Morphological investigation at the front and rear surfaces of fused silica processed with femtosecond laser pulses in air

Different responses for the front (entrance) and the rear (exit) surfaces of fused silica processed with femtosecond laser pulses at 807 nm were observed under tight focusing conditions (NA = 0.4). The morphology of the surface in the beam path is highly sensitive to the focus position. By adjusting the focus position, we can produce not only a submicrometer cavity but also a submicrometer bubble. We achieved higher-quality micromachining and a better spatial resolution (400 nm) by focusing the laser beam at the rear surface rather than at the front surface.