Anisotropic etching of dielectrics exposed by high intensity femtosecond pulses

Post-processing of crystalline and glass materials after the exposure to femtosecond pulses was carried out by wet etching in water solutions of hydrofluoric acid. Crystalline sapphire and quartz showed high (larger than 100) anisotropy of etching, which allowed to develop high-aspect-ratio three-dimensional structures in the volume of those dielectrics. In silicate glasses the anisotropy of wet etching can be achieved by a proper selection of the overlap of adjacent pulses during recording, their energy, and focusing. Three-dimensional structures in silica glass (viosil with OH concentration below 1200 ppm) with a high aspect ratio of 100 were achieved. The mechanism of anisotropy in wet etching is discussed. Surface irradiation of sapphire at irradiance close to that of surface ablation recorded structural modifications resembling the ripples. Those structures were made observable only after wet etching. Period of the ripples can be explained by the recently presented theory (Y. Shimotsuma et al., Phys. Rev. Lett. 91 247405-1 (2003)). Sub-micrometer structuring of surface is demonstrated. Electron temperature at the moment of structure recording can be estimated from the period of ripples (for sapphire T_e ≃ 11 keV was found).