Deposition of optical coatings by pulsed laser ablation

Abstract

Hafnia, zirconia and yttria films for optical applications were prepared by laser ablation using an excimer laser at 248 nm wavelength. Films were deposited at room temperature either in an oxygen atmosphere or with additional oxygen ion bombardment of the growing films. We will show that laser ablated oxide films have a high refractive index approaching that of the corresponding bulk material and, hence, a high packing density. Moreover, the films possess high laser damage thresholds at 1.06 micrometers wavelength, though they are still somewhat lower than those of good electron beam evaporated films. Oxygen ion bombardment leads above a certain threshold of ion energy and current density to a decrease in refractive index. In the case of hafnia, for example, it decreases from 2.15 down to 1.80 at 600 nm wavelength. Experimental proof will be given that this behavior is a result of ion induced modifications of microstructure. While films with high refractive index were of amorphous structure and had a high packing density with low porosity, increasing ion bombardment of the growing films leads to increasing crystallization within the films and, finally, to polycrystalline films combined with increasing grainlike film growth. Larger voids between the grains result in lower packing density and, therefore, lower refractive index. Based on these findings multilayer systems of only one material with ion controlled refractive index variations were prepared and investigated with regard to their laterally resolved absorption and their laser damage thresholds.