Influence of refocusing of femtosecond laser pulses in air on third harmonic generation

The refocusing of femtosecond laser pulses forming the filament in air is studied both numerically and experimentally. The evolution of fundamental as well as third harmonic pulses and spectra during propagation in the filament is discussed. It is shown that the refocusing of the pulse gives rise to enhanced spectral shifts of fundamental and third harmonic waves.