# Germanium Single Crystals for Photonics
## 2 Absorption and Scattering of Germanium in the IR Region of the Spectrum
This can apparently explain some excess of the spectrophotometrically measured value of \(\beta_{10.6}\) over the calorimetric measurements [10; 11; 58; 59], where only volume absorption was recorded at \(\lambda\) = 10.6 \(\mu\)m. These same effects probably led to a decrease in the value of the power coefficient \(\gamma\) in the measured frequency dependence \(\beta\)\(-\)\(\lambda^{\gamma}\)[59]. As shown in [2], in the region of small values of \(\beta\) for Ge, the value of the coefficient \(\gamma\) should be \(\approx\)2. With the advent of technology for forming optical parts using high-temperature deformation, an attempt was made to carry out this process when germanium optics are produced. It has been established [60] that plastic deformation leads to a noticeable increase in the absorption coefficient of germanium single crystals. It turned out that changes in the absorption spectrum and conductivity indicate that dislocations arising as a result of deformation at room temperature have acceptor properties. In the presence of scattering [21; 23], the Bouguer-Lambert law for light attenuation is not strictly satisfied, and the known formulas relating transmission and attenuation with noticeable scattering become inaccurate. The differences increase with the growth of the ratio of the probabilities of scattering and absorption of photons in a layer of unit length, and the greater the ratio of the length of the crystal to its diameter, the greater this ratio [26]. For a number of applications, primarily thermal imaging (receiving, processing, and transmitting images), scattering leads to a decrease in image contrast and resolution [15; 16]. In this range, small-angle Mie scattering occurs, and the proportion of scattered radiation can reach 20% or more, depending on the characteristics of the specific part made from germanium. Due to the scattering of radiation at the grain boundaries, Ge polycrystals scatter light several times more intensely than single crystals. The scattering coefficients of Ge (10\({}^{-3}\)-10\({}^{-1}\) cm\({}^{-1}\)), corresponding to these losses \(\beta_{\rm{sc}}\), are included in the light-weakening (attenuation) coefficients. In many samples, they are comparable with the extinction coefficients themselves and often exceed the value of \(\beta\)[23; 25]. Scattering of radiation in Ge occurs due to the presence of growth thermoelastic stresses, impurity inhomogeneity, and structural defects. Maximum scattering is observed in doped mono- and polycrystals with a large number of defects (low-angle boundaries, inclusions, block structure) [21; 61]. High-temperature annealing of crystals leads to a decrease in the intensity of light scattering by several times, which indicates a change in the size and shape of the scattering inhomogeneities, and, possibly, their partial disintegration. During heat treatment of germanium crystals, the following two processes occur: dissociation of oxygen complexes with antimony during heat treatment (with removal of antimony from the impurity cloud) and growth of oxygen clouds due to oxygen diffusion. The use of ultra-pureraw materials, improvement of Ge growth technologies, and high-temperature annealing of grown crystals makes it possible to reduce the loss coefficient [23]. The presence of a sharp temperature dependence of \(\beta\) limits the use of optical elements made of Ge in continuous lasers. Germanium windows in gas lasers operate satisfactorily at radiation power densities of 100-250 W/cm\({}^{2}\) if they can be effectively cooled [9]. However, the use of Ge in continuous lasers has virtually ceased after the creation of optical elements from polycrystalline zinc selenide obtained by the CVD method. Although this material has much worse mechanical properties, it has better optical speed and is much less sensitive to temperature changes.