## 4 Conclusions  
In the presented work, methods of modifying the optical properties of PbSe films were compared: laser modification in the bleaching mode and in the darkening mode using continuous radiation with a wavelength of 405 nm; laser modification using pulsed radiation with a wavelength of 1064 nm; and heat treatment in an oven. The mode of laser exposure that makes it possible to provide the maximum decrease in reflection and the maximum absorption coefficient value, \(\alpha\) = 9220 cm\({}^{-1}\)[25], was observed during scanning with continuous UV radiation in the darkening mode. As a result of the laser exposure and treatment in the furnace, the refractive index n decreased, reaching its minimum value in the sample modification case in the darkening mode using short pulses. Moreover, this modification mode was characterized by a maximum decrease in the band gap value, Eg = 1.3 eV, relative to the raw film, which has Eg = 1.59 eV. However, the band gap value for samples treated in the bleaching mode using short laser pulses was 1.45 eV, which exceeded the value of Eg = 1.33 eV for films after thermal exposure in an oven and after laser exposure at a wavelength of 405 nm in the darkening mode. The pulsed IR radiation used for modification turned out to be less effective, since the decrease in reflection is commensurate with modification upon annealing in a furnace. The next stage of this study is to measure the electrical lead selenide films' characteristics after treatment with laser-pulsed IR radiation and compare with the results after exposure to UV radiation. The results obtained show that film modification is a promising direction for improving optical characteristics. In the future, it will be possible to use processed chalcogenide films as organic substances sensors, as well as in gas analysis applications. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{Heat treatment} & A long-established method & Destructive influence \\  & Allows you to achieve & Uncontrollable \\  & increased absorption & Growth of crystalline oxide \\  & & Badly reproduced \\ \hline \multirow{4}{*}{NUV radiation} & \multirow{2}{*}{Darkening} & The highest absorption is & \\  & & achieved in the visible range & Possible film degradation \\  & & and in the range of 3-3.5 μm & over time \\  & & (CH\({}_{4}\) absorption peak) & \\ \cline{2-3}  & \multirow{2}{*}{Bleaching} & In contrast to heat treatment, a & \\  & & non-crystalline oxide is formed & Possible film degradation \\  & & Achieves the highest & over time \\  & & absorption in the range & Oxide formation on the surface \\  & & 4.5–5 μm (CO absorption peak) & \\ \hline \multirow{4}{*}{NIR radiation} & \multirow{2}{*}{Darkening} & Allows you to achieve & \\  & & increased absorption & \\ \cline{1-1} \cline{3-3}  & \multirow{2}{*}{Bleaching} & Less destructive mode than & \\ \cline{1-1} \cline{3-3}  & & darkening mode & Possibly destructive effect \\ \cline{1-1} \cline{3-3}  & & In contrast to heat treatment, a & Oxide formation on the surface \\ \cline{1-1} \cline{3-3}  & & non-crystalline oxide is formed & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of PbSe film processing methods. **Author Contributions:** Conceptualization, A.A.O. and M.M.S. ; methodology, A.A.O., N.K.K., N.V.N. and M.A.D. ; software, A.A.P. and M.A.D. ; validation, A.A.O., M.A.D., A.A.P. and M.M.S. ; formal analysis, A.A.P., N.K.K. and M.A.D. ; investigation, A.A.P. and M.A.D. ; resources, A.A.P. and M.A.D. ; data curation, A.A.O. ; writing--original draft preparation, A.A.O., A.A.P. and M.A.D. ; writing--review and editing, M.M.S. and A.A.O. ; visualization, A.A.O., N.K.K., N.V.N. ; supervision, M.M.S. and A.A.O. ; project administration, A.A.O. ; funding acquisition, M.M.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Russian Science Foundation grant (project No. 19-79-10208). **Institutional Review Board Statement:** Not applicable.