Mechanism of material removal through transient and selective laser absorption into excited electrons in fused silica

Transient and selective laser (TSL) processing is a method used for the rapid fabrication of transparent materials. In this method, electrons in glass are excited by an ultrashort-pulse laser. This electron-excited region selectively absorbs a long-pulse laser with low intensity, resulting in microfabrication. However, the mechanism of material removal during TSL processing is not completely understood. In this study, we measured the threshold of TSL processing in synthetic fused silica to investigate the details of the processing mechanism. The measurement revealed that the processing threshold was represented by the relationship between the intensity of the long-pulse laser and the electron density excited by the femtosecond laser. The calculations indicated that the absorption in synthetic fused silica was mainly due to linear absorption of the long-pulse laser into the excited electrons and that a simple threshold model can be used to explain the material removal volume. The revealed controlling factor from the identified threshold function will allow the shapes to be controlled during TSL processing.