Violation of axial symmetry during micro-modification by a linearly polarized Gaussian beam in the bulk of fused silica in the single ultrashort pulse regime

Research has demonstrated that a single ultrashort laser pulse creates axially asymmetric voxels when tightly focused within the bulk of fused silica. These voxels exhibit birefringence with the slow axis orientation aligning within the plane of polarization of the laser beam. Adjusting the polarization plane results in a corresponding rotation of the birefringence axis. The anisotropic voxel formation by a single laser pulse is attributed to the vector nature of the electric field, which becomes pronounced under focusing with high numerical aperture. The primary cause of axial asymmetry is that the longitudinal field component has two maxima equidistant from the optical axis in the beam polarization plane.