The ULIA-1 Conference, a scientific initiative of Antonio Giulietti, was the first of the ULIA Euroconferences Series, from the Istituto di Fisica Atomicae Molecolare (IFAM), Pisa, Italy. It was held in Elounda, Crete, Greece, May 7-11, 1999, one of the most attractive Mediterranean spots, characterized by beautiful surroundings rich with suggestions of the ancient Minoan civilization.

Simulations of the interaction of very intense short pulse lasers with solid density plasmas have indicated that there may be very high magnetic fields and electron temperatures in the vicinity of the focal spot. We investigate the propagation of an incident electromagnetic wave under these conditions, both analytically and by means of a particle code, and show that the evanescence length of the wave field in a highly overdense plasma may be much greater than is usually supposed. This effect could be important in understanding the penetration of energy into the overdense region and the particle heating and acceleration which takes place there.

We study fully relativistic nonlinear 1D fluid equations describing steady-state solutions for an electromagnetic wave interacting with plasma in the self-induced transparency regime. In addition to the well-known solution that corresponds to the transmission of the electromagnetic wave into plasma, another steady-state solution is shown to exist in a certain range of the amplitude of the wave. The latter one satisfies the same boundary conditions but corresponds to the full reflection of the incident wave. The co-existence of the two solutions indicates a rich structure of the interface between the opaque and transparent phases in ultraintense laser interactions with near-critical and overdense plasmas—in particular, one may expect hysteresis-like behavior when a current state of the system depends on its history.

The interaction of an intense laser field with a thin foil is studied within the framework of a 1D capacitor model with mobile negative point-charges. The model describes the excitation of plane electrostatic oscillations, taking into account the effects of particle–particle crossings and the presence of vacuum boundaries. It is demonstrated how vacuum excursions of particles cause wavebreaking within the foil. From wavebreaking points, charge density discontinuities are shown to propagate further along particle caustics. Absorption is calculated as a function of laser intensity and frequency. It is found that a thin overdense foil can absorb about 5 times more energy than a corresponding surface of an extended solid. This is explained by the doubling of surface charges and by electron heating in the potential well of the foil. A further absorption maximum is obtained below the plasma frequency due to resonance absorption.

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.

2D, low-frequency, relativistic solitary waves have been found recently in 2D. Particle-in-cell simulations of ultraintense laser pulses propagating in underdense plasmas, in regimes where the laser pulse undergoes energy depletion and downshifting of its frequency. These slowly propagating, spatially localized, electromagnetic structures represent an important channel of energy conversion from the laser pulse to the plasma. Pulse energy can also be converted into fast propagating structures, associated with collimated beams of ultrarelativistic electrons. Lienard–Wiechert magnetic field structures have been observed in PIC simulations to move together with focalized ultrarelativistic electron beams in the plasma.

A detailed investigation of many aspects of the physics of laser–plasma interaction at very high laser intensities is required in order to assess the feasibility and the promise of the fast ignitor scheme for inertial confinement fusion. Relevant results, obtained in a series of experiments carried out at the Rutherford Appleton Laboratory, Chilton (UK) and at the Centre d'Etudes Atomique, Limeil Valenton (France), are presented and discussed here. In particular, the formation of plasma channels was observed following the propagation of relativistically intense, ps laser pulses through underdense plasmas. The channels persist long after the interaction, and their expansion has been measured. Efficient guiding of ultraintense laser pulses, both through preformed density channels and through solid guides, has been demonstrated. Finally, indication of collimated fast electron propagation through solid targets has been obtained from the observation of filamentary ionization tracks in laser irradiated solid targets.

An investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a plasma is presented. Based on WKB and paraxial ray approximation the phenomenon of relativistic self-focusing (RSF) is analytically investigated. The differential equation for beamwidth parameter of rippled laser beam is evaluated. The ripple gets focused when the initial power of the ripple is greater than the critical power for focusing. The focusing is found to be considerably affected by the power of the main beam and the phase angle between the electric vectors of the main beam and the ripple. At higher intensities the saturation effects of nonlinearity become predominant, making the nonlinear refractive index in the paraxial region have slower radial dependence, and thus the ripple extract relatively less energy from its neighborhood. The case of magnetized plasmas is also preliminarily discussed.

Frequency doubling a 140 × 110 mm 40 J sub-ps 1054 nm beam for laser matter interaction studies was investigated at the Central Laser Facility, with efficiencies greater than 60% being achieved. The conversion characteristics (efficiency, beam quality, focusability, and pulse length) for two large 157 mm diameter aperture high quality KDP crystals of thickness 2 mm and 4 mm were studied. Using a fundamental drive beam of two times diffraction limited quality, focal spots of four times the diffraction limit in the frequency doubled beam were achieved and beam degradation effects shown to be minimal.

We investigate numerical effects related to “single-cycle” ionization of dense matter by an ultra-short laser pulse. The strongly nonadiabatic response of electrons leads to generation of a MG 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.

The nonlinear interaction of molecular matter with the intense field of a linearly-polarized laser beam, allows the study of not only the ionization dynamics of the parent molecule, but also the angular distribution of the exploding fragments. This is carried out by rotating the polarization vector of the laser with respect to the laboratory reference frame. The angular distributions for the CS2, CO2, N2O, H2S, and CH3I molecular ions, at intensities of about 1016 W cm−2, are presented. The distributions seem to be dependent on the molecule under consideration, but common features are that the peripheral atoms of the molecule are ejected along the ToF-axis, and the central atom perpendicularly to it, whenever the polarization vector and ToF-axis are collinear. It would seem that the distributions for the lighter of the molecules are partly due to alignment via dipole moments induced by the laser, as the distributions narrow as their charge-state increases. This is indicative of a larger torque acting on the higher-charged precursor molecular ion, via the interaction of the field with the laser-induced dipole moment. On the other hand, the angular dependence of the heavier molecules studied, are thought to originate from a dependence of the ionization/dissociation probability of the molecular ion on the initial angle made between the molecular axis and the polarization vector, that is, a preferential ionization/dissociation process. Spatial alignment in the laser pulse, in this case, is not thought to occur since the peaks do not narrow as the ionic charge increases. Finally, the results for H2S and also N2O are particularly interesting, since distributions for up to S7+ are presented, while the N-distributions show both a parallel and perpendicular component of the distribution. Neither of these results has, to the author's knowledge, been previously observed.

Calculations of the double ionization of a model 1D helium atom interacting with an intense short laser pulse are presented. A substantial signature of nonsequential double ionization is found for all the wavelengths that are considered, from 248 nm to 1064 nm, provided that sufficiently short pulses are employed. We show how one can modify the size of the “knee” structure of the double ionization yield curve by varying the laser pulse duration. The underlying mechanism of the correlated double electron ejection is investigated and discussed.

Detailed experiments on the energy and angular distributions of both electrons and protons formed from molecular hydrogen in strong femtosecond laser fields are reported. At the wavelengths 389 and 406 nm the ionization dynamics is governed by resonance shifting of molecular Rydberg states. Differences from this mechanism appear only at the very lowest intensities, when close lying resonances of the E, F-state determine the photoelectron pattern in resonant enhanced ionization, and at very high intensities, when tunnel ionization suppresses molecular details. At wavelengths below 400 nm at intensities in the range of ≈1·1013 W/cm2 femtosecond pulses are quite suitable for preparation of state selected H2+ targets in a restricted range of vibrational levels.

We present a numerical study of the stabilization process for a fully correlated two-electron model atom in an intense laser pulse. A comparison with calculations for a “real” 3D He atom is also done. We concentrate on the very high frequency regime, where the photon energy exceeds the ionization energy of both electrons, outer and inner. Our results show that when correlation effects are included the ionization probability (IP) is enhanced. Nevertheless, we still observe a decreasing IP within a certain intensity domain. The results from the fully correlated simulations are compared with those from simpler, approximate models. The full numerical treatment for the He atom is not yet possible. We therefore present results obtained with “single active electron” approximation and time-dependent density functional theory. Our numerical simulations can be useful for the future understanding of the stabilization phenomenon for more-than-one electron atoms.

Results for the energy spectra of the electrons scattered on a potential in the presence of a bichromatic field of frequencies ω and 2ω are obtained based on 1D numerical integration of the time dependent Schrödinger equation. Both the perturbative and the nonperturbative regimes are investigated for low scattering energies and a laser frequency that is close to that of CO2 lasers. Special attention is paid to the influence of the relative phase between the harmonic and the laser field on the features of the laser-assisted signals.

We present the results of ab initio, nonperturbative, R-Matrix Floquet calculations of laser-induced continuum structures in helium. We compare our results with recent experimental data and good agreement is found.

A simple model is proposed to study the field generated by a 3D plasma layer. The model neglects electron–electron interactions in the same way as occurs in the well known moving mirror models. However, the present model considers the radiation emitted by the moving electrons in a more precise way than in the mirror models, and allows the study of underdense or near critical plasmas. Results, including the transverse intensity profile of the driving field, are presented that show the possibility of generating trains of attosecond pulses.

We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated in a neon gas jet by a 30 fs Titanium:Sapphire pumping laser with intensities in the range 5–10 × 1014 W/cm2. The main feature of our observations concerns the possibility of continuously tuning the harmonic wavelength in the spectral region 20–7 nm, by taking advantage of the blue shift of harmonic wavelengths induced by the presence of free electrons to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift, which can be as large as 0.3–0.4 nm, are imparted to the given harmonic by simply changing the gas-jet-laser-beam-waist relative position. We have also interpreted our experimental results with a simple model for the generation process based on the “barrier suppression” ionization of an atom exposed to an ultraintense laser field.

We apply the average electron model to describe high harmonic generation in a collisional plasma in a laser field. The model is based on the coupled kinetic equations for electron velocity and electron temperature. The harmonic intensities are obtained from the Fourier coefficients of the nonlinear part of the electron velocity. Numerical calculations are reported for both strongly and weakly ionized plasmas in the nonstationary regime. We show the role of collisions frequencies as well as of quiver and thermal velocities in the harmonic generation efficiency.

Time-resolved photoemission spectroscopy, using high order harmonics, is used to measure the energy relaxation rate of hot electrons in α-SiO2 with sub-picosecond time resolution. Our results indicate that electrons of 30 eV kinetic energy in the conduction band relax at a rate which is at least two orders of magnitude lower than the one of photo-excited carriers of a few eV. As a result, we give insight in the relaxation process of hot electrons and show that impact ionization probability per unit time is only of the order of 1/40 ps−1, in very strong contrast with the much higher value generally assumed in models of optical breakdown.