Extended filamentation with temporally chirped femtosecond Bessel-Gauss beams in air.

We report experimental results on ultrafast filamentation with temporally chirped femtosecond Bessel-Gauss beams. We find that by chirping the pulses, the longitudinal range of the generated plasma channels can be extended relative to filaments generated by fully compressed, transform-limited femtosecond pulses. We find a clear correlation between the extent of filamentation and the intensity of the on-axis emission by the femtosecond Bessel-Gauss beam. The on-axis emission is negligible for fully compressed pulses, but it can become quite substantial (up to 10% of the input pulse energy) when chirped pulses are used. Under certain conditions, the on-axis emission becomes sufficient for generating its own plasma channel thus resulting in extended filamentation. This effect may offer means of remote control over filament formation with femtosecond Bessel-Gauss beams.We identify a four-wave mixing process, enhancement of which is likely to result in a maximum of the on-axis emission, and derive a simple expression for estimating the duration of the chirped pulse that is required for such enhancement. Our estimations are in good agreement with the experiment.