F. Burgy

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20–26u2009eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

Validation of modelled imaging plates sensitivity to 1-100 keV x-rays and spatial resolution characterisation for diagnostics for the “PETawatt Aquitaine Laser”

Thanks to their high dynamic range and ability to withstand electromagnetic pulse, imaging plates (IPs) are commonly used as passive detectors in laser-plasma experiments. In the framework of the development of the diagnostics for the Petawatt Aquitaine Laser facility, we present an absolute calibration and spatial resolution study of five different available types of IP (namely, MS-SR-TR-MP-ND) performed by using laser-induced K-shell X-rays emitted by a solid silver target irradiated by the laser ECLIPSE at CEntre Lasers Intenses et Applications. In addition, IP sensitivity measurements were performed with a 160 kV X-ray generator at CEA DAM DIF, where the absolute response of IP SR and TR has been calibrated to X-rays in the energy range 8-75 keV with uncertainties of about 15%. Finally, the response functions have been modeled in Monte Carlo GEANT4 simulations in order to reproduce experimental data. Simulations enable extrapolation of the IP response functions to photon energies from 1 keV to 1 GeV, of interest, e.g., for laser-driven radiography.

High-order harmonic transient grating spectroscopy of SF6 molecular vibrations

Strong field transient grating spectroscopy has shown to be a very versatile tool in time-resolved molecular spectroscopy. Here we use this technique to investigate the high-order harmonic generation from SF6 molecules vibrationally excited by impulsive stimulated Raman scattering. Transient grating spectroscopy enables us to reveal clear modulations of the harmonic emission. This heterodyne detection shows that the harmonic emission generated between 14 and 26xa0eV is mainly sensitive to two among the three active Raman modes in SF6, i.e. the strongest and fully symmetric υ1-A1g mode (774xa0cm−1, 43 fs) and the slowest mode υ5-T2g (524xa0cm−1, 63 fs). A time-frequency analysis of the harmonic emission reveals additional dynamics: the strength and central frequency of the υ1 mode oscillate with a frequency of 52xa0cm−1 (640 fs). This could be a signature of the vibration of dimers in the generating medium. Harmonic 11 shows a remarkable behaviour, oscillating in the opposite phase, both on the fast (774xa0cm−1) and slow (52xa0cm−1) timescales, which indicates a strong modulation of the recombination matrix element as a function of the nuclear geometry. These results demonstrate that the high sensitivity of high-order harmonic generation to molecular vibrations, associated to the high sensitivity of transient grating spectroscopy, make their combination a unique tool to probe vibrational dynamics.

Combined high-harmonic interferometries for vectorial spectroscopy

We present a new method to characterize transverse vectorial light produced by high-harmonic generation (HHG). The incoherent sum of the two components of the electric field is measured using a bi-dimensional transient grating while one of the components is simultaneously characterized using two-source interferometry. The combination of these two interferometric setups enables the amplitude and phase measurement of the two vectorial components of the extreme ultraviolet radiation. We demonstrate the potential of this technique in the case of HHG in aligned nitrogen, revealing the vectorial properties of harmonics 9-17 of a Ti:sapphire laser.

Near infrared few-cycle pulses for high harmonic generation

We report on the development of intense tunable few-cycle pulses with central wavelengths ranging from to . These pulses were used as a proof of principle for high harmonic generation in atomic and molecular targets. In order to generate such pulses, we produced a filament in a four-bar krypton cell. Spectral broadening by a factor of two to three of a 40 fs near-infrared input pulse was achieved. The spectrally broadened output pulses were then compressed by fused silica plates down to the few-cycle regime close to the Fourier limit. The auto-correlation of these pulses revealed durations of ~ three cycles for all investigated central wavelengths. Pulses with a central wavelength of and up to energy per pulse were employed to generate high-order harmonics in Xe, Ar, and N2. Moving to near-infrared few-cycle pulses opens the possibility of operating deeply in the non perturbative regime with a Keldysh parameter, . Hence, this source is suitable for the study of the non perturbative tunneling regime in most generating systems used for high-order harmonic generation and attoscience.

Proton acceleration measurements using fs laser irradiation of foils in the target normal sheath acceleration regime

We present experimental results obtained at the CELIA laboratory using the laser ECLIPSE to study proton acceleration from ultra-intense laser pulses. Several types of targets were irradiated with different laser conditions (focusing and prepulse level). Proton emission was characterized using time-of-flight detectors (SiC and diamond) and a Thomson parabola spectrometer. In all cases, the maximum energy of observed protons was of the order of 260u2009keV with a large energy spectrum. Such characteristics are typical of protons emitted following the target normal sheath acceleration mechanism for low-energy short-pulse lasers like ECLIPSE.We present experimental results obtained at the CELIA laboratory using the laser ECLIPSE to study proton acceleration from ultra-intense laser pulses. Several types of targets were irradiated with different laser conditions (focusing and prepulse level). Proton emission was characterized using time-of-flight detectors (SiC and diamond) and a Thomson parabola spectrometer. In all cases, the maximum energy of observed protons was of the order of 260u2009keV with a large energy spectrum. Such characteristics are typical of protons emitted following the target normal sheath acceleration mechanism for low-energy short-pulse lasers like ECLIPSE.

On the complex structures of spatially resolved XUV high-order-harmonic spectra

We study the far field spatial profiles of high order harmonics generated in gases with a high spectral resolution. It allows us to observe simultaneously several harmonics under diverse conditions including single shot imaging with spatio-spectral resolution at low and high intensities. This study gives access to the origin of the spatiospectral coupling appearing in the far field spatially resolved high-order-harmonic spectra.

D+D fusion reactions in 1018 W/cm2 intensity and repetitive laser-plasma interactions

We studied D + D fusion reactions at the CELIA/ECLIPSE laser facility. These reactions were triggered by focusing the ECLIPSE laser (up to 100 mJ on target, pulse duration of 40 fs and focal spot of FWHM) on a solid C2 D4 target. By detecting the 2.5 MeV neutrons, produced in the D + D fusion reaction, using C6 D6 scintillators in a time-of-flight experimental set-up, we measured a rate of a few hundred fusions per shot. At the same time, we detected also energetic ions (up to a few 100 keV) using time-of-flight diamond detectors. The experimental set-up will be presented and the results discussed and compared with dedicated simulations.

Aneutronic Nuclear Fusion Experiments by Laser-Plasma Energetic Particles

Protons at energies up to a few 100keV have been accelerated, by irradiating thin Al foils with an intense, fs laser pulses. The produced energetic protons, directed on a massive natural B sample, induced (Error rendering LaTeX formula) B fusion reactions accompanied by alpha particles emission, detected by a CR39 plate.

The GEMpix detector as new soft X-rays diagnostic tool for laser produced plasmas

Laser produced plasmas lend to several interesting applications. The study of X-ray emission from this kind of plasmas is important not only to characterize plasmas itself but also to study the application of these particular plasmas as intense X-ray sources. In particular several emission configurations can be obtained using different kinds of targets and tuning the characteristics of the laser pulse delivered to the target. Typically, laser pulse duration ranges between a few tens of femtoseconds and tens of nanoseconds, with energies from few mJ to tens of kJ. X-ray photon emissions last for times comparable to the laser pulses and during this time a great number of photons can be emitted. The following paper presents a measure of the soft-X-ray emission on the ECLIPSE laser facility realized with a new triple-GEM gas detector (GEMpix). It is a hybrid gas detector with a C-MOS front-end electronics based on Medipix chips. In the present work, different targets have been used in order to test X-rays of different energies. In this paper, in particular, we present results obtained for copper and iron targets. GEMpix is able to realize a 2D imaging of the X-ray emission from plasma with a signal proportional to the energy released in the gas of the detector active volume. Then through a preliminary single photon equalization realized at the NIXT lab (ENEA), also the number of photons reaching the area of the detector has been estimated.