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 $20\mu$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 $E_m\simeq 10$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

Efficiency of radiation friction losses in laser-driven "hole boring" of dense targets

In the interaction of laser pulses of extreme intensity ($>10^{23}~{\rm W cm}^{-2}$) 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 $\simeq 10^{24}$W/cm$^2$ 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

Low-Energy Structures in Strong Field Ionization Revealed by Quantum Orbits

Experiments on atoms in intense laser pulses and the corresponding exact ab initio solutions of the time-dependent Schr\"odinger equation (TDSE) yield photoelectron spectra with low-energy features that are not reproduced by the otherwise successful work horse of strong field laser physics: the "strong field approximation" (SFA). In the semi-classical limit, the SFA possesses an appealing interpretation in terms of interfering quantum trajectories. It is shown that a conceptually simple extension towards the inclusion of Coulomb effects yields very good agreement with exact TDSE results. Moreover, the Coulomb quantum orbits allow for a physically intuitive interpretation and detailed analysis of all low-energy features in the semi-classical regime, in particular the recently discovered "low-energy structure" [C.I. Blaga et al., Nature Physics 5, 335 (2009) and W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)].Comment: 4 pages, 3 figures, REVTe

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