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Papers by Aurelien Houard

New Journal of Physics, 2021

Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent ex... more Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800 nm. The mechanism responsible for the lasing is highly controversial. In this article we propose a model explaining the delayed lasing, which contains two parts: (i) ionization of neutral nitrogen molecules and subsequent excitation of nitrogen ions in a strong pump laser pulse, and (ii) coherent emission of excited ions due to the presence of long-lived polarizations maintained by a weak laser post-pulse and coupling simultaneously ground state X 2 Σ g + to states A2Π u and B 2 Σ u + of singly ionized nitrogen molecules N 2 + . Two regimes of signal amplification are identified: a signal of a few picosecond duration at low gas pressures and a short (sub-picosecond) signal at high gas pressures. The theoretical model compares favorably with results obtained by different experimental groups.

The Review of scientific instruments, 2013

We report on a compact energy-scalable device for generating high-fidelity femtosecond laser puls... more We report on a compact energy-scalable device for generating high-fidelity femtosecond laser pulses based on spatial filtering through a hollow-core fiber followed by a nonlinear crystal for cross-polarized wave (XPW) generation. This versatile device is suited for temporal pulse cleaning over a wide range of input energies (from 0.1 to >10 mJ) and is successfully qualified on different ultrafast laser systems. Full characterization of the XPW output is presented. In particular, we demonstrate the generation of 1.6 mJ energy pulses starting from 11 mJ input pulse energy. The temporal contrast of the pulses is enhanced by more than 4 orders of magnitude. In addition, pulse shortening from 40 fs down to 15 fs Fourier-transform limit yields an overall peak-power transmission of up to 50%. This device not only serves as an integrated pulse contrast filter inside an ultrafast laser amplifier but also as a simple back-end solution for temporal post-compression of amplified pulses.

Physical Review Letters, 2013

Optics Express, 2022

Experiments and simulations are performed to study filamentation and generation of acoustic waves... more Experiments and simulations are performed to study filamentation and generation of acoustic waves in water by loosely focused multi-millijoules laser pulses. When the laser pulse duration is increased from femtosecond to nanosecond duration, a transition is observed from a filamentary propagation with extended and low energy density deposition to a localized breakdown, related to high energy density deposition. The transition suggests that Kerr self-focusing plays a major role in the beam propagation dynamics. As a result, the shape, the amplitude and the spectrum of the resulting pressure wave present a strong dependence on the laser pulse duration.

Femtosecond lasers can form long plasma filaments over distances ranging from several centimetres... more Femtosecond lasers can form long plasma filaments over distances ranging from several centimetres to several meters which can be used advantageously for high speed flow control, in particular for drag reduction. Recently the first experimental demonstration of femtosecond energy deposition in a wind tunnel has been reported. This paper completes this report by presenting a parametric analysis of the effect of the femtosecond energy deposition in the flow. In particular, the effect stagnation pressure on the flow perturbation and the drag variation is investigated. Drag measurements show that higher reductions in drag are obtained for larger stagnation pressures.

2021 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), 2021

Significant efforts have been dedicated to control lightning with lasers [1] to protect sensitive... more Significant efforts have been dedicated to control lightning with lasers [1] to protect sensitive sites like rocket launching pads and airports. We report on a dedicated European large framework project towards this goal, called Laser Lightning Rod (llr-fet.eu) [2] . In this project, a unique laser system has been developed, providing 720 mJ, < 1 picosecond pulses at 1 kHz repetition rate, i.e., TW-class peak power and kW average power [3] . This thin disk laser has been further frequency doubled (SHG) and tripled (THG) to provide high intensity outputs at 1030 nm, 515 nm and 343 nm, with conversion efficiencies up to 45% and 30%, respectively. Nonlinear propagation and filamentation of this laser could be characterized over 130 meters in a dedicated hall (LAL in Orsay), both in a collimated geometry, and using a 40 cm diameter beam expander. The whole system will be transported to the top of the Säntis Mountain (2481 m altitude) in Switzerland in April 2021, one of the locations with the highest lightning occurrence in Europe. The site has been fully equipped to host the laser transmitter in the harsh meteorological conditions encountered on the top of the mountain. The site has also been fully equipped with state of the art lightning detection and measurement devices (a lightning mapping array, an interferometer, high-speed cameras, current sensors, a field mill, etc.) to register the triggering and guiding effects of the laser on the lightning process. This site is especially interesting as it generates mostly upward flashes from the 123 m tall tower communications tower. The laser will be aimed to the top of this tower in order to extend its height by the length of the filaments , realizing a "laser lightning rod".

Optics Letters, 2021

Pulses at 744 nm with 90 fs duration, 6 mJ energy, and a weakly divergent wavefront propagate for... more Pulses at 744 nm with 90 fs duration, 6 mJ energy, and a weakly divergent wavefront propagate for more than 100 m and generate a filament followed by an unprecedently long high intensity ( ≥ 1 T W / c m 2 ) light channel. Over a 20 m long sub-section of this channel, the pulse energy is transferred continuously to the infrared wing, forming spectral humps that extend up to 850 nm. From 3D+time carrier-resolved simulations of 100 m pulse propagation, we show that spectral humps indicate the formation of a train of femtosecond pulses appearing at a predictable position in the propagation path.

Physical Review Letters, 2007

Conference on Lasers and Electro-Optics, 2020

Amplitude of Terahertz radiation from air plasma is enhanced by one order of magnitude with a 3-c... more Amplitude of Terahertz radiation from air plasma is enhanced by one order of magnitude with a 3-color femtosecond sawtooth field. Moreover, the THz wave can be coherently controlled by the two relative phases.

Conference on Lasers and Electro-Optics, 2016

We report on experimental evidence showing that the intense 391nm radiation emitted by air plasma... more We report on experimental evidence showing that the intense 391nm radiation emitted by air plasma is superradiance. Electron recollision is speculated to play an essential role in this superradiant process.

Optics Letters

We study the filamentation in air of multi-millijoule optical vortices and compare them with the ... more We study the filamentation in air of multi-millijoule optical vortices and compare them with the classical filamentation regime. The femtosecond vortex beam generates multiple plasma filaments organized in a cylindrical geometry. This plasma configuration evolves into a meter-scale tubular neutral gas column that can be used as a waveguide for nanosecond laser pulses at 532 nm. It appears that optical vortices produce a more uniform heating along the propagation axis, when compared with Gaussian or super-Gaussian beams, and that the resulting low-density channel is poorly sensitive to the laser input power thanks to the combination of filamentation intensity clamping and phase vorticity.

Physical review letters, Jan 17, 2017

Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse giv... more Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm (2ω_{782.8 nm}=ω_{391.4 nm}). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.

Journal of Physics B: Atomic, Molecular and Optical Physics

Physical review. E, 2016

Acoustic signals generated by filamentation of ultrashort terawatt laser pulses in water are char... more Acoustic signals generated by filamentation of ultrashort terawatt laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.

Journal of Applied Physics, 2014

Proceedings of Spie the International Society For Optical Engineering, Dec 1, 2008