Research to improve the optical performance and laser-induced damage threshold of hafnium oxide/silica dichroic coatings

The high optical performance and laser-induced damage threshold (LIDT) of broadband hafnium oxide/silica (HfO2/SiO2) dichroic coatings with high transmittance and reflectance around 532 and 1064 nm, respectively, are essential for the efficiency of a harmonic separation system. However, the transmittance of dichroic coatings tends to drop rapidly at 532 nm because of the half-wave hole effect. Moreover, an intense electric field penetrates the coating (similar to 6 mu m thick) to the substrate, thereby generating a challenge to improve the LIDT at 532 nm. In this paper, we study the methods to improve the optical performance and LIDT of HfO2/SiO2 dichroic coatings prepared by electron-beam evaporation. The inhomogeneity in the growth characteristics of the HfO2 layer is studied and considered in the design of dichroic coatings, which can improve spectral performance, especially by broadening the transmittance bandwidth around 532 nm. Additionally, the effects of the different treatment procedures, including ultrasonic cleaning, acid-solution etching, and argon-ion-beam etching, of the fused silica substrate on the LIDT of dichroic coatings are investigated by the raster-scan damage test method. We observe that the LIDT of dichroic coatings at 532 nm is limited by the surface and subsurface defects of the substrate. Etching similar to 200 nm of the surface of the fused silica substrate can effectively inhibit the occurrence of low-energy catastrophic growth damage. Compared with acid-solution etching, argon-ion-beam etching can yield improved substrate surface quality and increased LIDTs of dichroic coatings at 532 nm.