Jean-Claude J. Gauthier

High-order harmonic generation by nonlinear reflection of an intense high-contrast laser pulse on a plasma

We demonstrate the use of a plasma mirror to obtain 60-fs 10-TW laser pulses with a temporal contrast of 10(8) on a nanosecond time scale and 10(6) on a picosecond time scale, and we use these high-contrast pulses to generate high harmonics by nonlinear reflection on a plasma with a steep electronic density gradient. Well-collimated harmonics up to 20th order are observed for a laser intensity of approximately equal to 3 x 10(17) W/cm2, whereas no harmonics are obtained without the plasma mirror.

Opacity Studies of Iron in the 15-30eV Temperature Range

Absorption of the 2p-3d transitions of iron has been measured using point projection spectroscopy. Thin C tamped Fe foils were heated around 20 eV by X-rays generated in gold spherical hohlraums irradiated by the high-power laser ASTERIX IV. Absorption of Fe V to Fe X has been observed in the spectral vicinity of 730 eV (17 A). The Ag backlighter source and absorbed spectra were recorded on the same shot by a TlAP crystal spectrograph. The experimental spectra have been reproduced by the two superconfiguration local thermodynamic equilibrium codes SCO and STA. Detailed statistical calculations of the different ionic structures have also been performed with the Spin Orbit Split Arrays method, allowing the determination of ion populations. The electron temperature and average ionization obtained by fitting the experiment with the different calculations were compared with radiative hydrodynamic simulations.

Femtosecond Laser-Produced Plasma X-Rays from Periodically Modulated Surface Targets

We have studied theoretically and experimentally the x-ray production above 1 keV from femtosecond laser plasmas generated on periodically modulated surface targets. Laser energy coupling to plasma surface waves has been modeled using a numerical differential method. Almost total absorption of incident laser radiation is predicted for optimized interaction conditions. Silicon gratings have been irradiated by a 120 fs Ti: sapphire laser at irradiances in excess of 1016W/cm2. X-ray intensities above 1.5 keV (K-shell lines) have been measured as a function of the incidence angle. Results show a distinct x-ray emission maximum for the first order diffraction angle and are in good qualitative agreement with our theoretical predictions.

Characterization of a femtosecond-laser-produced plasma x-ray source by electronic, optical, and x-ray diagnostic techniques

Short-pulse laser-produced plasmas look very promising for the generation of sub-picosecond X-rays. By combining several experimental techniques, we have significantly progressed towards a better understanding of ultrafast laser-matter interaction. The X-ray yield is a sensitive function of the electron density gradient scale length of the target plasma. In this work, the scale length has been changed by varying the temporal separation between the main laser pulse and a lower intensity prepulse. X-ray spectroscopic diagnostics of the plasma parameters have been used from the analysis of resonance and dielectronic satellite lines. The angular and energy distribution of suprathermal electrons emitted during the ultrafast laser- plasma interaction have been measured as a function of laser polarization and prepulse delay. Frequency-domain interferometry and optical measurements of the reflected probe pulse have been used to study the velocity and the gradient scale length of the expanding plasma. The Kα emission yield peaks for a scale length where resonant absorption is optimized. Hydrodynamic simulations have been performed to investigate the plasma dynamics and the basic processes which control the X-ray emission duration and intensity. Applications of ultrashort Kα X-rays to the diagnostic of solid plasma conditions and as a source for time-resolved diffraction and spectroscopy of transient chemical, biological or physical phenomena are underway.

Ultrafast X-ray sources and applications

Abstract The recent development of ultrafast X-ray sources makes conceivable the analysis of subpicosecond transient structures. This paper describes these new techniques and reports the first experiments dedicated to the analysis of atomic motions on this time-scale.

Subpicosecond dynamics of intense laser-cluster interaction: keV x rays and highly charged ion production

We have performed studies of keV x-ray production from (formula available in paper) rare gas clusters submitted to intense IR laser pulses. Up to (formula available in paper) per pulse at a moderate atomic density have been observed. High resolution spectroscopy studies in the case of (formula available in paper) clusters have also been performed, gibing unambiguous evidence of highly charge ions with K vacancies production. We have determined the photon energies and the absolute photon emission yields as a function of several physical parameters governing the interaction: size and atomic number of the clusters, peak intensity of the laser. Unexpectedly low laser intensity thresholds have been measured. The result obtained indicate nevertheless that x-rays may be emitted before cluster explosion on a subpicosecond time scale, and that several mechanisms must be involved in the first stage of the production of the hot nanoplasma induced from each cluster.

Impulse generated by a laser-produced x-ray source on a metallic layer

This study deals with the mechanic impulse transmitted to a solid target by soft x rays. We have measured the impulse transmitted to thin aluminum layers by x-ray radiation around 1.2 keV produced by a laser-irradiated copper target. The results are compared to radiative hydrocode simulations with the code XRAD.

Dense Ultrafast Plasmas

The interaction of high power lasers with plasmas has been an active field of research for over 30 years. Motivated primarily by inertial confinement fusion1 and x-ray laser research,2 most studies have used nanosecond pulse gas lasers — CO2 lasers in the early days, then excimer (KrF and XeCl) lasers — and solid state Nd:glass lasers, eventually frequency up-converted to 0.53µm (2ω0), O.34µm (3ω0), and O.26µm (4ω0). Laser-plasma interaction physics has been studied extensively to explore the efficiency of collisional absorption by inverse bremsstrahlung, parametric instability growth rates,3, 4 filamentation, x-ray conversion efficiency and hydrodynamic instabilities.5Numerous other applications of laser-produced plasmas do exist.6The development of bright incoherent short wavelength radiation7 is a major research topic that is being developed for numerous purposes,8ncluding x-ray sources for x-ray lithography around l0nm wavelength and hard (hν > 1keV) x-ray sources for material characterization and solid-state physics. Laser-driven shock wave research for equation of state measurements has also important developments in planetary research, geophysics and astrophysics.

Intense ion beams accelerated by relativistic laser plasmas

We have studied the influence of the target properties on laser-accelerated proton and ion beams generated by the LULI multi-terawatt laser. A strong dependence of the ion emission on the surface conditions, conductivity, shape and material of the thin foil targets were observed. We have performed a full characterization of the ion beam using magnetic spectrometers, Thompson parabolas, radiochromic film and nuclear activation techniques. The strong dependence of the ion beam acceleration on the conditions on the target back surface was found in agreement with theoretical predictions based on the target normal sheath acceleration (TNSA) mechanism. Proton kinetic energies up to 25 MeV have been observed.

Femtosecond time-resolved x-ray diffraction with a laser-produced plasma x-ray source

Optical pump, x-ray diffraction probe experiments have been used to study the lattice dynamics of organic materials using a laser-produced plasma x-ray source. The x-ray source is generated from a 10 Hz, 26 mJ, 120 fs laser beam focused on a silicon wafer target. The emitted K(alpha ) x-ray radiation is used to probe a cadmium arachidate Langmuir-Blodgett film and a TlAP crystal optically perturbed at laser fluences from 1.8 J/cm2 to 27 J/cm2. Ultrafast disordering inside the lattice -- within a time scale below 600 fs to few tens of picoseconds -- is clearly observed and produce a drop of the probe x-ray diffracted signal.