Investigation of optical damage mechanisms in hafnia and silica thin films using pairs of subnanosecond laser pulses with variable time delay

Abstract

Optical damage thresholds of submicron-thick, electron beam deposited HfO2 and SiO2 films on BK-7 substrates have been measured by monitoring the emission of neutral constituents during excitation with time-delayed pairs of 70-ps laser pulses at a wavelength of 1064 nm. The dependence of the optical damage threshold on time delay provides evidence of the optical damage mechanism. For SiO2, linear absorption is the mechanism for energy deposition into the films by the laser beams. The data for HfO2 are less definitive, although linear absorption is the most likely damage mechanism. The behavior of the single-layer films is compared to multilayer HfO2-SiO2 high-reflector coatings, for which a ‘‘conditioning’’ effect causes an increased optical damage threshold due to multiple pulse laser excitation at fluences below the single-pulse optical damage threshold.