Broadband optical spectral shift analysis of dielectric coatings.

High-energy laser facilities require high reflection multilayer coatings on meter-scale substrates. Due to stringent use specifications, a precise control of deposition parameters is necessary to tailor the optical and mechanical properties of components. The resulting coatings are sensitive to relative humidity variations, leading to a shift of their optical spectra called spectral shift. This spectral shift is generally observed on a narrow range, near the operating wavelength. Here we extend the concept of spectral shift to a broader spectral range. This analysis serves as a tool to study the behavior of a multilayer coating spectrum with relative humidity. To validate the spectral shift determination method, we compared the spectral shift of single layers induced by the relative humidity with simulated optical properties induced by either thickness or refractive index variations. In addition to the validation of the approach, the fitting results and the comparison between spectral shift shapes show that relative humidity variations mainly impact the refractive index of the layers and SiO2 is more sensitive than HfO2.

Influence of absorption-edge properties on subpicosecond intrinsic laser-damage threshold at 1053 nm in hafnia and silica monolayers.

Owing to their relatively high resistance to laser-induced damage, hafnia and silica are commonly used in multilayered optical coatings in high-power laser facilities as high- and low-refractive-index materials, respectively. Here, we quantify the laser-induced-damage threshold (LIDT) at 1053 nm in the short-pulse regime of hafnia and silica monolayers deposited by different fabrication methods, including electron-beam evaporation, plasma ion-assisted deposition and ion-assisted deposition. The results demonstrate that nominally identical coatings fabricated by different deposition techniques and/or vendors can exhibit significantly different damage thresholds. A correlation of the LIDT performance of each material with its corresponding absorption edge is investigated. Our analysis indicates a weak correlation between intrinsic LIDT and the optical gap of each material (Tauc gap) but a much better correlation when considering the spectral characteristics in the Urbach tail spectral range. Spectrophotometry and photothermal absorption were used to provide evidence of the correlation between the strength of the red-shifted absorption tail and reduced LIDT at 1053 nm.