# Germanium Single Crystals for Photonics
## 9 Acousto-Optic Effects in Germanium  
At present, optical radiation control devices, the operation of which is based on the acousto-optic effect (the interaction of light with sound in an optically transparent medium), are an important component of such a technological section as optical electronics. The acousto-optic effect (AO) [94; 95] is a nonlinear effect; the effect allows one to control all parameters of optical radiation: intensity, frequency, direction of propagation, polarization, frequency, and spatial spectrum. The passage of an acoustic wave in a photoelastic medium induces changes in the instantaneous value of the refractive index,  
Figure 10: Characteristics of a photodetector based on the photon drag effect of holes in germanium: (**a**) volt-watt characteristic of the photodetector; (**b**) dependence of the output signal of the photodetector on the resistance. Reprinted with permission from Ref. [5]. 2015, [PERSON] V.E. which leads to the formation of a phase grating with a period equal to the length of the acoustic wave and an amplitude proportional to the amplitude of the acoustic wave and the photoelastic constant of the crystal. Such capabilities, combined with purely technological advantages--reliability, compactness, and, as a rule, fairly low power consumption of AO devices, determine the wide application of AO devices. The modern trend in the development of acousto-optics is associated with the increasing introduction of AO devices into technology, which leads to a constant increase in the production volumes of AO devices in our country and in the world. In laser technology, acousto-optic devices are used as laser shutters; AO modulators are indispensable for modulating the amplitude and changing the frequency of optical radiation. AO deflectors are used both to control the direction of propagation of optical radiation (for example, in lidars) and for tunable cylindrical lenses of scanning systems in ultra-high-resolution optical microscopes. AO devices turned out to be useful in systems for generating short and ultrashort laser pulses of high and ultrahigh power. Widely used applications of AO devices (Q-switches) include their use for synchronization of longitudinal laser modes. AO filters are indispensable devices for managing and analyzing the spectral composition of optical radiation, creating compact spectrometers, including imaging ones. Until recently, the existence of powerful sources of optical radiation in the infrared spectral range was actually limited to the CO\({}_{2}\) laser. At present, quantum cascade lasers have appeared that operate in the spectral range of generated radiation from 2.5 to 300 \(\upmu\)m. For example, quantum cascade lasers are used to detect various gasses by absorption lines located in the range from 2.5 to 15 \(\upmu\)m. Free-electron lasers, capable of emitting in a wide range of wavelengths, are also being actively developed. They have made it possible to significantly expand the areas of practical use of infrared and terahertz radiation [96]. The sharp increase in the number of practical applications has exacerbated the existing problem of controlling the parameters of optical radiation with such wavelengths. As in the case of shorter wavelengths of optical radiation, the use of AO devices in the thermal IR range seems quite promising. However, the development of AO devices that effectively function in this spectral region is limited by the presence of suitable materials for creating AO cells. Germanium is one of the main materials in acousto-optics of the mid- and far-infrared ranges, due to its good physical and chemical properties and high values of the refractive index and photoelastic constants [50]; moreover, for CO\({}_{2}\) lasers, it is, today, the only acousto-optic material used. Acousto-optic modulators based on germanium for CO\({}_{2}\) lasers are mass-produced by many companies (see, for example, [51]).