Third harmonic beam profile generated in atmospheric air using femtosecond laser pulses

We present, both experimentally and theoretically, the results on third harmonic generation in air during the filamentation of a powerful femtosecond laser pulse. The third harmonic beam pattern shows a central spot on the propagation axis at lower pump energy and is then surrounded by bright third harmonic conical emission at higher pump energy. We argue that the combined action of self-focusing and plasma generation in air plays an important role for the on-axis and off-axis phase-matching conditions along the filament resulting in high conversion efficiency. The experimental results are in good qualitative agreement with numerical simulations.