<title>Investigation of neutral atom and ion emission during laser conditioning of multilayer HfO<formula><inf><roman>2</roman></inf></formula>-SiO<formula><inf><roman>2</roman></inf></formula> coatings</title>

Abstract

It has been recently shown that the optical damage thresholds of multilayer coatings can be increased by up to a factor of two by conditioning the coating at laser fluences below the optical damage threshold. We have investigated the laser conditioning mechanism of Hf02-Si02 coatings by observing the laser-induced emission of neutral constituents of the coatings during laser conditioning. We observe the ejection of Hf, Si and oxygen (0 or 02, or both) neutrals starting at fluences of about 10 J/cm2, considerably below our measured unconditioned damage threshold of about 16-18 J/cm2. We observed a threshold pulse fluence for laser conditioning, which is also about 10 J/cm2. This identical threshold for neutral emission and laser conditioning suggests that the neutral emission is caused by the laser conditioning process. The emitted neutrals have extremely high kinetic energies, on the order of several tens of eV. On the basis of this information, we propose that the laser conditioning process is caused by microscopic damage that involves cracking of the coating layers, resulting in the ejection of the high-energy particles via a "fractoemission" process. This microscopic damage apparently raises the damage threshold by either reducing the absorption of coating defects or by relieving the thermal stresses that cause cracking, particle emission, and possible plasma formation by laser ionization of the neutrals.