Polarization influence on femtosecond laser-induced surface damage and electron plasma ultrafast dynamics in sapphire

This study investigates the impact of the polarization direction of femtosecond laser relative to the sapphire crystal axis on surface damage threshold and ultrafast electron plasma dynamics, which were characterized by using damage area extrapolation and pump–probe shadowgraphy methods, respectively. It was found that the surface damage threshold, electron plasma density, and nonlinear focal-point position exhibit π/2 periodic oscillations with changes in the laser polarization angle when the laser is incident along the a axis. However, such dependence is not obvious when the laser is incident along the c axis. It is proved from the absorption coefficient and relaxation time that the periodic variation is relative to the equivalent electron mass and crystal symmetry. These findings provide important insights into optimizing femtosecond laser processing of anisotropic materials such as sapphire.