Time-resolved microscopy of femtosecond laser filaments in fused quartz

Detailed picture of continuous shape evolution of femtosecond laser pulse has been recorded directly in the process of filament formation in fused silica using time-resolved polarization microscopy with ∼70 fs temporal resolution. The main stages of the pulse transformation (temporal self-compression and splitting, formation of conical wave) have been studied. The maximum temporal compression has been found to be achieved by the leading subpulse after the time splitting event. It was found that the Bessel zone of conical wave formation is shifted backwards from the pulse front. Sub- and superluminal propagation velocities of the pulse maxima after the time splitting have been measured.