Energy deposition in air by moderately focused femtosecond laser filaments

Filamentation of high-power femtosecond laser pulses in air is accompanied by a fairly strong release of optical energy into the propagation medium due to laser-induced ionization of air molecules and production of an underdense plasma of charged species. We present the results of our laboratory experiments and numerical simulations aimed to the estimation of energy deposition amount by laser filament upon propagation in air depending on the conditions of spatial focusing, pulse energy, and radiation wavelength. For the first time to our knowledge, our study reveals a more than 50% decrease in the filament energy deposited in air in the range of moderate numerical aperture values, approximately from 0.003 to 0.007, at the carrier wavelengths of 740 nm and 470 nm. We attribute such a considerable reduction in the laser pulse energy release for femtosecond plasma to the competing effects of Kerr self-focusing and geometric divergence of focused laser pulse.