## 5 Discussion and Conclusions
Two crystals, used in our computer simulation, possess the following absorption coefficients for the radiation with wavelengths 4 mm and 10 mm: about 1.2 cm\({}^{-1}\) for the crystal AgGaS\({}_{2}\)[44]; and for the radiation with wavelength 24 mm: about 1.7 cm  
Figure 7: Two possible schemes of the frequency down-conversion to the THz range of the frequencies. for the crystal Te [36]. In dependence of a ratio between the intensities of the incident pulses the required crystals' lengths can change in the interval (2 mm, 7 mm) or or even longer if the nonlinear coupling coefficient \(\gamma\) is equal to unity. To decrease the absorption of the laser energy, one can increase the intensity of the incident pulses that it is possible because we used the low-intensive pulses. In this case, we can decrease the crystal length in two or even more times. Moreover, there is other possibility to decrease the laser energy losses caused by the absorption by the crystals.At present time, many new nonlinear crystals were developed and they are transparent up to the wavelength 31 \(\upmu\)m (Cs\({}_{2}\)Hg\({}_{3}\)I\({}_{8}\), [45]), 41 \(\upmu\)m (Cs\({}_{2}\)HgI\({}_{2}\)Cl\({}_{2}\), [46]), respectively, or even 50 \(\upmu\)m (6 THz, Cs\({}_{2}\)Hg\({}_{2}\)Br\({}_{2}\)I\({}_{4}\).H\({}_{2}\)O, [47]) (full review of this research can be seen in [48]). It should be mentioned that there are various IR laser, such as CO\({}_{2}\) lasers with the wavelength 10.6 \(\upmu\)m and the duration 200 ns [49], the fiber lasers with the wavelength 3.9 \(\upmu\)m and with the pulse duration 160  fs with maximal power 200 kW [50; 51], the quantum cascade laser that can generate a pulse with the wavelength \(\lambda_{1}=8\)\(\upmu\)m and with the pulse duration up to 630 fs possessing maximal power \(\max P(t)\) about of 4.5  W [52] and some others. Our scheme of the frequency down-conversion allows us to obtain the pulse with shorter duration and various wavelength and possessing broadband spectrum and with much high power density. Therefore, this scheme supplements the existence sources of the IR radiation and can be used for a generation of the THz broadband pulse. Additional remark about the application of the converted frequency to the THz of the frequencies. As a rule, the THz radiation refers to the range of frequencies belonging to interval 0.1-10 THz. Figure 7 illustrates how one can reach this frequency range by using proposed approach. One can also use the quantum cascade laser and sources of the IR pulsed radiation and the chosen crystal for the frequency down-conversion. We emphasize that the many dangerous substances have the absorption frequencies in the range of the frequencies 10-20 THz (see, for example, [53], Table 1 in [54], Table 9.1 in [55], Figure 1 in [56]. Evidently, that these frequencies can be used for the problem of the detection and identification of such substances that is very important for practice. Thus, the proposed scheme of the frequency down-conversion can be covered this frequency range. **Author Contributions:** V.A.T. : conceptualization, methodology, writing--review and editing, supervision, discussion; D.M.K. : software, investigation, writing--original draft, discussion; M.V.F. : formal analysis, discussion; Y.Y. : discussion, draft writing. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by Key Program of Joint Fund of the National Natural Science Foundation of China (U2001218); International (regional) Cooperation and Exchange Program of the National Natural Science Foundation of China (51875215). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** All available data is within the paper. **Conflicts of Interest:** The authors declare no conflict of interest.