Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses.

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.

Underwater acoustic signals induced by intense ultrashort laser pulse.

Acoustic signals generated in water by terawatt (TW) laser pulses undergoing filamentation are studied. The acoustic signal has a very broad spectrum, spanning from 0.1 to 10 MHz and is confined in the plane perpendicular to the laser direction. Such a source appears to be promising for the development of remote laser based acoustic applications.

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.

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.

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.

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.

Ciliary white light: optical aspect of ultrashort laser ablation on transparent dielectrics.

We report on a novel nonlinear optical phenomenon, coined as ciliary white light, during laser ablation of transparent dielectrics. It is observed in 14 different transparent materials including glasses, crystals, and polymers. This phenomenon is also universal with respect to laser polarization, pulse duration, and focusing geometry. We interpret its formation in terms of the nonlinear diffraction of the laser generated white light by the ablation crater covered by nanostructures. It carries rich information on the damage profile and morphology dynamics of the ablated surface, providing a real time in situ observation of the laser ablation process.