Influence of LN2 substrate cooling on optical properties of laser-pulse-deposited oxide films

In this paper we will present the results of investigations of the influence of substrate cooling by liquid nitrogen during film deposition on the optical properties of oxide films prepared by means of laser pulse deposition. A CO2-TEA laser at 10.6 μm wavelength (pulse duration 200 ns, repetition rate 40 Hz, laser fluence 5 × 107 W/cm2, monomode) and an excimer laser at 248 nm wavelength (pulse duration 20 ns, repetition rate 200 Hz, laser fluence 5 × 107 W/cm2) were used for the evaporation and, respectively, the ablation of sintered oxide targets. Moreover, the growing films were bombarded by oxygen ions of varying energy and current density. As will be shown, the variation of refractive index with ion bombardment changed only imperceptibly due to substrate cooling, i.e. films deposited at relatively low ion energy and current density possess a high bulk-like refractive index and films deposited at relatively high ion energy and current density possess a rather low refractive index. We could observe, however, that, in comparison with uncooled substrates, cooling of the substrates by liquid nitrogen results in a marked decrease in absorption together with a marked increase of the laser damage thresholds of the films at 1.06 μm wavelength. For example, the laser damage threshold of hafnia films increased by a factor of 1.5.