3D photothermal microscope for the detection of nano-sized absorbing defects responsible of laser-induced damage initiation

摘要

The recent progresses in optical components manufacturing have permitted to increase strongly the laser-induced damage threshold. However, in high power laser applications, the slightest inhomogeneity of the material can lead to an irreversible breakdown. Considering the difficulty to eliminate the whole defects, it is important to have an accurate tool to exhibit the smallest absorbing centers assumed to be precursors of laser damage. In this paper we describe a non destructive technique based on the photothermal effect induced by local absorbing inhomogeneities in order to detect nano-scale absorbing defects. The purpose will be illustrated by the detection of artificial isolated metallic inclusions of a few ten nanometers in the bulk of transparent substrates and thin-film coatings. The high spatial resolution of detection is obtained thanks to a three-dimensional piezolectric translation stage. Moreover, the photothermal setup coupled with a laser damage facility, permits to follow with high accuracy the evolution of these defects under laser irradiation and determine a pre-damage stage ten times lower than the surface damage.