Ultrashort pulse-induced nanogratings: Temperature stable optically active phase elements

This paper presents a femtosecond direct written optical components exhibiting circular birefringence. Based on tightly focused ultrashort laser pulses in the bulk of fused silica, self-organized modifications, so called nanogratings can be induced. Due to their anisotropic structure and sub-wavelength periodicity, these nanogratings show a strong form-birefringence. Emerging after several laser pulses they are always oriented perpendicular to the laser polarization. So by combing the tunable orientation of the nanogratings with the outstating three-dimensional writing ability allows for the fabrication of almost arbitrary phase elements. Here, multiple layers of anisotropic orientation were inscribed in fused silica in order to imitate the morphology of cholesteric liquid crystals. By analyzing optical parameters as the retardance of the fabricated devices at different wavelengths we found linear as well as circular birefringence. In order to reduce the amount of linear retardance (of each layer), the laser parameters were tuned, resulting in almost pure polarization rotating devices.