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We discuss harmonic generation in the case of laser field-dressed Hamiltonians that are invariant under so-called dynamical symmetry operations. Examples for such systems are molecules which exhibit a discrete rotational symmetry of order N (e.g. benzene with N=6) interacting with a circularly polarized laser field and single atoms in a bichromatic field, with the two lasers having circular polarizations. Within a general group theory approach we study the harmonics one obtains from the interaction of a laser pulse and a circular molecule. When the system is in a pure field-dressed state the known selection rule kN \pm 1, k=1,2,3, ... results. However, other lines are observed when recombinations with states of a symmetry different from the initial one become important. This is the case for realistic laser pulses (i.e., with a finite duration). In particular when the fundamental laser frequency (or one of its multiples) is resonant with a transition between field-dressed states. Numerical ab initio simulations, confirming our analytical calculations and illustrating the power of the group theory approach, are presented., Comment: Submitted to Journal of Physics B

The problem of the Thomas-Fermi atom in strong static homogeneous electric fields is revisited. The static field situation is a reasonable approximation to the otherwise fully time-dependent problem of an atom in a laser pulse, provided that the laser frequency is not too high. Exact results for the ionization degree versus the applied electric field, obtained numerically, are compared with those from an approximate model which is much easier to solve. Very good agreement is found. Although the approximate model was published ten years ago, an unexplained discrepancy with both experimental results and the exact numerical solution of about one order of magnitude in the ionization degree had remained. In this paper it is shown that this discrepancy does not exist. A comparison with the simpler `over the barrier ionization' model and available experimental data for rare gases is made. A very appealing feature of the Thomas-Fermi model is its universality thanks to the scaling behaviour with respect to the nuclear charge Z .

A class of nonlinear optical effects related to fast field ionization in an interference pattern is investigated by numerical simulations. Interference between counter-propagating ultra-short pulses slightly below the ionization threshold produces a layered distribution of free-electron density. In a dense dielectric target, this effect allows us to trap light between plasma layers creating a sort of optical microcavity. Other peculiar features include frequency upshift, pulse lengthening and self-generated relativistic ionization fronts.

Absorption of intense femtosecond laser pulses in targets having either self-generated or imposed deformations is investigated with Vlasov simulations. Absorption attains high values and scales strongly with the deformation depth but weakly with density and intensity. Fast electron transport is mainly lateral for thin foils with small ratios of target thickness to deformation depth. For thick foils with larger ratios a single magnetic channel forms.

Ponderomotive (or radiation pressure) acceleration is a route to the generation of high-energy ions in laser–solid interactions, which does not rely on the generation of fast electrons. It may be optimized by the use of circularly polarized (CP) laser pulses rather than linearly polarized (LP) ones, since for CP, fast electron generation is strongly inhibited, quenching competing mechanisms such as target normal sheath acceleration. We present a comparison of 1-D and 2-D particle-in-cell simulations for CP and LP, which shows the potential of CP pulses for ion acceleration. The comparison also enables us to discriminate “pure” ponderomotive effects from those due to fast electrons, aiding the understanding of basic mechanisms of ion acceleration and surface rippling.

In recent experiments of laser pulse interaction at relativistic intensities with a low density plasma, the proton radiography technique showed evidence of long--lived field structures generated after the self-channeling of the laser pulse. We present 2D particle-in-cell simulations of this interaction regime, where the dynamics of similar structures has been resolved with high temporal and spatial resolution. An axially symmetrical field pattern, resembling both soliton-like and vortex structures, has been observed. A study of the physics of such structures and a comparison with experimental data is in progress., To appear in the Proceedings of the International Workshop "Collective phenomena in macroscopic systems", Villa Olmo, Como, Italy, December 4-6, 2006; 9 pages, 8 figures submitted in GIF format

The propagation of a superintense laser pulse in an underdense, inhomogeneous plasma has been studied numerically by two-dimensional particle-in-cell simulations on a time scale extending up to several picoseconds. The effects of the ion dynamics following the charge-displacement self-channeling of the laser pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of the plasma channel walls, causing an inversion of the radial space-charge field and the filamentation of the laser pulse. At later times a number of long-lived, quasi-periodic field structures are observed and their dynamics is characterized with high resolution. Inside the plasma channel, a pattern of electric and magnetic fields resembling both soliton- and vortex-like structures is observed., Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download a high-resolution version), to appear in Plasma Physics and Controlled Fusion (Dec. 2007), special issue containing invited papers from the 34th EPS Conference on Plasma Physics (Warsaw, July 2007)

We have theoretically investigated the acceleration of ions in the interaction of high intensity, circularly polarized laser pulses with overdense plasmas. By using 1D and 2D particle‐in‐cell (PIC) simulations we find that high‐density, short duration ion bunches moving into the plasma are promptly generated at the laser‐plasma interaction surface. This regime is qualitatively different from ion acceleration regimes driven by fast electrons, such as sheath acceleration at the back of the target or shock acceleration at the front, which occur for linear polarization. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch velocity and generation time as a function of pulse intensity and plasma density. The present mechanism based on circular polarization of the laser pulse leads to moderate ion energies (in the 100 keV–1 MeV range) but very high ion densities and low beam divergence. These ion bunches might be of interest for problems of compression and...

The generation of harmonics by atoms interacting with two circularly polarized and frequency related laser fields is addressed through ab initio numerical simulations. A detailed charaterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far-off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range., Submitted to Appl. Phys. B, 4 pages

Two-surface wave decay (TSWD), i.e. the parametric excitation of electron surface waves, was recently proposed as an absorption mechanism in the interaction of ultrashort, intense laser pulses with solid targets. We present an extension of the fluid theory of TSWD to a warm plasma which treats boundary effects consistently. We also present test-particle simulations showing localized enhancement of electron acceleration by TSWD fields; this effect leads to a modulation of the current density entering into the target and may seed current filamentation instabilities., 4 figures, submitted to Appl.Phys.B (special issue from HFSW X conference, Biarritz, France, Oct 12-15 2003); slightly revised text

The dynamics and generation of surface structures in intense laser interaction with overdense plasmas is studied numerically and analytically. The dynamics of two-dimensional (2D) electron surface oscillations (ESOs) has been resolved in particle-in-cell simulations, showing a “period doubling” effect with respect to the driving magnetic force of the laser. A new parametric process, in which a 1D electrostatic oscillation decays into two electromagnetic surface waves, has been investigated analytically and provides a mechanism for the generation of ESOs. For oblique laser incidence and p-polarization generation of subharmonic surface waves is also predicted. The possible impact of surface deformations on fast electron generation and disruption of plasma “moving mirrors” for high harmonic generation is discussed.

Using an analytical model we discuss the parametric excitation of pairs of electron surface waves (ESW) in the interaction of an ultrashort, intense laser pulse with an overdense plasma which has a step-like density profile. The ESWs can be excited either by the electric or by the magnetic part of the Lorentz force exerted by the laser and, correspondingly, have frequencies around $\omega/2$ or $\omega$, where $\omega$ is the laser frequency., Comment: 4 EPS figures, Revtex

By analytical modeling and numerical simulation, we show that surface modes in moderately overdense plasmas may be excited parametrically by an intense, ultrashort laser pulse. This process has a feedback effect on fast electron generation and may seed a fast distortion of plasma “moving mirrors.”

SUMMARY We study the interaction of strong femtosecond laser pulses with the C$_{60}$ molecule employing time-dependent density functional theory with the ionic background treated in a jellium approximation. The laser intensities considered are below the threshold of strong fragmentation but too high for perturbative treatments such as linear response. The nonlinear response of the model to excitations by short pulses of frequencies up to 45eV is presented and analyzed with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond laser pulses of 800nm wavelength ionization is found to occur multiphoton-like rather than via excitation of a ``giant'' resonance.

The generation of electron surface oscillations in overdense plasmas irradiated at normal incidence by an intense laser pulse is investigated. Two-dimensional (2D) particle-in-cell simulations show a transition from a planar, electrostatic oscillation at $2\omega$, with $\omega$ the laser frequency, to a 2D electromagnetic oscillation at frequency $\omega$ and wavevector $k>\omega/c$. A new electron parametric instability, involving the decay of a 1D electrostatic oscillation into two surface waves, is introduced to explain the basic features of the 2D oscillations. This effect leads to the rippling of the plasma surface within a few laser cycles, and is likely to have a strong impact on laser interaction with solid targets., Comment: 9 pages (LaTeX, Revtex4), 4 GIF color figures, accepted for publication in Phys. Rev. Lett

We investigate numerically effects related to ``single-cycle'' ionization of dense matter by an ultra-short laser pulse. The strongly non-adiabatic response of electrons leads to generation of a megagauss steady magnetic field in laser-solid interaction. By using two-beam interference, it is possible to create periodic density structures able to trap light and to generate relativistic ionization fronts, 12 pages, 6 figures, to be published in Laser and Particle Beams

The effect of surface deformations on laser absorption and transport in overdense plasma targets is investigated with 2D Vlasov simulations. Fractional absorption increases strongly with the deformation depth and scales weakly with density or intensity. Fast electrons generated in the skin layer have high tangential velocity directed toward the deformation center. In the case of a single, smooth deformation, they may be collimated by magnetic fields into a single jet which penetrates deeply into the plasma. Small-scale surface corrugations are able to destroy the collimation of a single jet leading to multiple jets or diffuse electron sprays.

By discussing the results of Yu et al. [Phys. Rev. E 59, 3583 (1999)] on the near-total transmission of laser pulses through highly overdense plasma foils, we show that they actually cannot explain the experimental results of Giulietti et al. [Phys. Rev. Lett. 79, 3194 (1997)]. Simple analytical calculations as well as particle-in-cell simulations support our assertion.

Ponderomotive (or radiation pressure) acceleration is a route to the generation of high-energy ions in laser-solid interactions, which does not rely on the generation of fast electrons. It may be optimized by the use of circularly polarized (CP) laser pulses rather than linearly polarized (LP) ones, since for CP, fast electron generation is strongly inhibited, quenching competing mechanisms such as target normal sheath acceleration. We present a comparison of 1-D and 2-D particle-in-cell simulations for CP and LP, which shows the potential of CP pulses for ion acceleration. The comparison also enables us to discriminate "pure" ponderomotive effects from those due to fast electrons, aiding the understanding of basic mechanisms of ion acceleration and surface rippling.

Relativistic Vlasov simulation is employed for the study of ultrashort, superintense laser-solid interaction. A novel absorption mechanism is evidenced in 1D2V simulations. Analytical calculations elucidate the simulation results. Implementation of the Vlasov code on a parallel platform and progress towards a full Boltzmann approach are discussed.

We discuss a kinetic model for a system of free and bound electrons undergoing field ionization. Fluid equations are derived and used to study transient ionization effects on short laser pulse propagation. We finally discuss a suitable semiclassical fluid approach to heavy atoms ionization.

In the variety of processes occurring in matter under the action of superstrong laser fields rapid field ionization and ponderomotive effects play a dominant role. We investigate field ionization by solving the time-dependent Schrodinger equation numerically and compare the results with classical calculations. We obtain the ionization rates of hydrogen-like ions, the ejection energy spectrum of the electrons, and we show the transition to classical behavior at high energies. In addition, we are able to give a physical explanation of non-sequential ionization. Our ionization rates differ by more than an order of magnitude from the standard expressions. A model is presented which allows to calculate the back action of field ionization on the laser beam. Finally, we present the most general relativistic derivation of ponderomotive forces on single particles and present their transformation properties. Ponderomotive forces are of non-Newtonian character.

The dynamics of electric field generation and radial acceleration of ions by a laser pulse of relativistic intensity propagating in an underdense plasma has been investigated using an one-dimensional electrostatic, ponderomotive model developed to interpret experimental measurements of electric fields [S. Kar et al, New J. Phys. *9*, 402 (2007)]. Ions are spatially focused at the edge of the charge-displacement channel, leading to hydrodynamical breaking, which in turns causes the heating of electrons and an "echo" effect in the electric field. The onset of complete electron depletion in the central region of the channel leads to a smooth transition to a "Coulomb explosion" regime and a saturation of ion acceleration., Comment: 9 pages, 7 figures, final revised version, to appear on Plasma Phys. Contr. Fus., special issue on "Laser and Plasma Accelerators", scheduled for February, 2009

Large-scale self-focusing effects are evidenced by focusing a 300 MW Nd laser beam with aF/8 spherical lens in a fairly low-density gas (such thatne≈nc/100). The include important beam recollimation and formation of an «empty» channel which was visualized by holographic interferometry. In these conditions plasma expansion and laser energy deposition were both larger ahead than behind the focal point.

Our aim is to study the two-photon induced luminescence in CdS crystals by using a low power CW laser (5 mW). We discuss briefly the possibility given by the electron-pulse-counting technique to detect weak optical signals. This technique enables us to investigate the luminescence spectra (which show two bands at 2.4 and 2.0 eV) and the dependence of the luminescence intensity on the laser power. The discussion of the experimental results makes it clear that the level responsible for the red emission is partially saturated at a laser intensity of 1023 photon/cm2 s.

The interaction of a GW laser beam ( λ= 1. 06 µm) with a plasma far from critical density (ne /nc ≤ 10-2) has been studied at an intensity of 1013 W/cm2. Experimental results showed a net redistribution of the laser energy both in space and time; the formation of a low density channel was also detected. These results are discussed in terms of self-focusing in an inhomogeneous plasma, where inhomogeneities are produced by hydrodynamic motion.

Electron density distribution and shock front profiles are obtained, by holographic interferometry, from a plasma produced by a laser in a 500 Torr helium atmosphere, with an f N = 1.4 lens. For a long time, compared with the duration of the laser pulse, interesting features are observed, such as a three-folded shock front and hot plasma jets. A model based on Mach reflection of two spherical shocks allows us to explain those structures.