# Germanium Single Crystals for Photonics
## 12 Transmission of Germanium in the THz Spectral Region  
Due to the extreme shortage of optical materials, especially nonlinear crystals, for the terahertz (THz) range of the electromagnetic spectrum (\(\sim\)3 mm-30 \(\mu\)m, 3 cm\({}^{-1}\)-300 cm\({}^{-1}\)) [104], the possibilities of using germanium in this spectral region have recently been considered. It turned out that the active elements of germanium acousto-optic devices are very interesting for use in this range [105]. In [12], the optical properties of pure and doped Ge in the THz range were studied. It is shown that germanium can be used in multispectral infrared imaging devices of the IR + THz range, as well as for the production of output windows of THz gas lasers with optical pumping by CO\({}_{2}\) laser radiation.  
Figure 12: SEM images of the {111} surface of dislocation Ge after exposure to scanning nanosecond radiation of a UV laser with an energy density of E = 1.14 J/cm\({}^{2}\): magnification—\(\times\)1000. Reprinted with permission from Ref. [101]. 2023, Pleiades Publishing, Ltd.  
The transmission spectra of Ge with different resistivity \(\rho\) are shown in Figure 13[12]. The results showed a noticeable increase in absorption in the doped Ge crystals in the THz range compared to undoped (\(\rho\) = 47 Ohm\(\cdot\)cm). An increase in the concentration of doping impurities, both electron and hole, leads to an increase in absorption. If in the range of 25-50 \(\upmu\)m, the effect of \(\rho\) on the transmission of Ge(Sb) is not noticeable, then in the range of \(\geq\)220 \(\upmu\)m, it is observed directly.  
The main contribution to the absorption of radiation in germanium single crystals in the THz range is made by free-charge carriers (intrinsic and impurity). If we compare the transmission spectra data for Ge (spectral region \(\lambda\)\(\sim\)160-220 \(\upmu\)m) and for silicon [12], they show that the attenuation coefficient in the region of 160-220 \(\upmu\)m is approximately the same and is \(\sim\)0.5 cm\({}^{-1}\).  
In contrast to the IR range, in the THz region, minimal losses of \(\sim\)0.5 cm\({}^{-1}\) are observed in undoped germanium crystals in the region of \(\lambda\)\(\sim\)160-220 \(\upmu\)m. Significant losses due to Fresnel reflection can be largely compensated by creating periodic relief structures on the surface of the optical element with a high degree of regularity and a period shorter than the radiation wavelength. Therefore, optical products made of undoped single-crystal germanium can be used quite effectively to control radiation in the THz range (100-300 \(\upmu\)m range).  
In [106], the transmission spectra of all five single crystals of isotopically pure germanium were investigated in the THz spectral region. It was shown that the absorption minimum in the wavelength range of 30-3000 \(\upmu\)m is observed in the range of 200-800 \(\upmu\)m, and the corresponding absorption coefficient for this range is less than 1 cm\({}^{-1}\) for most of the investigated single crystals of isotopes. In the range of 1000-3000 \(\upmu\)m, a tendency was found for the absorption coefficient to increase with increasing mass number of the isotope.