## 4 Discussions  
We compared our results on refractive index measurement of the GaSe:S crystals at a wavelength of 1547 nm and at a frequency of 1 THz with previously published reports. In graphical form, the data are presented in Figures 8 and 9, respectively. The numerical data for \(\lambda=1547\) nm are presented in supplementary (Table S1) for convenience. Additionally, it reflects the refractive index measurement technique and the method of crystal growth for each reference, if they were indicated in the article. Ordinary refractive indexes measured in the IR and THz ranges are generally in good agreement with previous work. Figure 8: Ordinary refractive index of the GaSe:S crystals at a frequency of 1 THz. Data, measured in this work, are presented in bars and compared with the results published in other works: [PERSON] (2015) [25], [PERSON] (2017) [38], [PERSON] (2015) [53], [PERSON] (2009) [54], [PERSON] (2005) [63]. Some data for undoped GaSe is shifted horizontally for better readability. Figure 7: Estimated electro-optic coefficient \(r_{22}\) of GaSe:S crystals. In the terahertz range, the value of the ordinary refractive index is in good agreement between most studies. For example, the values for undoped crystals lie in the range from 3.24 to 3.264, which approximately corresponds to an interval of 0.78%. An exception is the work of [PERSON] et al. (2005) [63] in which the refractive index differs significantly from other studies (Figure 8). The authors also report a rather high absorption coefficient for the crystal, which is inconsistent with data from other works in which high-quality crystals were studied. In our case, the crystals also show an extremely low level of absorption for THz waves. The samples in [63] have a high DC conductivity, possibly due to the defects in the crystals. Such conductivity makes a significant contribution to the dielectric susceptibility of the material and, as a consequence, leads to an increase in the refractive index and absorption coefficient at THz frequencies. Figure 9 shows ordinary refractive index of the GaSe:S crystals at \(\lambda=1547\) nm. Since no study has performed direct measurements at this wavelength, the value of the refractive index has been estimated using the suggested in the works Sellmeier equations. It can be seen that the old studies carried out by [PERSON] et al. (1975) [42] and [PERSON] et al. (1982) [34] reveal a significant deviation from our results, which may be explained by the imperfection of the crystals due to the limitations of the growth technology at the time. However, in earlier work [PERSON] et al. (1972) [64] report on the refractive index value agrees with our findings. According to the study [42], after a more precise measurement, the authors received results that sharply deviated from the present results. It may also be due to the variation in quality between samples, and the measurement accuracy. [PERSON] and [PERSON] (2011) [56] proposed to use an effective medium model, based on the values of the refractive indices of pure GaSe and GaS crystals, for estimation of the refractive index of GaSe:S intermediate compounds. From our perspective, the model is effective up to \(x=0.12\). At higher \(x\) values, the divergence starts to increase. The reliability of this approach at high sulfur concentrations has to be tested further. \(r_{22}=0.975\) pm/V, while the crystal with \(x=0.12\) has \(r_{22}=1.262\) pm/V and demonstrates the greatest detection efficiency. These data are in good agreement with the values of 1.3 pm/V [47] and 1.1 pm/V [48] obtained at a wavelength of 1.064 micrometers when studying the Pockels effect in undoped GaSe. Our study also confirms that the value of the optimal sulfur concentration \(x\) is in the range from 0.04 to 0.12, which is consistent with the results obtained in [66]. Despite having a comparable electro-optical coefficient, GaSe:S has half the coherence length of GaAs. According to the expression (2), this reduces the detection efficiency of THz waves by half.