a practicable method of calculation, in general. The method is applied to the calculation of high-order above-threshold ionization spectra with various binding potentials: Coulomb, Yukawa, and shell potentials which may model C60 molecules or clusters. The shell potentials generate rescattering spectra that are more structured and may feature an apparently higher cutoff