Spectral analysis and control to obtain sub-5 fs pulses by femtosecond filamentation.

A spectral-filter method is numerically demonstrated to obtain sub-5 fs pulses by using femtosecond filamentation in fused silica. Instead of employing spectral phase compensation, by properly employing a high-pass filter to select the broadened high-frequency spectra that are located almost in phase in the tailing edge of the self-compressed pulses owing to self-steepening, as short as single-cycle pulses can be obtained. For instance, for an input pulse with a duration of 50 fs and energy 2.2 microJ, the minimum pulse duration can reach to approximately 4 fs (about 1.5 cycles) by applying a proper spectral filter.