261 research outputs found
Electronic dynamics and frequency-dependent effects in circularly polarized strong-field physics
We analyze, quantum mechanically, the dynamics of ionization with a strong,
circularly polarized, laser field. We show that the main source for
non-adiabatic effects is connected to an effective barrier lowering due to the
laser frequency. Such non-adiabatic effects manifest themselves through
ionization rates and yields that depart up to more than one order of magnitude
from a static-field configuration. Beyond circular polarization, these results
show the limits of standard instantaneous - static-field like - interpretation
of laser-matter interaction and the great need for including time dependent
electronic dynamics
Circularly Polarized Molecular High Harmonic Generation Using a Bicircular Laser
We investigate the process of circularly polarized high harmonic generation
in molecules using a bicircular laser field. In this context, we show that
molecules offer a very robust framework for the production of circularly
polarized harmonics, provided their symmetry is compatible with that of the
laser field. Using a discrete time-dependent symmetry analysis, we show how all
the features (harmonic order and polarization) of spectra can be explained and
predicted. The symmetry analysis is generic and can easily be applied to other
target and/or field configurations
Resonantly enhanced pair production in a simple diatomic model
A new mechanism for the production of electron-positron pairs from the
interaction of a laser field and a fully stripped diatomic molecule in the
tunneling regime is presented. When the laser field is turned off, the Dirac
operator has resonances in both the positive and the negative energy continua
while bound states are in the mass gap. When this system is immersed in a
strong laser field, the resonances move in the complex energy plane: the
negative energy resonances are pushed to higher energies while the bound states
are Stark shifted. It is argued here that there is a pair production
enhancement at the crossing of resonances by looking at a simple 1-D model: the
nuclei are modeled simply by Dirac delta potential wells while the laser field
is assumed to be static and of finite spatial extent. The average rate for the
number of electron-positron pairs produced is evaluated and the results are
compared to the single nucleus and to the free cases. It is shown that
positrons are produced by the Resonantly Enhanced Pair Production (REPP)
mechanism, which is analogous to the resonantly enhanced ionization of
molecular physics. This phenomenon could be used to increase the number of
pairs produced at low field strength, allowing the study of the Dirac vacuum.Comment: 11 pages, 4 figure
Quantum-classical correspondence in circularly polarized high harmonic generation
Using numerical simulations, we show that atomic high order harmonic
generation, HHG, with a circularly polarized laser field offers an ideal
framework for quantum-classical correspondence in strong field physics. With an
appropriate initialization of the system, corresponding to a superposition of
ground and excited state(s), simulated HHG spectra display a narrow strip of
strong harmonic radiation preceded by a gap of missing harmonics in the lower
part of the spectrum. In specific regions of the spectra, HHG tends to lock to
circularly polarized harmonic emission. All these properties are shown to be
closely related to a set of key classical periodic orbits that organize the
recollision dynamics in an intense, circularly polarized field
Clusters of Exceptional Points for a Laser Control of Selective Vibrational Transfer
When a molecule is exposed to a laser field, all field-free vibrational
states become resonances, with complex quasi energies calculated using Floquet
theory. There are many ways to produce the coalescences of pairs of such quasi
energies, with appropriate wavelength-intensity choices which define
Exceptional Points (EP) in the laser parameter plane. We dress for the
molecular ion H an exhaustive map of these exceptional points which
appear in clusters. Such clusters can be used to define several vibrational
transfer scenarios implying more than a single exceptional point, exchanging
single or multiple vibrational quanta. The ultimate goal is molecular
vibrational cooling by transferring an initial (thermal, for instance)
population on a final (ground, for instance) single vibrational state. When a
molecule is exposed to a laser field, all field-free vibrational states become
resonances, with complex quasi energies calculated using Floquet theory. There
are many ways to produce the coalescences of pairs of such quasi energies, with
appropriate wavelength-intensity choices which define Exceptional Points (EP)
in the laser parameter plane. We dress for the molecular ion H an
exhaustive map of these exceptional points which appear in clusters. Such
clusters can be used to define several vibrational transfer scenarios implying
more than a single exceptional point, exchanging single or multiple vibrational
quanta. The ultimate goal is molecular vibrational cooling by transferring an
initial (thermal, for instance) population on a final (ground, for instance)
single vibrational state.Comment: 16 pages, 7 figures, 1 tabl
Visualizing quantum entanglement and the EPR paradox during the photodissociation of a diatomic molecule using two ultrashort laser pulses
We investigate theoretically the dissociative ionization of a H2+ molecule
using two ultrashort laser (pump-probe) pulses. The pump pulse prepares a
dissociating nuclear wave packet on an ungerade surface of H2+. Next, an UV (or
XUV) probe pulse ionizes this dissociating state at large (R = 20 - 100 bohr)
internuclear distance. We calculate the momenta distributions of protons and
photoelectrons which show a (two-slit-like) interference structure. A general,
simple interference formula is obtained which depends on the electron and
protons momenta, as well as on the pump-probe delay on the pulses durations and
polarizations. This interference can be interpreted as visualization of an
electron state delocalized over the two-centres. This state is an entangled
state of a hydrogen atom with a momentum p and a proton with an opposite
momentum. -p dissociating on the ungerade surface of H2+. This pump-probe
scheme can be used to reveal the nonlocality of the electron which intuitively
should be localized on just one of the protons separated by the distance R much
larger than the atomic Bohr orbit
Asymmetry of above-threshold ionization of metal clusters in two-color laser fields: A time-dependent density-functional study
URL:http://link.aps.org/doi/10.1103/PhysRevA.69.063415
DOI:10.1103/PhysRevA.69.063415Above threshold ionization (ATI) spectra of small metal clusters (e.g., Na4 and Na4+) are calculated numerically using a spherical jellium model and time-dependent density functional theory for two-color (1064 and 532 nm) ultrashort (25 fs) laser pulses as a function of phase difference between the two fields. ATI spectra and ionized electron fluxes are obtained in the two opposite directions of the linearly polarized laser fields. The asymmetry, defined as the difference in electron yield, is shown to depend strongly on the carrier-envelope phase of the second-harmonic (2ω) field. The ATI spectra allow one to identify the range of kinetic energies of the ionized electrons where the asymmetry mainly occurs. Comparisons are made between calculations with and without self-interaction correction and also with previous exact numerical solutions of the one-electron systems H and H2+ [A. D. Bandrauk and S. Chelkowski, Phys. Rev. Lett. 84, 3562 (2000)] where such asymmetry effects had first been observed. We find that ATI spectra in the clusters generally have much longer energy plateaus than in previously studied one-electron systems, with cutoffs up to 30-40 times the ponderomotive energy Up. In high-harmonic generation spectra, on the other hand, no extended plateaus are observed.Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, as well as to the University of Missouri Research Board, for partial support of
this research
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