Guillaume Méjean

Long-distance remote laser-induced breakdown spectroscopy using filamentation in air

We demonstrate remote elemental analysis at distances up to 90m, 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.

Electric events synchronized with laser filaments in thunderclouds

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.

Ultraintense light filaments transmitted through clouds

We demonstrate that ultrashort and ultraintense light filaments survive their interaction with water droplets as large as 95 μm and that they are transmitted through water clouds having an optical thickness as high as 3.2 (transmission 5%). In contrast with linear optics, this remarkable transmission through optically dense media results from a dynamic energy balance between the quasisolitonic structure and the surrounding laser photon bath, which acts as an energy reservoir. Implications for free-space laser communications, remote sensing, and telemetry are discussed.

Remote LIBS with ultrashort pulses: characteristics in picosecond and femtosecond regimes

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.

Improved laser triggering and guiding of meqavolt discharges with dual fs-ns pulses

We demonstrate that the capacity of ultrashort high-power laser pulses to trigger and guide high-voltage discharges can be significantly enhanced by a subsequent visible nanosecond laser pulse. The femtosecond pulse induces a bundle of filaments, which creates a conducting channel of low density and cold plasma connecting the electrodes. The subsequent laser pulse photodetaches electrons from O2− ions in the electrode leader. The resulting electrons allow efficient heating by Joule effect in a retroaction loop, resulting in a 5% reduction of the breakdown voltage.

Laser filaments generated and transmitted in highly turbulent air

The initiation and propagation of a filament generated by ultrashort laser pulses in turbulent air is investigated experimentally. A filament can be generated and propagated even after the beam has propagated through strongly turbulent regions, with structure parameters C(n)2 as many as 5 orders of magnitude larger than those encountered in the usual atmospheric conditions. Moreover, the filaments position within the beam is not affected by the interaction with a turbulent region. This remarkable stability is allowed by the strong Kerr refractive-index gradients generated within the filament, which exceed the turbulence-induced refractive-index gradients by 2 orders of magnitude.

Triggering and guiding of megavolt discharges by laser-induced filaments under rain conditions

We demonstrate laser control of high-voltage discharges over a gap of 1.2 m filled with a dense water cloud. Self-guided filaments generated by ultrashort laser pulses are transmitted through the cloud and ionize a continuous plasma channel. The cloud typically reduces the discharge probability in given experimental conditions by 30%, but has almost no influence on the threshold required to trigger single discharge events, both in electrical field and laser energy. This result is favorable for real-scale lightning control applications.

White-light filaments for multiparameter analysis of cloud microphysics

We present a lidar technique using femtosecond-terawatt laser pulses to perform a multiparameter analysis of cloud microphysics. Particle size and density within the cloud are deduced from the multispectral multiple scattering pattern of an ultrashort laser pulse. Furthermore, the spectral analysis of the atmospheric transmission of the white-light continuum from the same laser source yields temperature and relative humidity.

Frequency Comb Based Spectrometer for in Situ and Real Time Measurements of IO, BrO, NO2, and H2CO at pptv and ppqv Levels

We report an instrument designed for trace gas measurement of highly reactive halogenated radicals, such as bromine oxide and iodine oxide, as well as for nitrogen dioxide and formaldehyde. This compact and robust spectrometer relies on an alternated injection of a frequency-doubled femtosecond radiation at 338 and 436 nm into two parallel high-finesse cavities, for measuring BrO + H(2)CO, and IO + NO(2), respectively. The transmission of the broadband radiation through the cavity is analyzed with a high resolution, compact spectrograph consisting of an echelle grating and a high sensitivity CCD camera. The transportable instrument fits on a breadboard 120 × 60 cm size and is suitable for in situ and real time measurements of these species. A field campaign at the Marine Boundary Layer in Roscoff (in the northwest of France, 48.7°N, 4.0°W) during June 2011 illustrates the outstanding performance of the instrument, which reaches a bandwidth normalized minimum absorption coefficient of 1.3 × 10(-11) cm(-1) Hz(-1/2) per spectral element, and provides detection levels as low as 20 parts per quadrillion of IO in 5 min of acquisition.

Measurement of reactive atmospheric species by ultraviolet cavity-enhanced spectroscopy with a mode-locked femtosecond laser

We demonstrate the possibility of measuring parts in 10(12) by volume concentrations of radicals of high atmospheric interest, such as IO or BrO, as needed for monitoring these species in the environment. We apply cavity-enhanced absorption spectroscopy in the near UV range using a frequency-doubled Ti:Sa mode-locked femtosecond laser. Efficient broadband injection of a high-finesse cavity is obtained by matching this optical frequency-comb source to the comb of cavity transmission resonances. A grating spectrograph and a detector array disperse and detect the spectrum transmitted by the cavity carrying the absorption features of intracavity molecules. Spectra recorded over ~4 nm with 10 s averaging display a noise level of 8 x 10(-10)/cm.