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Collective processes in plasmas often induce microinstabilities that play an important role in many space or laboratory plasma environments. Particularly notable is the Weibel-type current filamentation instability, which is believed to drive the creation of collisionless shocks in weakly magnetized astrophysical plasmas. Here, this instability class is studied through interactions of ultraintense and short laser pulses with solid foils, leading to localized generation of megaelectronvolt electrons. Proton radiographic measurements of both low- and high-resistivity targets show two distinct, superimposed electromagnetic field patterns arising from the interpenetration of the megaelectronvolt electrons and the background plasma. Particle-in-cell simulations and theoretical estimates suggest that the collisionless Weibel instability building up in the dilute expanding plasmas formed at the target surfaces causes the observed azimuthally symmetric electromagnetic filaments. For a sufficiently high resistivity of the target foil, an additional resistive instability is triggered in the bulk target, giving rise to radially elongated filaments. The data reveal the growth of both filamentation instabilities over large temporal (tens of picoseconds) and spatial (hundreds of micrometres) scales.

Les processus dynamiques se produisant lors de transitions de phase ultra-rapide peuvent être déduits à partir de mesures de diffraction ou d'absorption de rayonnement X. Les lasers femtosecondes ont récemment été utilisés pour étudier la dynamiques de la matière au moyen d'une pompe optique et d'une sonde X : du rayonnement X K alpha produit par interaction laser plasma. Nous présentons nos plus récents résultats concernant le développement d'un sytème de spectroscopie d'absorption du rayonnement X (XAS) basée sur une source laser-plasma large bande dans la gamme 1-5 nm permettant d'atteindre une résolution temporelle femtoseconde. Le système est conçu pour sonder les dynamiques électroniques ayant lieu durant la transion de phase semiconducteur-métal du dyoxide de vanadium (VO2) lorsque celle-ci est initiée par une impulsion laser femtoseconde. Dans la présente expérience, un spectre large bande proche du seuil L du vanadium (511 eV) et du seuil K de l'oxygène (525 eV) du VO2 a été généré et mesuré avec un haut rapport signal sur bruit (100), une grande résolution spectrale ($\Delta {\rm E/E} = 4.2\times 10^{-3})$, et une résolution temporelle de 1,2 ps.

We have compared the acceleration of high-energy ions from the rear-surface of thin foils for various contrast conditions of the ultra-intense laser pulse irradiating the targets. The experiments were performed using the LULI 100 TW facility. We used Al targets of variable thicknesses and the laser pulse contrast ratio ahead of the main pulse was varied using either a fast Pockels cell or a single or double plasma mirror. The latter was installed at an intermediate field position, in between the focusing optics and the target, so that its effect was optimized. By improving with these two methods the laser pulse contrast, we have observed that we could significantly reduce the thickness of the target used for proton acceleration and at the same time increase both the cut-off energy of the accelerated protons and the energy conversion efficiency of the process.

Focusing a high intensity laser pulse, onto a thin foil target generates a plasma and energetic proton and ion beams from the target rear and front sides, propagating along the target normal. Such laser produced collimated and energetic protons beams are of high interest because of the wide range of applications: ion based fast ignitor schemes, probing of electromagnetic fields in plasma, isotopes production or hadron therapy. The 100 TW class laser system at the Advanced Laser Light Source facility, is used with an intensity close to 1019W/cm2, to study protons acceleration with femtosecond laser pulses, ultra thin foil target and high contrast laser pulse ratio. To characterize the plasma expansion, we monitor it with an imaging technique using a femtosecond laser probe. In this configuration we were able to reach a proton critical energy of 12 MeV and to work with target foil thickness as small as 15 nm.

We ionized He, Ne and Ar in the tunnelling regime with a linearly polarized laser pulse and measured the ion-recoil momentum distribution on an axis perpendicular to laser polarization for two different wavelengths (? = 800 nm and ? = 1800 nm). We observed a significantly narrower distribution for 800 nm than for 1800 nm. Classical simulations of the electron wave packet's evolution under the combined influence of the ion's Coulomb potential and the laser field show that the narrowing is caused by Coulomb focusing of the electron wave packet during recollisions. The narrowing is sensitive on the longitudinal momentum of the wave packet after tunnelling, which suggests a way of measuring it. Measurements at 800 nm in circular polarization-for which no recollision occurs-do not exhibit this narrowing of the perpendicular momentum distribution. Simulations for circular polarization show another aspect of Coulomb focusing: the Coulomb potential also affects the wave packet's transverse momentum as it leaves the saddle point immediately after tunnelling.

Time-resolved visible spectroscopy of plasmas produced by laser breakdown of air using femtosecond Nd laser pulses (50–300 mJ, 500 fs) reveals features not observed with nanosecond laser pulses. Emission is initially dominated by molecular lines, specifically the second positive system of N2and the first negative system of N2+. This is followed by continuum emission with a growth time of ∼3 ns and a decay time of ∼30 ns. Atomic lines of N and O emerge from the decay of the continuum and last up to 1 μs; only faint ionization lines are observed. Several of the atomic lines are initially strongly broadened, narrowing over a period of 100 ns.

Results on the deposition of aluminum nitride thin films on silicon (111) substrates by pulsed laser deposition are reported. A KrF excimer laser was used to ablate in vacuum an AlN target. The intensity and the repetition rate of the laser as well as the substrate temperature were varied and their effects on film properties were determined. The AlN films were characterized by several techniques such as in situ laser interferometry, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and transmission electron microscopy. At a deposition temperature of 500 $^{\circ}$C and a laser intensity of $1\times 10^8$ W/cm$^2$, highly oriented AlN layers are deposited. Increasing the laser intensity leads to the presence of metallic Al in the films. As temperature is increased from 500 to 920 $^{\circ}$C, the growth rate is found to decrease but the crystalline structure as well as the smoothness of the films are both significantly improved. It is also found that a reduction of the growth rate by varying the repetition rate from 30 to 10 Hz leads to a further improvement of the crystalline quality of the AlN layers.

Results on the deposition of aluminum nitride thin films on silicon (111) substrates by pulsed laser deposition are reported. A KrF excimer laser was used to ablate in vacuum an AlN target. The intensity and the repetition rate of the laser as well as the substrate temperature were varied and their effects on film properties were determined. The AlN films were characterized by several techniques such as in situlaser interferometry, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and transmission electron microscopy. At a deposition temperature of 500 °C and a laser intensity of 1 × 108W/cm2, highly oriented AlN layers are deposited. Increasing the laser intensity leads to the presence of metallic Al in the films. As temperature is increased from 500 to 920 °C, the growth rate is found to decrease but the crystalline structure as well as the smoothness of the films are both significantly improved. It is also found that a reduction of the growth rate by varying the repetition rate from 30 to 10 Hz leads to a further improvement of the crystalline quality of the AlN layers.

We have used a novel beam-averaging technique that makes use of a segmented wedge array (SWA) to overlap several line foci and to produce a much more uniform linear plasma than has been obtained to date. With this focusing arrangement, laser gain was observed for the first time to our knowledge on a 2s2p^63d–2s2p^63p, J = 2–1 Ne-like transition at 198.99 Å. The experiments were carried out on the LP2 Nd:glass laser system operating at 1.06 μm with incident energies of ~125 J/cm of line focus. We also observed gain on five 3p–3s transitions, using both the SWA and crossed cylindrical lenses to produce the line focus.

We describe a technique for spectrally and temporally resolving the kilo-electron-volt emission from ultrashort plasmas produced from solid targets with a tabletop terawatt 400-fs laser. The first time-resolved Al spectra (near 8 Å) obtained with a 2-ps time resolution are presented. The results clearly demonstrate that the resonance emission width decreases as the plasma density increases. The ultrafast K_α emission component is also measured in our experimental conditions.

Little-exploited properties of silver halide films are used to record the spatial distribution of 10.6 μm-CO_2 laser pulses with a dynamic range of at least 20. The IR radiation is shown to sensitize commonly available silver halide films to subsequent visible radiation. A permanent, high-resolution image is thus formed of radiation too low in energy to eject directly photoelectrons. The method is calibrated for the film found most useful for this technique.

Dynamic wave-front correction is applied before each shot on a 100-TW, 30-J/300-fs high-power laser facility by use of an adaptive-optics system. This system allows us to increase the repetition rate of high-energy lasers while maintaining excellent and constant beam focusability with a Strehl ratio of >0.75 despite the amplifiers’ not being in thermal equilibrium. The best results in terms of the highest Strehl ratio and intensities are obtained when locking the system on wave-front sensing after pulse recompression.

A novel interferometry technique is presented by which, in one shot, one can measure phase changes with a resolution of tens of femtoseconds while extending the measurement over picoseconds or even longer. The method is based on spectral (frequency-domain) interferometry with a pair of linearly chirped pules as probes. With this technique we obtained single-shot measurements of the rapid phase changes induced by optical field ionization of air. This allowed us to calculate the time profile of the electron density created by an intense short laser pulse.

A convenient method is described for optical characterization of thin films during growth. The method has been demonstrated on lead zirconate titanate (PZT) films deposited by pulsed laser ablation for various temperatures. The optical constants of the PZT films as well as the film growth rate were determined in situ by fitting (with three free parameters) the calculated reflectance as a function of film thickness to the experimental reflectance curve as a function of deposition time, as obtained by unpolarized laser reflectometry. The fitted parameters are the uniform complex PZT refractive index and the layer thickness (assumed proportional to time), with the complex refractive index of the platinum substrate being measured previously. These results compare well with the subsequent ellipsometric measurements made to assess the precision of the reflectometry technique.