Optical Properties of Nanogratings Inscribed with Conical Phase Fronts

Femtosecond laser direct writing in fused silica can lead to different modification types, one of them being the so-called nanogratings [1] - [3] . This modification type consists of self-assembled, grating-like, nanoscopic structures, that exhibit form-birefringence and a higher etching rate than pristine material. These characteristics have been exploited in various applications, including microfluidic devices [4] , data storage [5] , waveplates [6] and quantum gates [7] , to name just a few. The nanograting properties can be tuned using the laser parameters during inscription, such as number of overlapping pulse exposures, focussing condition and repetition rate. Most commonly, nanogratings are inscribed using traditional Gaussian beams. Further manipulation of the nanograting properties may arise from tuning of the inscription beam shape, which has not been systematically explored to date.