Efficient T1I/KC1/T1I antireflection (AR) coatings (< 0.1% reflectance per surface), which are highly resistant to laser radiation damage (< 0.04 % absorption per AR coating at A = 10.6 um) have been fabricated. The three-layer AR coatings were designed by replacing one of the layers of a two-layer AR coating with a Herpin equivalent structure. Thickness of the layers of the resultant three-layer AR coatings can be varied continuously over ranges defined by the indices of the coating materials and the substrate. The ability to vary the thickness of the layers enables the optimization of the coating design with respect to one or more requirements, for example, low absorption, environmental durability or ease of fabrication. The TlI films deposit with an orthorhombic structure, the orientation of which is dependent on the KC1 substrate orientation. Therefore, the refractive index of the TlI films depends on the substrate orientation. The influence of the substrate orientation on the TlI refractive index can be eliminated by coating the KC1 substrate with a thin layer (~200 A) of SrF2 prior to depositing the TlI film.
W. T. Boord,
H. Y. B. Mar,
M. C. Ohmer,
"Low Absorption T1I/KC1/T1I Antireflection Coatings For KC1 Surfaces," Optical Engineering 18(6), 186586 (1 December 1979). https://doi.org/10.1117/12.7972439