Interaction of intense IR laser radiation with a-C:H protective coatings

The results of a determination of the optical breakdown thresholds of air near the surface of copper laser mirrors with a-C:H protective coating by intense pulsed 10.6 µm radiation are analyzed and systematized. It is shown that there is no correlation between the breakdown threshold of a coated mirror, the reflectance of the coating, and the breakdown threshold of the initial copper surface. Experimental dependences of the threshold of optical breakdown of coated mirrors on the a-C:H thickness and deposition rate as well as the storage time of the mirrors are given. Estimates are made of the rise in the surface temperature in the irradiation zone for the case of an ideal adhesive contact and calculations are made of the damage threshold of the coating in the case that the adhesive contact between the mirror and the coating is impaired. The effect of the a-C:H coating properties and the conditions at the polished metal surface-protective coating interface on the optical breakdown threshold is discussed.