Room-temperature transmittance measurements at 10.6 µm were made of germanium single-crystal samples. The samples were cut from very large doped single-crystal slabs that had been grown from +40-Ω-cm charge material of unknown purity for use as infrared windows and image-forming elements. The dependence of the absorption coefficient on sample resistivity was calculated from measured transmittances of ninety-two samples with resistivities ranging from 0.9 Ω-cm to 57 Ωcm. Maximum transmittance at 10.6 µm was obtained for the samples that were doped to a resistivity of 5–10 Ω-cm and n-type conductivity.
Evan D. Capron and Orville L. Brill, "Absorption Coefficient as a Function of Resistance for Optical Germanium at 10.6µm," Appl. Opt. 12, 569-572 (1973)