Gradient enhanced third harmonic generation in a femtosecond filament.

The third harmonic generated during femtosecond filamentation in air is studied. By establishing a gradient from atmospheric pressure to vacuum conditions, we truncate the filament abruptly at defined positions. The introduction of the pressure gradient leads to an enhancement of the generated third harmonic radiation by 3 orders of magnitude. This effect is attributed to an improved on-axis phase-matching condition. We investigate the spectral shape and the conversion efficiency of the third harmonic during the propagation in the filament.