Self-Shortening Dynamics Measured along a Femtosecond Laser Filament in Air

摘要

The filamentation-induced temporal shortening of a 40 femtosecond pulse propagating in air is traced using impulsive vibrational Raman scattering and measurement of the power spectrum as a function of position along the propagation axis. The N_2, O_2, and H_2O vibrational Raman responses reveal self-shortening of pulse features to 14 fs during the first filamentation cycle and to at least 9 fs in the second cycle. Spectral measurements further demonstrate that the coherent bandwidth generated in the region from 470 to 330 nm during the self-shortening process forms the ～9 fs pulse.