Stefano Henin

Higher-order Kerr terms allow ionization-free filamentation in gases

We show that higher-order nonlinear indices (n(4), n(6), n(8), n(10)) provide the main defocusing contribution to self-channeling of ultrashort laser pulses in air and argon at 800 nm, in contrast with the previously accepted mechanism of filamentation where plasma was considered as the dominant defocusing process. Their consideration allows us to reproduce experimentally observed intensities and plasma densities in self-guided filaments.

Field measurements suggest the mechanism of laser-assisted water condensation

Because of the potential impact on agriculture and other key human activities, efforts have been dedicated to the local control of precipitation. The most common approach consists of dispersing small particles of dry ice, silver iodide, or other salts in the atmosphere. Here we show, using field experiments conducted under various atmospheric conditions, that laser filaments can induce water condensation and fast droplet growth up to several μm in diameter in the atmosphere as soon as the relative humidity exceeds 70%. We propose that this effect relies mainly on photochemical formation of p.p.m.-range concentrations of hygroscopic HNO3, allowing efficient binary HNO3–H2O condensation in the laser filaments. Thermodynamic, as well as kinetic, numerical modelling based on this scenario semiquantitatively reproduces the experimental results, suggesting that particle stabilization by HNO3 has a substantial role in the laser-induced condensation.

Production of ozone and nitrogen oxides by laser filamentation

We have experimentally measured that laser filaments in air generate up to 1014, 3×1012, and 3×1013 molecules of O3, NO, and NO2, respectively. The corresponding local concentrations in the filament active volume are 1016, 3×1014, and 3×1015 cm−3, and allows efficient oxidative chemistry of nitrogen, resulting in concentrations of HNO3 in the parts per million range. The latter forming binary clusters with water, our results provide a plausible pathway for the efficient nucleation recently observed in laser filaments.

On negative higher-order Kerr effect and filamentation

As a contribution to the ongoing controversy about the role of higher-order Kerr effect (HOKE) in laser filamentation, we first provide thorough details about the protocol that has been employed to infer the HOKE indices from the experiment. Next, we discuss potential sources of artifact in the experimental measurements of these terms and show that neither the value of the observed birefringence, nor its inversion, nor the intensity at which it is observed, appear to be flawed. Furthermore, we argue that, independently on our values, the principle of including HOKE is straightforward. Due to the different temporal and spectral dynamics, the respective efficiency of defocusing by the plasma and by the HOKE is expected to depend substantially on both incident wavelength and pulse duration. The discussion should therefore focus on defining the conditions where each filamentation regime dominates.

Contribution of water droplets to charge release by laser filaments in air

We measured the electric charge release from single watermicrodroplets illuminated by ultrashort laser filaments in air. This charge is up to 600 times larger than from a comparable filament volume in air. In contrast, for atmospheric droplet concentrations and sizes, the volume-averaged overall droplet contribution to the charge is small as compared with that of the filaments along its whole propagation path.

Influence of pulse duration, energy, and focusing on laser-assisted water condensation

We investigate the influence of laser parameters on laser-assisted water condensation in the atmosphere. Pulse energy is the most critical parameter. Nanoparticle generation depends linearly on energy beyond the filamentation threshold. Shorter pulses are more efficient than longer ones with saturation at ∼1.5 ps. Multifilamenting beams appear more efficient than strongly focused ones in triggering the condensation and growth of submicronic particles, while polarization has a negligible influence on the process. The data suggest that the initiation of laser-assisted condensation relies on the photodissociation of the air molecules rather than on their photoionization.

Laser-induced plasma cloud interaction and ice multiplication under cirrus cloud conditions.

Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10−9 fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.

Multijoule scaling of laser-induced condensation in air

Using 100 TW laser pulses, we demonstrate that laser-induced nanometric particle generation in air increases much faster than the beam-averaged incident intensity. This increase is due to a contribution from the photon bath, which adds up with the previously identified one from the filaments and becomes dominant above 550 GW/cm2. It appears related to ozone formation via multiphotondissociation of the oxygen molecules and demonstrates the critical need for further increasing the laser energy in view of macroscopic effects in laser-induced condensation.

Higher-order Kerr improve quantitative modeling of laser filamentation

We test numerical filamentation models against experimental data about the peak intensity and electron density in laser filaments. We show that the consideration of the higher-order Kerr effect improves the quantitative agreement without the need of adjustable parameters.

Cooperative effect of ultraviolet and near-infrared beams in laser-induced condensation

We demonstrate the cooperative effect of near infrared (NIR) and ultraviolet (UV) beams on laser-induced condensation. Launching a UV laser after a NIR pulse yields up to a 5-fold increase in the production of nanoparticles (25–300 nm) as compared to a single NIR beam. This cooperative effect exceeds the sum of those from the individual beams and occurs for delays up to 1 μs. We attribute it to the UV photolysis of ozone created by the NIR pulses. The resulting OH radicals oxidize NO2 and volatile organic compounds, producing condensable species.