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 μm is at a level of ~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.
The spectral dependences of the transmittance of CsI, AgCl, KRS-5 and KRS-6 single crystals in the infrared (IR) and terahertz (THz) ranges have been measured. The spectral dependences of the absorption (attenuation) coefficient of these crystals were calculated in the spectral range of 200–3000 μm. It was found, that these crystals are transparent in the millimeter range, which makes it possible to use materials when working with THz radiation. It should be noted that, in contrast to the IR spectral region, the optical quality of the surface of the crystal samples practically does not affect the transmission in the millimeter range.
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