95 research outputs found
Strong Field Ionization Rate for Arbitrary Laser Frequencies
A simple, analytical, nonrelativistic ionization rate formula for atoms and
positive ions in intense ultraviolet and x-ray electromagnetic fields is
derived. The rate is valid at arbitrary values of the Keldysh parameter and
confirmed by results from ab initio numerical solutions of the single active
electron, time-dependent Schroedinger equation. The proposed rate is
particularly relevant for experiments employing the new free electron laser
(FEL) sources under construction worldwide.Comment: 4 pages, 1 figure, REVTe
Strong Field Approximation for Systems with Coulomb Interaction
A theory describing above-threshold ionization of atoms and ions in a strong
electromagnetic field is presented. It is based on the widely known strong
field approximation and incorporates the Coulomb interaction between the
photoelectron and the nucleus using the method of complex classical
trajectories. A central result of the theory is the Coulomb-corrected
ionization amplitude whose evaluation requires little extra numerical effort.
By comparing our predictions with the results of ab initio numerical solutions
for two examples we show that the new theory provides a significant improvement
of the Coulomb-free strong field approximation. For the case of above-threshold
ionization in elliptically polarized fields a comparison with available
experimental data is also presented.Comment: submitted to Journal of Modern Optics (Heraeus Seminar "Novel Light
Sources and Applications", February 3-9, 2008, Obergurgl, Austria
Bright single-cycle terahertz source based on gas cells irradiated by two-color laser pulses
We study the excitation of electron currents in a transparent cell of
sub-millimeter size filled by an atomic gas and illuminated by an intense
two-color femtosecond laser pulse. The pulse consists of a strong fundamental
component and its second harmonic of low intensity, both circularly polarized.
We show that for sufficiently small m cells the plasma oscillation
excited by asymmetric ionization is almost spatially homogeneous within the
interaction volume. This coherent dipole plasma oscillation results in a
remarkably efficient conversion of the electron energy into that of radiation
emitted in the terahertz frequency domain. Simultaneously, strong quasi-static
electric fields of maximal strength MV/cm are shown to exist
inside the cell during several hundred femtoseconds after the ionizing
two-color laser pulse has gone.Comment: 7 pages, 2 figures, 1 table. XXXIst International Conference on
Photonic, Electronic, and Atomic Collisions (ICPEAC 2019), Deauville, July
23-30, 201
Manipulation by Photoelectron Currents for the Generation of Terahertz Light Pulses
Using the strong field approximation we calculate photoelectron momentum distributions generated in the interaction of low-frequency two-color laser fields with atomic gases. The field consists of an infrared linearly or circularly polarized pulse of intensity close to 1014W/cm2 and its second linearly polarized harmonic whose intensity does not exceed 10% of the fundamental. Our calculations aim to find a field configuration, which maximizes the photoelectron current left after the interaction. Such net currents result from asymmetries of photoelectron distributions in non-monochromatic coherent fields with fixed phases between the frequency components. We show that combining a circularly polarized intense pulse with a linearly polarized pulse of the second harmonic one could approach the highest possible asymmetry of the photoelectron distribution and therefore the highest value of the net current.
Keywords: terahertz radiation, strong-field ionization, photoelectron currents, strong field approximatio
Treating branch cuts in quantum trajectory models for photoelectron holography
Most implementations of Coulomb-distorted strong-field approaches that contain features such as tunneling and quantum interference use real trajectories in continuum propagation, while a fully consistent approach must use complex trajectories throughout. A key difficulty in the latter case are singularities of the Coulomb potential in the complex time plane. These singularities have the form of branch points which generate corresponding branch cuts. We present a method for treating branch cuts in quantum-trajectory models, which is subsequently applied to photoelectron holography. Our method is not numerically intensive and is applicable to Coulomb-free and Coulomb-distorted trajectories. We show that the presence of branch cuts leads to discontinuities and caustics in the holographic fringes in above-threshold ionization (ATI) photoelectron momentum distributions. These artifacts are removed with our method, provided no hard recollision takes place during the interaction. A comparison with the full solution of the time-dependent Schrodinger equation is also performed, and a discussion of the applicability range of the present approach is provided
Harmonic Generation from Laser-Irradiated Clusters
The harmonic emission from cluster nanoplasmas subject to short, intense
infrared laser pulses is analyzed by means of particle-in-cell simulations. A
pronounced resonant enhancement of the low-order harmonic yields is found when
the Mie plasma frequency of the ionizing and expanding cluster resonates with
the respective harmonic frequency. We show that a strong, nonlinear resonant
coupling of the cluster electrons with the laser field inhibits coherent
electron motion, suppressing the emitted radiation and restricting the spectrum
to only low-order harmonics. A pump-probe scheme is suggested to monitor the
ionization dynamics of the expanding clusters.Comment: 4 pages, ReVTeX
Efficiency of radiation friction losses in laser-driven "hole boring" of dense targets
In the interaction of laser pulses of extreme intensity () with high-density, thick plasma targets, simulations show
significant radiation friction losses, in contrast to thin targets for which
such losses are negligible. We present an analytical calculation, based on
classical radiation friction modeling, of the conversion efficiency of the
laser energy into incoherent radiation in the case when a circularly polarized
pulse interacts with a thick plasma slab of overcritical initial density. By
accounting for three effects including the influence of radiation losses on the
single electron trajectory, the global `hole boring' motion of the laser-plasma
interaction region under the action of radiation pressure, and the
inhomogeneity of the laser field in both longitudinal and transverse direction,
we find a good agreement with the results of three-dimensional particle-in-cell
simulations. Overall, the collective effects greatly reduce radiation losses
with respect to electrons driven by the same laser pulse in vacuum, which also
shift the reliability of classical calculations up to higher intensities.Comment: 15 pages, 3 figure
Quantum effects on radiation friction driven magnetic field generation
Radiation losses in the interaction of superintense circularly polarized
laser pulses with high-density plasmas can lead to the generation of strong
quasistatic magnetic fields via absorption of the photon angular momentum (so
called inverse Faraday effect). To achieve the magnetic field strength of
several Giga Gauss laser intensities W/cm are required
which brings the interaction to the border between the classical and the
quantum regimes. We improve the classical modeling of the laser interaction
with overcritical plasma in the "hole boring" regime by using a modified
radiation friction force accounting for quantum recoil and spectral cut-off at
high energies. The results of analytical calculations and three-dimensional
particle-in-cell simulations show that, in foreseeable scenarios, the quantum
effects may lead to a decrease of the conversion rate of laser radiation into
high-energy photons by a factor 2-3. The magnetic field amplitude is suppressed
accordingly, and the magnetic field energy - by more than one order in
magnitude. This quantum suppression is shown to reach a maximum at a certain
value of intensity, and does not grow with the further increase of intensities.
The non monotonic behavior of the quantum suppression factor results from the
joint effect of the longitudinal plasma acceleration and the radiation reaction
force. The predicted features could serve as a suitable diagnostic for
radiation friction theories.Comment: 10 pages, 3 figure
Boosting terahertz-radiation power with two-color circularly polarized midinfrared laser pulses
A way to considerably enhance terahertz radiation, emitted in the interaction of intense midinfrared laser pulses with atomic gases, in both the total energy and the electric-field amplitude is suggested. The scheme is based on the application of a two-color field consisting of a strong circularly polarized midinfrared pulse with wavelengths of 1.6-4 mu m and its linearly or circularly polarized second harmonic of lower intensity. By combining the strong-field approximation for the ionization of a single atom with particle-in-cell simulations of the collective dynamics of the generated plasma, it is shown that the application of such two-color circularly polarized laser pulses may lead to an order-of-magnitude increase in the energy emitted in the terahertz frequency domain as well as in a considerable enhancement in the maximal electric field of the terahertz pulse. Our results support recently reported experimental and numerical finding
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