Thin film laser damage mechanisms at the YAG third harmonic

The morphology of the first damage occurring on either single layer materials or multilayer HR coatings are studied using Scanning Electron Microscopy. For single layers obtained by electron beam evaporation or ion beam sputtering, the damages are always centered on a small size center of about 0.1 micrometers . This center triggers the degradation which presents a well defined shape and size. Two kinds of damage mechanisms occur, either melting of the layer or mechanical breaking of the layer. The same behavior is observed on HR coatings with damage centers located at the interfaces between the high and low index materials of the stack. From these experiments an impurity dominated model is proposed which enables the main features of UV damages to be understood.