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We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity (approximately 5 x 10(18) W/cm(20), ultrahigh contrast (>10(10)) laser pulse interacts with foils of thickness ranging from 0.08 to 105 microm. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the p component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.

Two-color two-photon femtosecond ionization experiments have been performed on NaI. The wave packet evolution of the A excited state has been followed by detecting photoions and photoelectrons. The results indicate that the Na+ ions are formed when the wave packet is located at the outer turning point of the excited state. Surprisingly, the NaI+ ions are also observed to be in phase with the Na+ signal. Photoelectron spectra show that high kinetic energy electrons are produced when ionizing around the outer turning point, in agreement with the NaI+ formation. The absence of signal corresponding to ionization from the covalent part of the excited state potential can only be understood if the absolute ionization cross section is much smaller in the covalent region of the A state (where the molecule can be considered as a van der Waals complex) than in the ionic Na+···I- part of the A state potential (where the interatomic distance is such that the ionization process may be considered as a photodetachment of the electron from I- anion). Simulations taking into account that ionization occurs only when the wave packet is in the ionic region of the A state are in good agreement with experimental data.

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.

A general approach for optically controlled spatial structuring of overdense plasmas generated at the surface of initially plain solid targets is presented. We demonstrate it experimentally by creating sinusoidal plasma gratings of adjustable spatial periodicity and depth, and study the interaction of these transient structures with an ultraintense laser pulse to establish their usability at relativistically high intensities. We then show how these gratings can be used as a "spatial ruler" to determine the source size of the high-order harmonic beams produced at the surface of an overdense plasma. These results open new directions both for the metrology of laser-plasma interactions and the emerging field of ultrahigh intensity plasmonics.

A gamma-ray source with an intense component around the giant dipole resonance for photonuclear absorption has been obtained via bremsstrahlung of electron bunches driven by a 10-TW tabletop laser. 3D particle-in-cell simulation proves the achievement of a nonlinear regime leading to efficient acceleration of several sequential electron bunches per each laser pulse. The rate of the gamma-ray yield in the giant dipole resonance region (8

We demonstrate that frequency-domain interferometry can be performed in the extreme ultraviolet range using high-order harmonics. We first show that two phase-locked harmonic sources delayed in time can be generated in the same medium despite ionization. This gives insight into the dynamics of the generation and ionization processes. We then apply the technique to the study of the temporal evolution of an ultrashort laser-produced plasma at the femtosecond time scale. PACS numbers: 42.65.Ky, 32.80.Rm Recently, high-order harmonic generation (HHG) has attracted considerable interest, not only as an intriguing and spectacular phenomenon, but also as a potential source presenting unique properties in the extreme ultraviolet (XUV) range. The two main characteristics of this radiation are the ultrashort pulse duration and the good coherence that are both unprecedented in this spectral region. The short pulse duration (down to a few tens of femtosecond) has been characterized [1-3] and is already used in atomic and molecular spectroscopy [4]. On the other hand, a number of experiments in the last few years have shown that good coherence properties could be obtained in some generating conditions [5][6][7][8] but no applications of this coherence have been performed yet. XUV interferometry, developed so far with x-ray lasers [9-12], would benefit a lot from the HHG properties. First, the tunability of the radiation allows one to adapt the wavelength to the probed medium, for example, far from resonances. In plasmas, different electron densities can be probed by changing the harmonic order, leading to a precise density mapping. Note that beside this coarse tunability, a fine adjustment of the wavelength can be obtained by generating the harmonics with a chirped laser [13] or with a laser mixed with an optical parametric amplifier [14]. Second, the ultrashort harmonic pulse duration is well adapted to the study of ultrafast processes, and could prevent, for example, the blurring of the fringes due to fast evolution of the density profile of a plasma close to the critical surface, as is the case when using x-ray lasers [9]. Finally, HHG is a tabletop XUV source, naturally synchronized with the generating laser at the same repetition rate (10 Hz to 1 kHz) allowing systematic experiments. In this work, we perform frequency-domain interferometry with high-order harmonics. We first demonstrate that the technique can be transposed to harmonics, and then apply it to probe the electron density of a laser-produced plasma in a high density gas jet with a femtosecond temporal resolution.Frequency-domain interferometry is now a widely used technique in the infrared for femtosecond time-resolved studies in solid-state and plasma physics [15][16][17]. It can be described as the temporal analog of the Young two-slit experiment [18]. In the latter, two spatially separated phase-locked sources lead to interferences in the far field after the beams have diffracted. In the former, two temporally separated phase-locked pulses interfere in ...