Ultrathin Ge films with thickness of about 15 nm at different deposition temperatures were prepared by electron beam evaporation. Spectral measurement results showed that as the deposition temperature increased from 100°C to 300°C, the transmittance of the films in the wavelength range from 350 nm to 2100 nm decreased. After annealing in air at 500°C, the transmittance significantly increased and approached that of uncoated fused quartz. Based on the Tauc plot method and Mott–Davis–Paracrystalline model, the optical band gap of Ge films was calculated and interpreted. The difference in optical band gap reveals that the deposition temperature has an effect on the optical band gap before annealing, while having little effect on the optical band gap after annealing. Furthermore, due to oxidation of Ge films, the optical band gap was significantly increased to ～5.7 eV after annealing.

Laser resistance and stress-free mirrors, windows, polarizers, and beam splitters up to 400 mm×400 mm are required for the construction of the series SG facilities. In order to improve the coating quality, a program has been in place for the last ten years. For the small-aperture pick-off mirror, the laser-induced damage threshold (LIDT) is above 60 J/cm2 (1064 nm, 3 ns), and the reflected wavefront is less than λ/4 (λ=633 nm). The Brewster-angle polarizing beam splitter (Φ50×10 mm) shows the best LIDT result, up to 29.8 J/cm2 (1064 nm, 10 ns) for a p-polarized wave in the 2012 damage competition of the XLIV Annual Boulder Damage Symposium. For the larger-aperture mirror and polarizer, the LIDT is above 23 J/cm2 (1064 nm, 3 ns) and 14 J/cm2 (1064 nm, 3 ns), respectively. The reflected wavefront is less than λ=3 (λ=633 nm) at the used angle.