Optical properties of single-crystal germanium in the THz range

The transmission of intrinsic, antimony-doped, and gallium-doped Ge single crystals in the THz spectral range have been experimentally investigated. It is shown that the attenuation coefficient of intrinsic germanium in the range of 160–220 $\mu$m is at a level of $\sim$0.5 cm$^{-1}$, a value comparable with that for silicon. The free-carrier absorption cross sections of silicon and germanium are significantly different, which may be caused by the difference in the mechanisms of carrier–phonon interaction in these materials.