Chirp-induced dynamics of femtosecond filaments in air.

We investigate the influence of a chirped phase on femtosecond pulses propagating in air. Pulses with an initially negative chirp are temporally compressed by compensation with group-velocity dispersion. We demonstrate that this property, combined with plasma defocusing, can be used to trigger filamentation at different foci, increase self-guiding ranges, or even shorten pulse duration.