Observation of inner surface roughness in fused silica microholes with varying the number of femtosecond laser pulses

Inner walls of microhole in a thin fused silica plate were observed after changing ablating laser pulse shots of a focused femtosecond laser at the wavelength of 400 nm with an energy of 20 μJ in a pulse width of 350 fs. Using an objective lens with an NA of 0.28, it was revealed that the inner surface of the microhole was melted with 10 laser pulse shots. By increasing the pulse numbers to 100, however, deposition of fused silica particles on the melted inner surface was observed. In order to minimize the inner surface roughness, the objective lens was changed. After 50 laser pulse shots, the inner surface structure was brought close to optical quality using an objective lens with NA of 0.65.