Plasma kinetics in ultrashort pulse laser filament: Time resolved spectral measurement

Filamentation of ultrafast laser pulses in air has been studied extensively. The very intense light channels, small in diameter for a long distance (widely believed to be an equilibrium of Kerr self-focusing, linear diffraction and plasma defocusing) leave a low density plasma in its wake [1]. Over the years, researchers have tried to characterize these plasma channels for applications. Such applications range from filament guided discharges to remote sensing. In order to use the filament plasma for applications, the plasma properties and dynamics need to be understood in great detail. In the air, the plasma densities are on the order of 10 16 cm −3 [2] Until now, there are only few experimental reports that have measured the temperature of the plasma, yielding a temperature between 3000 and 5000 K [3,4]. One approach focused on the absorption and diffraction of a probe beam propagating perpendicularly to the filament [3], a second approach studied spectral line broadening and emission intensities [4]. The first approach is a pump probe experiment with good time resolution, but is not doable in single shot. Whereas the second approach integrates over the lifetime of the filament and therefore does not capture the time dynamics of the plasma. In our approach, we are looking at certain spectral lines of molecules to study the time dependent kinetics of the filament plasma produced by a 800nm, 1.5 mJ, 35 fs laser pulse by using a spectrometer coupled to a streak camera. The time resolution of the streak camera is 25 ps, and can capture a streak of more than 1 ns. Investigating the spectral lines in molecular nitrogen (337.1 nm, 357.6 nm and 353.6 nm) of the second positive system C 3 Π u -B 3 Π g , we observed the buildup of the light emission. The maximum of the emission is reached in the order of hundred picosecond. These results are expected since the excitation of these levels is driven by inelastic electron-molecule collisions and therefore closely correlated to the plasma kinetics. A computer model for the plasma kinetics is currently being developed to explain the observed experimental data and to extract the plasma temperature and density.