Triggering rain on demand is an old dream of mankind, with a huge potential socio-economical benefit. To date, efforts have mainly focused on cloud-seeding using silver salt particles. We demonstrate that self-guided ionized filaments generated by ultrashort laser pulses are also able to induce water-cloud condensation in the free, sub-saturated atmosphere. Potential contributing mechanisms include photo-oxidative chemistry and electrostatic effects. As well as revealing the potential for influencing or triggering water precipitation, laser-induced water condensation provides a new tool for the remote sensing of nucleation processes in clouds
International audienceWe demonstrate remote elemental analysis at distances up to 90 m, using a laser-induced breakdown spectroscopy scheme based on filamentation induced by the nonlinear propagation of unfocused ultrashort laser pulses. A detailed signal analysis suggests that this technique, remote filament-induced breakdown spectroscopy, can be extended up to the kilometer range
Abstract:We investigated the possibility to trigger real-scale lightning using ionized filaments generated by ultrashort laser pulses in the atmosphere. Under conditions of high electric field during two thunderstorms, we observed a statistically significant number of electric events synchronized with the laser pulses, at the location of the filaments. This observation suggests that corona discharges may have been triggered by filaments. R. Fieux, C. Gary, and P. Hubert, "Artificially Triggered Lightning above Land," Nature 257, 212-214 (1975 199-202 (1999).
5.A. Braun, G. Korn, X. Liu, D. Du, J. Squier, and G. Mourou, "Self-channeling of high-peak-power femtosecond laser pulses in air," Opt. Lett. 20, 73-75 (1995
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.
Using a container-integrated mobile femtosecond terawatt laser system with integrated detection unit (Teramobile), we have demonstrated remote laser-induced breakdown spectroscopy (R-LIBS) on copper and aluminium samples with targets located at 25 m away from the container. The ability of our laser system to generate pulses in the femtosecond, picosecond and nanosecond regimes allowed us to perform direct comparisons between these three pulse durations. The dependence of the fluorescence signal on laser pulse energy showed a nonlinear behavior with a threshold, which is consistent with the previous observations for laser ablation. Such nonlinear behavior leads to a dependence of the LIBS signal on the temporal-spectral shape of the laser pulse. We showed especially that the transform-limited pulse does not optimize the fluorescence. A properly applied chirp allows an increase of the LIBS signal. Understanding and optimization of the chirp effect would improve the detection limit of the LIBS using a femtosecond laser (Femto-LIBS) and lead to a larger detection distance. Furthermore the use of pulse shaping should enhance the detection specificity for the cases of spectral overlapping between several elements to be identified.
International audienceThe propagation of femtosecond terawatt laser pulses at reduced pressure (0.7 atm) is investigated experimentally. In such conditions, the non-linear refractive index n 2 is reduced by 30%, resulting in a slightly farther filamentation onset and a reduction of the filament number. However, the filamentation process, especially the filament length, is not qualitatively affected. We also show that drizzle does not prevent the filaments from forming and propagating
We experimentally and numerically characterize multiple filamentation of laser pulses with incident intensities of a few TW/cm 2 . Propagating 100 TW laser pulses over 42 m in air, we observe a new propagation regime where the filament density saturates. As also evidenced by numerical simulations in the same intensity range, the total number of filaments is governed by geometric constraints and mutual interactions among filaments rather than by the available power in the beam.
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