The morphology of pulsed laser-induced damage on the surface of a copper film mirror

A copper film mirror was irradiated using a pulsed Nd:YAG laser (1.06 mum, 10 ns) through an aperture. After laser irradiation, the damaged spot was analyzed using an optical microscope and periodic damage rings were found at the laser irradiation spot, the average spacing of these rings being about 40-50 mum. The damage ring could be observed in the periphery of the laser spot under lower laser energy densities. As the laser fluence increased, the periodic damage rings grew to cover the whole laser spot. Under higher laser energy density, the whole film was damaged and the rings disappeared. A Fresnel diffraction model is used to explain the appearance of these rings. The laser beam is diffracted by the aperture in the optic path, the laser intensity distribution at the copper film mirror becomes periodic rings and damage ripple pattern forms instantly with the laser ablation. The laser intensity distribution at the mirror surface is calculated using Collins formula, with which the period of the damage rings and other phenomena can be explained. (C) 2003 Elsevier Science Ltd. All rights reserved.