Multilayer dielectric high-reflectance coatings were investigated for potential application in a high-average-power, 1-μm wavelength, free-electron laser oscillator. Of concern are the ultraviolet harmonics of the primary wavelength generated within the oscillator that tend to degrade the dielectric coatings. These coatings are required to be high reflecting and low absorbing with respect to the 1-μm wavelength and resist degradation/damage from the ultraviolet harmonics.

A KrF excimer laser was used to generate the low fluence ultraviolet light at 248-nm wavelength, a wavelength that simulates the fourth harmonic of the 1-μm free-electron laser. A variety of candidate dielectric oxide thin film materials are compared in this report. In each multilayer dielectric high reflector measured, a candidate high-index dielectric material is used in combination with SiO² as the low-index material. Damage test measurements are reported for the 248-nm wavelength with HfO² being the high-index material with significantly the best results.