1,249 research outputs found
Resonant enhancements of high-order harmonic generation
Solving the one-dimensional time-dependent Schr\"odinger equation for simple
model potentials, we investigate resonance-enhanced high-order harmonic
generation, with emphasis on the physical mechanism of the enhancement. By
truncating a long-range potential, we investigate the significance of the
long-range tail, the Rydberg series, and the existence of highly excited states
for the enhancements in question. We conclude that the channel closings typical
of a short-range or zero-range potential are capable of generating essentially
the same effects.Comment: 7 pages revtex, 4 figures (ps files
Threshold effects in strong-field ionization: Energy shifts and Rydberg structures
The behavior of strong-field ionization rates of neutral atoms in the vicinity ofmultiphoton ionization thresholds is analyzed using formal collision theory.Our approach,which accounts nonperturbatively for effects of an intense laser field, shows that the ionization rates have a nearly constant behavior below and above each multiphoton threshold and that between such thresholds there are an apparently finite number of rapid oscillations due to resonances with laser-field-modified Rydberg states. This pattern is typical for any atomic target, as we illustrate specifically for hydrogen and neon atoms. The flat behavior of the ionization yield near multiphoton thresholds gives the appearance of an energy shift of the ionization thresholds, which have been postulated in a number of recent studies concerning diverse aspects of above-threshold ionization and high-harmonic generation of atoms. The flat behaviors of the rates near threshold exhibit only a rather weak dependence on the laser-field intensity. Other aspects of the near-threshold behavior of ionization rates and their dependence on the laser-field parameters are also discussed
Two-Electron Effects in the Multiphoton Ionization of Magnesium with 400 nm 150 fs Pulses
The multiphoton ionization and photoelectron spectra of magnesium were
studied at laser intensities of up to 6x10^{13} Wcm^{-2} using 150 fs laser
pulses of a wavelength of 400 nm. The results indicated that a variety of
different ionization mechanisms played a role in both types of spectra. A
theoretical model describing the processes is presented and the routes to
ionization are identified. The work demonstrates the significance of the
two-electron nature of the atom in interpreting the experimental results.Comment: 14 pages, 9 figures, submitted to Physical Review
Strong-field above-threshold photoemission from sharp metal tips
We present energy-resolved measurements of electron emission from sharp metal
tips driven with low energy pulses from a few-cycle laser oscillator. We
observe above-threshold photoemission with a photon order of up to 9. At a
laser intensity of 2*10^11 W/cm^2 suppression of the lowest order peak occurs,
indicating the onset of strong-field effects. We also observe peak shifting
linearly with intensity with a slope of around -1.8eV / (10^12 W/cm^2). We
attribute the magnitude of the laser field effects to field enhancement taking
place at the tip's surface.Comment: 5 pages, 3 figure
Ultrashort filaments of light in weakly-ionized, optically-transparent media
Modern laser sources nowadays deliver ultrashort light pulses reaching few
cycles in duration, high energies beyond the Joule level and peak powers
exceeding several terawatt (TW). When such pulses propagate through
optically-transparent media, they first self-focus in space and grow in
intensity, until they generate a tenuous plasma by photo-ionization. For free
electron densities and beam intensities below their breakdown limits, these
pulses evolve as self-guided objects, resulting from successive equilibria
between the Kerr focusing process, the chromatic dispersion of the medium, and
the defocusing action of the electron plasma. Discovered one decade ago, this
self-channeling mechanism reveals a new physics, widely extending the frontiers
of nonlinear optics. Implications include long-distance propagation of TW beams
in the atmosphere, supercontinuum emission, pulse shortening as well as
high-order harmonic generation. This review presents the landmarks of the
10-odd-year progress in this field. Particular emphasis is laid to the
theoretical modeling of the propagation equations, whose physical ingredients
are discussed from numerical simulations. Differences between femtosecond
pulses propagating in gaseous or condensed materials are underlined. Attention
is also paid to the multifilamentation instability of broad, powerful beams,
breaking up the energy distribution into small-scale cells along the optical
path. The robustness of the resulting filaments in adverse weathers, their
large conical emission exploited for multipollutant remote sensing, nonlinear
spectroscopy, and the possibility to guide electric discharges in air are
finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure
Model atomic systems in intense laser fields: Exact time-dependent density functional and Floquet theory
The ab initio solution of the time-dependent Schrödinger equation for a many-body system is feasible for only a few constituents. Success of TDDFT when it comes to highly correlated electron dynamics is very limited. In this thesis, the exact exchange-correlation potential for the highly correlated process of autoionization is constructed.
One may try to employ the time-periodicity of external drivers. If this could also be applied to the time-dependent Kohn-Sham equation the time-dependent many-body problem could be reduced to a time-independent one. It is investigated if this is possible
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