Laser processing of dielectrics using spatiotemporally tuned ultrashort pulses

The authors report on the theoretical and experimental studies of laser-induced optical breakdown on the surface of fused silica to elucidate the influence of time delay and spatial separation between two ultrashort pulses on the position and size of the modification. Carriers involved in the damage formation including free electrons in the conduction band and self-trapped excitons (STEs) are investigated. The relationship between damage morphology and time delay shows that the seeding carriers (free electrons and STEs) generated from the first pulse are found to play a significant role for the second pulse—which is temporally and spatially separated from the first pulse—in creating the critical electron density needed for an optical breakdown. Consequently, processing outcomes, such as accuracy (position of the hole) and resolution (size of the hole), depend on the interplay of various laser-induced physics that can be tailored for specific goals. As a demonstration, laser lithography with resolution below the diffraction limit is achieved by exploiting multipulse induced physics. This work is a step toward repeatable laser processing of dielectrics beyond the diffraction limit and provides insights into ultrafast laser-matter interaction under the condition of an extremely high pulse repetition rate.