Study on the fourth harmonic beam splitter at 266 nm for ND : YAG laser

We design a fourth harmonic beam splitter for ND : YAG laser fabricated by electron beam evaporation with HfO₂/SiO₂. At 45° incident angle, the transmittance of the as-deposited film is 99.49% at 1064 nm and 98.96% at 532 nm, while the reflectivity is 99.80% at 266 nm. The films are annealed in air at temperatures from 200 °C to 400 °C. We study the optical properties, microstructure, and surface morphology of as-deposited and annealed films. The transmission spectra of the films show blue shift with increasing annealing temperature. X-ray diffraction (XRD) results show that all the films exhibit polycrystalline with a monoclinic structure of HfO₂ and an amorphous structure of SiO₂. The crystallite size and crystallinity of the film increase at annealing temperature of 400 °C, and there is no obvious change at annealing temperatures of 200 °C and 300 °C. According to the results of atomic force microscopy (AFM), the surface root-mean-square (RMS) roughness of the film significantly decreases at annealing temperatures of 400 °C.