## 5 Conclusions  
In this work, we measured the ordinary refractive index at a wavelength of 1547 nm and in the range of 136-1500 \(\upmu\)m (2.2-0.2 THz, respectively) of GaSe\({}_{(1-x)}S_{x}\) (GaSe:S) crystals, where \(x=0\), 0.03, 0.12, 0.16, and 0.22. The dispersion of the refractive index in the THz range is in good agreement with previous studies [41; 52; 53; 54]. The refractive index at \(\lambda=1547\) nm is in good agreement with the data presented in modern works [34; 39; 40; 41; 42; 56; 64; 65]. It is demonstrated that the refractive index consistently reduces in both the near-IR and THz ranges when the sulfur concentrations increase.  
We evaluated the electro-optic coefficient of the crystals. It is shown that at \(x=0\), the value of the electro-optical coefficient is \(r_{22}=0.975\) pm/V, while the crystal with \(x=0.12\) has \(r_{22}=1.262\) pm. The results are in good agreement with the values previously published data at a wavelength of 1.064 microns [47; 48].  
The results obtained in this work will be useful for accurate estimates of the efficiency of down conversion of the laser frequencies into the terahertz range, for terahertz electro-optic effects, as well as for the engineering of modulators for telecommunication wavelengths in the vicinity of 1.55 \(\upmu\)m and for the development of nonlinear optical devices for integrated photonics, based on crystals of GaSe doped with sulfur.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13042045/s1](https://www.mdpi.com/article/10.3390/app13042045/s1)]([https://www.mdpi.com/article/10.3390/app13042045/s1](https://www.mdpi.com/article/10.3390/app13042045/s1)), Table S1: GaSe:S refractive index for ordinary wave at the wavelength of 1547 mn.  
Conceptualization, N.A.N.; Crystal growth, Samples preparation K.A.K.; Investigation, O.N.S. (THz spectroscopy, assembling optical installation, THz detection efficiency measurement, refractive index measurements at 1547 nm), K.A.K. (EDX), S.L.M. (Refractive index measurements at 1547 nm), and N.A.N. (THz spectroscopy); Project administration, N.A.N.; Resources, N.A.N., S.L.M.; Validation, N.A.N., S.L.M.; Writing--original draft, O.N.S.; Writing--review and editing, N.A.N. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of the Russian Federation: project No. FSUS-2020-0029 (in terms of assessing the potential of employing GaSe crystals in future telecom devices for big data transmission systems); project No. 121032400052-6 (in terms of studying the linear and nonlinear optical properties of the crystals for the terahertzfrequency range); and within the state assignment project of the Sobolev Institute of Geology and Mineralogy (in terms of improving the growth technology of doped GaSe crystals).  
Not applicable.  
The data presented in this study are available on request from the corresponding authors.  
The authors acknowledge the Shared Research Center \"VATN\" of the Novosibirsk State University supported by the Ministry of Science and Higher Education of the Russian Federation (agreement #075-12-2021-697). The authors acknowledge the Shared Equipment Center \"Spectroscopy and Optics\" of the Institute of Automation and Electrometry SB RAS. The authors express their gratitude to [PERSON] for help with translation and [PERSON] for help with review and editing.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.