A. Dubrouil

Post-compression of high-energy femtosecond pulses using gas ionization

We present a new optical post-compression technique designed for high-energy ultrashort pulses. A large spectral broadening is achieved through rapid ionization of helium by an intense pulse (>10(15) W/cm(2)) propagating in a capillary filled with low-pressure helium. The blueshifted pulses are re-compressed with chirped mirrors and silica plates. From a terawatt Ti:sapphire laser chain providing pulses of 40 fs, 70 mJ, we demonstrate the compression of pulses down to 11.4 fs (FWHM) with a total output energy of 13.7 mJ.

Spatial shaping of intense femtosecond beams for the generation of high-energy attosecond pulses

We generate high-order harmonics with a spatially shaped TW laser beam. We present and analyse in detail a new approach for shaping an intense laser field to a flat-top intensity profile near focus. We show that this approach is well adapted for high harmonic generation with high-energy fundamental pulses and highlight the possibilities for generating high-energy attosecond pulses.

Controlling high harmonics generation by spatial shaping of high-energy femtosecond beam

We demonstrate controlled high-order harmonic generation in gas using high-energy femtosecond pulses (50 fs-50 mJ on target) by performing spatial shaping of the terrawatt fundamental laser beam. We have developed a two optical paths mirror that can withstand high power and shape the pump beam into a quasi-flat-top profile (super Gaussian) near focus. We observe clear signatures of the spatial shaping on the harmonic beam in terms of profile, divergence, level of signal, and spectrum. The harmonic generation in neon with a quasi-flat-top beam results in a broadband extreme UV beam with extremely low divergence (~340 μrad).

Application of optical-field-ionization-induced spectral broadening in helium gas to the postcompression of high-energy femtosecond laser pulses

We report a detailed numerical study on the postcompression of high-energy (∼30 mJ) femtosecond laser pulses (∼40 fs), using the spectral broadening induced by optical-field ionization of a low-pressure helium gas in a capillary. Our numerical results are in very good agreement with previously published experimental data on spectral shapes, transmitted energies and recompressed pulse durations in the full range of laser energies and gas pressures investigated. Then, we calculate the performances of the method with shorter input pulses (∼20 fs) and demonstrate that few optical cycles recompressed pulses with 5 to 8.5 mJ energy could then be achieved.

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.

Spatio-temporal coupling and self-induced diffraction structures in XUV continua from harmonic generation with 10-mJ 10-fs post-compressed pulses

We generate high-order-harmonics with ultrashort high energy pulses (10-fs, 10-mJ) and perform single shot characterization of the XUV beam. We observe spatially structured continuous spectra and demonstrate direct observation of spatio-temporal coupling.

Optimization of high harmonic generation driven by an ytterbium-doped fiber chirped pulse amplification at a repetition of 100 kHz

High harmonic generation (HHG) in gas occurs when atoms are excited by an intense laser field. This technique enables to generate short coherent and directional extreme UV (XUV) radiation. Due to these unique properties, HHG presents a promising source for many applications such as time-dependent XUV spectroscopy. So far, mainly high energy systems such as Ti-sapphire laser amplifier have been used but are associated with a low repetition rate (a demonstration has been still reported at 100 kHz [1]). However, some applications require detecting in practice one event per 10 shots or less [2]. With low repetition rate systems, acquisition times may become unacceptable for recording exploitable statistics. Fiber laser-based HHG technology offers now the possibility to drive such experiments at high repetition rate. Indeed, it has been demonstrated that Yb-doped fiber chirped-pulse amplification (FCPA) systems can deliver both high average power and high energy subpicosecond laser pulses at higher repetition rate (100 kHz – 1 MHz) with good enough characterization for HHG [3,4]. Very high harmonics were recently observed despite moderate conversion efficiency [5].