Philippe Martin

Double plasma mirror for ultrahigh temporal contrast ultraintense laser pulses

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.

Formation of Self-Organized Regular Nanostructures upon Femtosecond Laser Ablation

At the bottom of ablation craters produced in many materials, e.g. dielectric and silicon crystals, by the impact of femtosecond laser radiation, regular periodic structures are observed with a feature size at the order of a few 100 nanometers, much smaller than the incident wavelength. Their orientation depends strongly on the laser polarization but not on any intrinsic crystalline parameters. An increasing number of shots results in higher contrast, better developed structures, indicating a positive feedback. The region around the impact is shown, by micro Raman spectroscopy, to undergo phase transformations like under high pressure. The structure spacing appears to depend crucially on the depth of the perturbed volume, i.e. the incident (and absorbed) energy. All observations suggest that the structures form by self-organization from instabilities induced in the material by the laser input. A general picture suggests that the irradiation results in a rapid, non-equilibrium destabilization of the crystal structure, which should not be confused with melting as a classical thermodynamic process (i.e. temperatures defined as equilibrium properties). Relaxation from this instability results in the self-assembly of the observed structures. Theoretical simulations demonstrate the feasibility of this model, which also is corroborated by comparison to other unstable situations.

Proton Acceleration With High-Intensity Laser Pulses in Ultrahigh Contrast Regime

We investigate the interaction of a high-intensity (~5.1018 W/cm2) and short (~65 fs) laser pulse with thin foils (from 0.08 to 105 mum) in a regime of ultrahigh contrast (> 1010). This paper shows that for thicknesses less than about 10 mum, proton acceleration from both sides of the target presents quite symmetric features. Proton bunches emitted from each side show similar maximum energies and spatial characteristics. Moreover, we show that for ultrahigh-contrast pulses, the efficient acceleration mechanism is related to the Brunei effect and not to the ponderomotive force. Simulations performed with a 2-D particle-in-cell code are in close agreement with all experimental data.

NEURAL NETWORKS FOR THE RECOGNITION OF ENGRAVED MUSICAL SCORES

The image analysis levels of a recognition system for engraved musical scores are described. Recognizing musical score images requires an accurate segmentation stage to isolate symbols from staff lines. This symbols/staves segregation is achieved by the use of inscribed line (chord) information. This information, processed by a multilayer perceptron, allows an efficient segmentation in terms of the remaining connected components. Some of these components are then classified, using another network, according to a coding of their skeleton graph. Special attention is paid to the design of the networks: the architectures are adapted to the specificities of each task. Multilayer perceptrons are employed here together with other more classical image analysis techniques which are also presented.

Interaction of an intense laser field with a dielectric containing metallic nanoparticles

In order to understand the role played by nanodefects in optical breakdown of dielectrics, the interaction of an intense laser field with model dielectric samples containing metallic nanoparticles is studied both theoretically and experimentally. A theoretical study of the metal conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conduction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time-resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.

Utilization of a plasma mirror for the production of high-order harmonics from a planar surface

We investigate the harmonics generation from a pure dielectric target when submitted to laser intensities in the 1018W/cm2. We demonstrate the negative influence of the prepulses and ASE by addressing the direct comparison of the harmonic spectra with and without the introduction of a perfectly controlled plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free of any plasma expansion.

Subpicosecond studies of carrier dynamics in laser-induced breakdown

In this paper, we present measurements of the excited carrier density in various wide band gap oxides irradiated by short laser pulses, at intensities below and above breakdown threshold. This is achieved with the help of time resolved interferometry in the frequency domain, a technique which was successfully used to study the dynamics of photoexcited carriers in insulators. The result obtained in different experimental conditions, distance from the surface, pump intensities and duration, during or after the pump pulse, are discussed and compared to the models recently developed to explain optical breakdown.

Laser-dielectric interactions

We present the results of recent experiments concerning the interaction of high intensity-short laser pulses with dielectric materials. The laser pulse creates a high density of free carriers, and two complementary experiments have been carried out to investigate the relaxation mechanisms in the solid, during and just after the excitation pulse. The first is a photoemission experiment, in which we measure the kinetic energy distribution of photoelectrons emitted by the surface of quartz ((alpha) -SiO2) samples irradiated by femtosecond laser pulses. We observe a high energy tail in the photoelectron spectra, which extend far beyond the photon energy. This shows that electrons photoexcited in the conduction band can absorb multiple photon and that this laser heating process is efficient even with sub-picosecond pulses. In the second experiment, we observe interferences in the frequency domain between two probe pulses. This technique allows us to measure the density of electrons photoexcited by a high intensity pump pulse in the conduction band within the sample, with a temporal resolution of 120 fs. The lifetime of the carriers is found to be 50 ps in MgO, 100 ps in Al2O3, and 150 fs in SiO2. We explain such a contrasted behavior in different oxides by the ultrafast formation of intrinsic defects in SiO2.

Laser-Driven Ion Generation with Short, Intense, and High Contrast Pulses

About 10 years ago, first experimental works on high intensity( > 1018 W cm − 2) laser-driven proton acceleration showed the great potential of these ions bunches for different application domains (as, for instance, high resolution radiography, isochore heating, and fast ignition). Moreover, the prospect of a future exploitation of such high energy ions in hadron-therapy certainly further incited the scientific research in this domain. One of the goal of the present research lies in increasing the peak proton energy as this is a critical point for a number of applications. Reducing the target thickness is one of the suggested ways to get higher peak proton energies. Nevertheless, that can be realized using only very high contrast laser pulses.

Control of Relativistic and Non-Relativistic High-Harmonic Generation from Overdense Laser Plasmas

High harmonic generation from ultra-intense laser-matter interaction can be generated by both linear and relativistic means. In experiments with intensity up to a few 10 19 Wcm -2 , we show the distinctions and means to control each.