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 in 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 high numerical aperture focusing. The longitudinal component of the field, which induces axial asymmetry, manifests two maxima equidistant from the optical axis within the plane of beam polarization.