The effect of thermal annealing on the optical properties, microstructure, and laser-induced damage threshold (LIDT) of HfO₂/Ta₂O₅/SiO₂ HR films has been investigated. The transmission spectra shift to a short wavelength and the X-ray diffraction peaks of monoclinic structure HfO₂ are enhanced after thermal annealing. The calculated results of the m( 111) diffraction peak show that the HfO₂ grain size is increased, which is conducive to increasing the thermal conductivity. Thermal annealing also reduces the laser absorption of high-reflection films. The improvement of thermal conductivity and the decrease of laser absorption both contribute to the improvement of LIDT. The experimental results show that the highest LIDT of 22.4 J/cm² is obtained at 300°C annealing temperature. With the further increase of annealing temperature, the damage changes from thermal stress damage to thermal explosion damage, resulting in the decrease of LIDT.

In our investigation, lead germanium telluride, which is a pseudo-binary alloy of IV-VI narrow-gap semiconductor compounds of PbTe and GeTe, can be used in the fabrication of mid-wavelength infrared narrow bandpass filters as a high-index coating material, due to its high refractive index, lower absorption, and tunability of fundamental absorption edges. It is demonstrated that a half-width of 160 nm and a better rejection ratio can be obtained for a simple 8-layer double cavity filter with a central wavelength at 4 μm, compared with a half-width of 390 nm for those conveniently fabricated using Ge as high-index material.