Few-cycle shock X-wave generation by filamentation in prealigned molecules

We investigate the evolution of an intense femtosecond laser pulse in prealigned molecules of nitrogen following impulsive rotational Raman excitation by a weak pump pulse. The spatiotemporal and spectral dynamics of the probe pulse are readily controlled by matching the delay between pump and probe to various revivals of the molecular alignment. This control allows for intense filamentation of the probe and generation of self-shortened few-cycle pulses with angular spectra featuring shock X -waves. We identify the phase modulation originating from the sharp revivals of the molecular alignment as responsible for the generation of controllable rising or falling shock edges associated with on-axis redshifted or blueshifted supercontinuum spectrum and off-axis blueshifted or redshifted tails. Pulse self-steepening promotes the formation of the conical emission associated with the off-axis red tails, the blueshifted supercontinuum and the shock in the trailing part of the pulse.