Spatial coherence of a Raman wake excited inside a uniform filament

Abstract

We present results of an investigation of the spatial-temporal structure of the delayed Kerr nonlinearity excited during filamentation of a femtosecond laser pulse inside a crystalline target. It is found that the pulse splitting, which is an inherent property of the filamentation process, constrains the length of the spatial coherence of the Raman wake wave which follows the pump pulse. Each subpulse induces lattice vibrations, which interfere either constructively or destructively depending on the delay between the subpulses. This results in abrupt ``switching'' of the phase of the wake wave along and across the filament volume. For the same reason, the maximal amplitude of the Raman response is observed not at the point of the maximal laser intensity and electron concentration\char22{}it is shifted forward to the end of the filament. This effect should be observed if the period of Raman oscillations is close to the duration of the pump pulse.