139 research outputs found
Fine control of terahertz radiation from filamentation by molecular lensing in air
International audienceWe demonstrate a method to control remotely the terahertz (THz) source in air based on the bifilamentation of femtosecond laser pulses. By fine tuning the time delay between the two pulses, a significant modulation of the THz intensity from bifilamentation is observed. The phenomenon is attributed to the molecule quantum lensing effect around the air molecule revival time, which changes the separation between the two neighboring plasma producing filaments
Calorimetric detection of the conical THz radiation from femtosecond laser filaments in air
International audienceThe spectral distribution of the conical terahertz (THz) emission from a femtosecond laser filament in air is measured with a bolometric detector and a set of filters, confirming that the main part of the emission lies between 0.5 and 3 THz. The efficiency of this THz emission is compared with that obtained in air via Four Waves Mixing of femtosecond laser pulses at ω and 2 ω in the presence of a plasma
Polarization analysis of terahertz radiation generated by four-wave mixing in air
International audienceWe examine the generation of terahertz by optical rectification of fundamental infrared beam with its first harmonic in ionized air. From polarization measurements we identify an important, yet so far unreported cross term (X(3)xyxy + X(3)xyyx) of the nonlinear susceptibility tensor. Omission of this term leads to an error by a factor 10^6 of the THz intensity obtained with certain polarization configurations
Experimental observation of a traveling plasma grating formed by two crossing filaments in gases
International audienceThe spatial motion and effective duration of a traveling plasma grating formed by two interfering femtosecond laser filaments in gases is characterized by its Doppler effect imparted on a probe pulse. The shift velocity determined experimentally agrees with the theoretical calculations
Maker fringes in the Terahertz radiation produced by a 2-color laser field in air
International audienceThe terahertz radiation produced by a 2-color femtosecond laser scheme strongly saturates and develops an oscillatory behavior with increasing power of the driving femtosecond laser pulses. This is explained by the formation of a plasma channel due to filamentation. Due to dispersion inside the filament and the Gouy phase shift, the phase difference between the 800 nm and 400 nm pulses varies along this plasma emitter. As a result, the local THz radiations generated along the filament interfere destructively or constructively, which manifests itself in the form of Maker fringes
Plasma dynamics of a laser filamentation-guided spark
International audienceWe investigate experimentally the plasma dynamics of a centimeter-scale, laser filamentation-guided spark discharge. Using electrical and optical diagnostics to study monopolar discharges with varying current pulses we show that plasma decay is dominated by free electron recombination if the current decay time is shorter than the recombination characteristic time. In the opposite case, the plasma electron density closely follows the current evolution. We demonstrate that this criterion holds true in the case of damped AC sparks, and that alternative current is the best option to achieve a long plasma lifetime for a given peak current
Backward stimulated radiation from filaments in Nitrogen gas and air pumped by circularly polarized 800 nm femtosecond laser pulses
We report on strong backward stimulated emission at 337 nm in Nitrogen gas
pumped by circularly polarized femtosecond laser pulses at 800 nm. A distinct
dependence of the backward UV spectrum on pump laser polarization and intensity
is observed, pointing to the occurrence of backward amplified spontaneous
emission inside filaments. We attribute the population inversion to inelastic
collision between the free electrons produced by the pump laser and neutral N2
molecules. The addition of Oxygen molecules is detrimental for the gain,
reducing it to near threshold at atmospheric concentration
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 one 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.Comment: 5 pages, 4 figure
Amplification of transition-Cherenkov terahertz radiation of femtosecond filament in air
International audienceThe transition-Cherenkov terahertz radiation from a femtosecond laser filament in air is enhanced by three orders of magnitude in the presence of a longitudinal static electric field, while the radiation pattern and the polarization remain the same. An amplified longitudinal electron current inside the filament is responsible for this amplified terahertz emission
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