Study on effect of polarization and frequency chirp of incident pulse on femtosecond-laser-induced modification inside silica glass

Abstract

The effects of polarization and frequency chirp of femtosecond laser pulses focused inside silica glass on plasma density generated in a femtosecond modification process are numerically studied. The vector four-dimensional nonlinear Schrödinger equation coupled with the Drude plasma equation are simultaneously solved for that purpose. The evolution of polarization along the filament is investigated for different polarizations of the incident pulse. It is observed that there is a sharp variation of polarization ellipse at the vicinity of the focus for an incident pulse with elliptical polarization. For a linearly or circularly polarized incident beam the polarization along the filament remains unchanged. On the other hand, it is found that the magnitude and the sign of the frequency chirp of the incident pulse effectively change the plasma density generated in the process of laser-induced modification. In particular, for incident peak powers near the threshold, by changing the sign of the input chirp, maximum plasma density can be altered by several orders of magnitude. Furthermore, by adjusting the value of the input chirp, the magnitude and the location of plasma density can be adjusted. The results reveal that polarization and frequency chirp of the incident femtosecond laser pulses are two appropriate parameters for controlling the plasma density in the process of laser-induced modification.