Influence of the beam-focus size on femtosecond laser-induced damage threshold in fused silica

Abstract

In this work, spot-size dependence of surface femtosecond laser-induced damage threshold in fused silica is put in evidence when the damage reach the micrometer scale. Measurements of the threshold with various numerical apertures and different techniques are performed, revealing a noticeable threshold increase while decreasing the laser beam-focus size below ~10 μm. This unexpected result could be explained by the presence of micrometer-sized defects pre-existing in the SiO₂ sample.