K. Schiessl

Quantum path interference in the wavelength dependence of high-harmonic generation

We investigate the fundamental-wavelength dependence of high-harmonic generation yield. Superimposed on a smooth power-law dependence, we find surprisingly strong and rapid fluctuations on a fine wavelength scale, due to quantum-path interferences.

Electron guiding through insulating nanocapillaries

We simulate the electron transmission through insulating Mylar (polyethylene terephthalate, or PET) capillaries. We show that the mechanisms underlying the recently discovered electron guiding are fundamentally different from those for ion guiding. Quantum reflection and multiple near-forward scattering rather than the self-organized charge up are key to the transmission along the capillary axis irrespective of the angle of incidence. We find surprisingly good agreement with recent data. Our simulation suggests that electron guiding should also be observable for metallic capillaries.

Intracycle and Intercycle Interferences in Above-Threshold Ionization: the Time Grating

Within a semiclassical description of above-threshold ionization (ATI) we identify the interplay between intracycle and intercycle interferences. The former is imprinted as a modulation envelope on the discrete multiphoton peaks formed by the latter. This allows one to unravel the complex interference pattern observed for the full solution of the time-dependent Schroedinger equation (TDSE) in terms of diffraction at a grating in the time domain. These modulations can be clearly seen in the dependence of the ATI spectra on the laser wavelength. Shifts in energy modulation result from the effect of the long Coulomb tail of the atomic potential.

Simulation of charged particle guiding through insulating nanocapillaries

We report on our model for transport of charged particles through insulating nanocapillaries. The dynamics of transmitted ion beams prior to equilibrium is investigated. Extension of the transport theory to electron projectiles reveals that different mechanisms are responsible for their deflection in capillaries. Charge-up is no longer a prerequisite to establish guided transmission.

Energy dependence of ion guiding through nanocapillaries

We present first simulation results for the energy dependence of the effective guiding of ions through nanocapillaries in insulating materials based on classical transport theory. We also investigate the dependence of guiding on the effective dielectric shielding near the insulating surface. We find agreement with experimental data over a range of energies without any freely adjustable parameter.

Wavelength dependence of high-harmonic generation from ultrashort pulses

We investigate the dependence of the intensity of radiation due to high-harmonic generation (HHG) as a function of the wavelength λ of a few-cycle driver field. Superimposed on a smooth power-law dependence observed previously, strong and rapid fluctuations on a fine λ scale are observed. The origin of these fluctuations can be identified in terms of quantum path interferences with several orbits significantly contributing. The dependence on the pulse shape and pulse length of the driver is analyzed. We discuss the relation to well-known channel closing effects with emphasis on effects of the ultrashort duration of the pulse.

Enhancement of high-order harmonic generation by a two-color field: Influence of propagation effects

Recent calculations of the response of a single atom subjected to a two-color laser pulse with the higher frequency being resonant with an excitation of the target atom revealed a significant enhancement of photoionization as well as high-order harmonic generation [K. Ishikawa, Phy. Rev. Lett. 91, 043002 (2003)]. We investigate the problem in the framework a fully quantum-mechanical pulse propagation algorithm and perform calculations for rare gases in the single-active-electron approximation. The enhancement of harmonic output compared to the corresponding one-color pulse remains intact for short propagation lengths, promising the feasibility of experimental realization. We also study weak second colors resonant via a two-photon transition where significant enhancements in harmonic yields can be observed as well.

Diffraction at a time grating in above-threshold ionization: The influence of the Coulomb potential

We analyze the photoelectron emission spectrum in atomic above-threshold ionization by a linearly polarized short-laser pulse. Direct electrons can be characterized by both intracycle and intercycle interferences. The former results from the coherent superposition of two different electron trajectories released in the same optical cycle, whereas the latter is the consequence of the superposition of multiple trajectories released in different cycles. In the present article, a semiclassical analytical expression for the complete (both intracycle and intercycle) interference pattern is derived. We show that the recently proposed semiclassical description in terms of a diffraction process at a time grating remains qualitatively unchanged in the presence of the long-range Coulomb potential. The latter causes only a phase shift of the intracycle interference pattern. We verify the predictions of the semiclassical model by comparison with full three-dimensional (3D) time-dependent Schroedinger equation (TDSE) solutions. One key finding is that the subcycle interference structures originating from trajectories launched within a time interval of less than 1 femtosecond should be experimentally observable also in low-resolution spectra for longer multicycle pulses.

Interference Patterns in the Wavelength Dependence of High-Harmonic Generation

We investigate the dependence of the intensity of radiation due to high-harmonic generation (HHG) as a function of the wavelength of a few-cycle driver field. Superimposed on a smooth power-law dependence observed previously, strong and rapid fluctuations on a fine scale are observed. The origin of these fluctuations can be identified in terms of quantum path interferences with several orbits significantly contributing

Wavelength dependent channel-closing enhancements in multi-photon ionization

We analyze the enhancements of the total yield of a hydrogen atom ionized by a short laser pulse as a function of the wavelength of the laser field. We study the dependence of these enhancements with the pulse duration and pulse envelope and investigate on the relation between the enhancements of the ionization and the high harmonic generation yields.