Yohann Brelet

Superfilamentation in air.

The interaction between a large number of laser filaments brought together using weak external focusing leads to the emergence of few filamentary structures reminiscent of standard filaments, but carrying a higher intensity. The resulting plasma is measured to be 1 order of magnitude denser than for short-scale filaments. This new propagation regime is dubbed superfilamentation. Numerical simulations of a nonlinear envelope equation provide good agreement with experiments.

Self-seeded lasing in ionized air pumped by 800 nm femtosecond laser pulses

We report on the lasing in air and pure nitrogen gas pumped by a single 800 nm femtosecond laser pulse. Depending on gas pressure, incident laser power and beam convergence, different lasing lines are observed in the forward direction with rapid change of their relative intensities. The lines are attributed to transitions between vibrational and rotational levels of the first negative band of the singly charged nitrogen molecule-ion. We show that self-seeding plays an important role in the observed intensity changes.

Radiofrequency plasma antenna generated by femtosecond laser filaments in air

We demonstrate tunable radiofrequency emission from a meter-long linear plasma column produced in air at atmospheric pressure. A short-lived plasma column is initially produced by femtosecond filamentation and subsequently converted into a long-lived discharge column by application of an external high voltage field. Radiofrequency excitation is fed to the plasma by induction and detected remotely as electromagnetic radiation by a classical antenna.

Lasing of ambient air with microjoule pulse energy pumped by a multi-terawatt infrared femtosecond laser

We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ∼1.5  TW. The maximum lasing pulse energy is measured at 2.6 μJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of 3.5×10(-5) is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be in excess of 0.7/cm. Lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity.

Laser-induced periodic annular surface structures on fused silica surface

We report on the formation of laser-induced periodic annular surface structures on fused silica irradiated with multiple femtosecond laser pulses. This surface morphology emerges after the disappearance of the conventional laser induced periodic surface structures, under successive laser pulse irradiation. It is independent of the laser polarization and universally observed for different focusing geometries. We interpret its formation in terms of the interference between the reflected laser field on the surface of the damage crater and the incident laser pulse.

A simple high-voltage high current spark gap with subnanosecond jitter triggered by femtosecond laser filamentation

We describe a simple, sturdy, and reliable spark gap operating with air at atmospheric pressure and able to switch currents in excess of 10 kA with sub-nanosecond jitter. The spark gap is remotely triggered by a femtosecond laser filament.

Tesla coil discharges guided by femtosecond laser filaments in air

A Tesla coil generator was designed to produce high voltage pulses oscillating at 100 kHz synchronisable with a nanosecond temporal jitter. Using this compact high voltage generator, we demonstrate reproducible meter long discharges in air at a repetition rate of 1 Hz. Triggering and guiding of the discharges are performed in air by femtosecond laser filaments.

Effect of input pulse chirp on nonlinear energy deposition and plasma excitation in water

We analyze numerically and experimentally the effect of the input pulse chirp on the nonlinear energy transfer from 5 µJ fs-pulses at 800 nm to water. Numerical results are also shown for pulses at 400 nm, where linear losses are minimized, and for different focusing geometries. Input chirp is found to have a big impact on the transmitted energy and on the plasma distribution around focus, thus providing a simple and effective mechanism to tune the electron density and energy deposition. We identify three relevant ways in which plasma features may be tuned.

Two-color interferometer for the study of laser filamentation triggered electric discharges in air

We present a space and time resolved interferometric plasma diagnostic for use on plasmas where neutral-bound electron contribution to the refractive index cannot be neglected. By recording simultaneously the plasma optical index at 532 and 1064 nm, we are able to extract independently the neutral and free electron density profiles. We report a phase resolution of 30 mrad , corresponding to a maximum resolution on the order of 4×10(22) m(-3) for the electron density, and of 10(24) m(-3) for the neutral density. The interferometer is demonstrated on centimeter-scale sparks triggered by laser filamentation in air with typical currents of a few tens of A.

Rigorous Prediction of the Ground Wave Above Flat and Rough Highly-Conducting One-Dimensional Sea Surfaces in HF-VHF Band

For horizontally and vertically polarized line sources in HF-VHF band, a detailed analysis of the propagation over one-dimensional highly-conducting smooth and rough sea surfaces is addressed from an efficient rigorous numerical method: the method of moments combined with the BMIA-CAG approach and with the impedance boundary condition (Leontovitch approximation). This method can treat a huge problem, typically, ranging from 200 000 to 300 000 for the number of unknowns on the surface, which allows us to show, for the TM polarization, the ground wave propagation over a long distance. The contribution of the surface wave is then exhibited for a smooth sea surface and compared with the Collin asymptotic formulation deduced from the Sommerfeld integral. The surface roughness effect on the propagation is also investigated.