We have studied laser induced material modification in a frequency tripling mirror (FTM) consisting of alternating hafnia and silica layers. The third-harmonic signal generated by a train of femtosecond laser pulses (791 nm, 55 fs, 110 MHz) drops over time until it reaches about 20% of the initial value. From the observed changes in reflection and transmission of the mirror a refractive index change of 0.07 was estimated, which occurs in the layer with the highest field enhancement. This index change triggers a drop in the field enhancement, which reduces the efficiency of nonlinear optical processes. The estimated value of ▵n allowed us to explain the 80% reduction in conversion efficiency and as well as an observed decrease in two-photon absorption.
The laser damage behavior of high quality coatings under nanosecond pulse illumination is controlled by statistically distributed defects, whose physical nature and defect mechanisms are still largely unknown. Defect densities are often retrieved by modeling the fluence dependence of the damage probability measured by traditional damage test (TDT) methods, based on ‘damage’ or ‘no damage’ observations. STEREO-LID (Spatio-TEmporally REsolved Optical LaserInduced Damage) allows the determination of the damage fluence (and intensity) in a single test by identifying the initiation of damage both temporally and spatially. The advantages of this test method over the TDT are discussed. In particular, its ability to retrieve detailed defect distribution functions is demonstrated by comparison of results from HfO<sub>2</sub> films prepared by ion-assisted electron beam evaporation, ion-beam sputtering, and atomic layer deposition.