# Germanium Single Crystals for Photonics
## 8 The Effect of Photonic Drag of Current Carriers in Germanium  
When studying the time characteristics of powerful-pulsed CO\({}_{2}\) lasers, the active medium of which consists of a mixture of gasses (CO\({}_{2}\); N\({}_{2}\); He), an interesting problem was unexpectedly discovered. The fact is that such lasers in the free generation mode emit powerful pulses with an energy in the range of 1-1000 J, and sometimes more. The pulse duration in this case is several microseconds, and their shape is very specific (Figure 6) [79; 80]. The pulse is characterized by an initial peak with a duration of 100-200 ns, the amplitude of which is determined by the concentration of CO\({}_{2}\), and a \"tail\", the time duration of which depends on the concentration of nitrogen and helium in the working mixture. When working with such a laser, it is necessary to change the composition of the gradually degrading mixture quite often. It turned out that the pulse shape can be noticeably distorted; so, when conducting experiments, it is necessary to register it as accurately as possible. In addition, there are various methods for converting the pulse duration of CO\({}_{2}\) lasers, allowing this value to be changed in a wide range (10\({}^{-3}\)-10\({}^{-12}\) s) [93], which also stimulates the improvement of the process of measuring the shape of the laser pulse. When conducting laser experiments, a portion of the radiation is usually isolated for measurements using an optical wedge, which usually reflects \(\sim\)4% of the radiation. When using a highly sensitive photodetector for measurements, it is often necessary to additionally weaken the radiation entering it many times in order not to go beyond the dynamic range, which reduces the reliability of the measurements. Initially, photoresistors based on germanium alloyed with impurities of gold, mercury, zinc, etc., or based on CdHgTe compounds were used to measure the duration and shape of CO\({}_{2}\) laser pulses. Their use for these purposes is complicated by the fact that they only allow obtaining high-quality results, since these photodetectors, originally intended for recording weak signals, have excessively high sensitivity for this case, on the order of 3 V/W. When working with powerful lasers (10\({}^{6}\)-10\({}^{10}\) W and more), the radiation must be weakened many times, which naturally does not allow one to count on an accurate quantitative result. In addition, the dynamic range of such photodetectors usually does not exceed 2-3 orders of magnitude. This is clearly not enough for the pulse shape shown in Figure 6. In addition, they typically operate at liquid nitrogen temperature, and in some cases, even lower. Finally, they have a small area of reception platform (usually \(\sim\)2 \(\times\) 2 mm\({}^{2}\)) and an opaque input window made of a clear germanium plate, which significantly complicates alignment. The photodetectors described below, based on the effect of photon drag of current carriers, are free from these disadvantages. The effect of photon drag of current carriers is one of the manifestations of nonlinear phenomena in semiconductors. It was theoretically predicted in [37] and was initially discovered in p-type germanium single crystals under the influence of a pulse of radiation from a powerful CO\({}_{2}\) laser [38; 39]. Subsequently, this effect was also discovered in some other crystals when excited by powerful lasers of other frequencies. This effect is of considerable practical interest, since it turned out to be interesting for recording powerful laser pulse radiation [5; 40]. In particular, photodetectors for recording powerful pulse radiation of CO\({}_{2}\) lasers were made from p-type germanium single crystals. The IR radiation incident on the photodetector is absorbed, and in this case, the electromagnetic wave pulse can be transmitted to the charge carriers in the semiconductor, causing the appearance of a directed flow of charge carriers and thereby forming a current of entrainment [38; 39].