Redshift of few-cycle infrared pulses in the filamentation regime

By focusing infrared (IR) pulses of low energy (similar to 0.4 mJ) into an argon cell at a pressure of a few bars, a supercontinuum is generated with a long-wavelength tail that can exceed 1500 nm for initial pulse durations of similar to 5 fs in the single-filamentation regime. Numerical calculations simulating the propagation of single-or few-cycle IR pulses show that this red-shift is enhanced by a sharp leading edge appearing in the pulse temporal profile, as the pulse undergoes break-up due to the interplay between Kerr self-focusing, strong dispersion and plasma generation.