Jean-Paul Geindre

Femtosecond time-resolved X-ray diffraction from laser-heated organic films

The extension of time-resolved X-ray diffraction to the subpicosecond domain is an important challenge, as the nature of chemical reactions and phase transitions is determined by atomic motions on these timescales. An ultimate goal is to study the structure of transient states with a time resolution shorter than the typical period of vibration along a reaction coordinate (around 100 fs). Biological processes that can be initiated optically have been studied extensively by ultrafast infrared, visible and ultraviolet spectroscopy. But these techniques probe only electronic states, whereas time-resolved crystallography should be able to directly monitor atomic positions. Here we show that changes in the X-ray diffraction pattern from an organic film heated by a laser pulse can be monitored on a timescale of less than a picosecond. We have studied the response of a Langmuir–Blodgett multilayer film of cadmium arachidate to laser heating by observing changes in the intensity of one Bragg peak for different delays between the perturbing optical pulse and the X-ray probe pulse. A strong decrease in intensity is seen within a picosecond of heating, resulting from disorder introduced to the layers of cadmium atoms before thermal expansion of the film (which ultimately leads to its destruction) has time to occur.

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.

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.

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.

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.

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.

Subpicosecond laser-produced plasma dynamics

To simulate the interaction of high laser intensity with solid targets, we have used the 1D code FILM in which the collisional plasma absorption is calculated by solving the linear electromagnetic field for p and s polarization. For p-polarized light the collision frequency is adjusted so that the field in the critical region of the plasma never exceeds the maximum field allowed by the wave breaking limit. Energy transport by thermal conduction is described with the help of the delocalized heat flux theory. The ponderomotive force resulting from the huge filed is taken into account. The calculated temperatures and ion densities are used as an input to a time-dependent atomic physics code. Non-stationary ionization dynamics is demonstrated.

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.

Time-resolved x-ray diffraction with subpicosecond x-ray pulses

The emission from plasmas created with fs-lasers provides sub-picosecond x-ray pulses in the keV-range. Intense emission of K(alpha) lines as well as quasi continuum x-rays can be used for time-resolved diffraction and spectroscopy, i.e. to study lattice or atomic dynamics with sub-picosecond resolution by using a laser pump x-ray probe technique. The x-ray yield and x-ray pulse duration of the laser plasma source depend on the laser parameters and the target design, such as intensity, laser wavelength, pulse duration and prepulse level. To accumulate as many photons as possible of the isotropic source an efficient large aperture optic has to be used to select an x-ray line or a wavelength range and focus the radiation onto the sample. It is shown that the use of toroidally bent crystals provides the possibility to refocus 10-4 of the photons emitted in the whole solid angel to spot size of around 80 micrometers with a temporal broadening below 100 fs. Combinations of bent focusing crystals with a flat sample crystal for fast x-ray diffraction application are discussed. Experiments showing the temporal response of laser heated crystals are presented and compared with theoretical simulations based on Takagi-Taupin theory.