## 3 Results and Discussion  
### Optical Characterization  
At the beginning of the study, an analysis of the initial PbSe film structure and samples subjected to heat treatment in an oven was carried out, and their optical characterization is also given (Figure 3). Figure 3: Images of a PbSe film obtained using optical microscopy in bright field reflected light: (**a**) initial film; (**b**) film after heat treatment. Pictures of the PbSe film obtained with the SEM secondary electron detector: (**c**,**e**) initial film; (**d**,**f**) film after heat treatment. A picture of the PbSe film obtained using an X-ray diffractometer (**g**) initial film; (**h**) after heat treatment. During the film's heat treatment in the oven, the inhomogeneities of its structure became the centers of the second phase nucleation (PbSeO\({}_{3}\)), while the material matrix became denser, which led to a decrease in its transmission. Since the oxide is transparent, an increase in the second phase concentration during heating leads to film bleaching. Moreover, the thermal effect contributes to an increase in the concentration of defect centers, which become secondary sources of free charge carriers in the film, leading to an increase in the conductivity of chalcogenide films. The second phase of high concentration provides a significant increase in the entire film transmission (approximately 30 times). The reflection of the second phase is less, which provides a decrease in the entire sample reflection. An X-ray of a PbSe film that has undergone heat treatment in an oven (Figure 3h) demonstrates the presence of two phases: PbSe and PbSeO\({}_{3}\), whose crystal sizes are 60 nm and 68 nm, respectively. At the same time, for the raw film, the size of the PbSe crystal phase was about 52 nm (Figure 3g). When films are irradiated, the maximum effects from laser exposure will be observed when the radiation wavelength is chosen close to the film's intrinsic absorption edge. The wavelengths of 405 nm and 1064 nm used in the study fall within the intrinsic absorption region of PbSe films, which provides high absorption of the incident radiation in the absence of transmission. The most common photoinduced optical changes in chalcogenide films are darkening and bleaching. Upon bleaching, the transmission edge shifts towards higher energies (blue shift), and a corresponding decrease in the absorption coefficient occurs with an increase in the band gap. With darkening, a shift of the transmission edge to lower energies (red shift) is observed, while the absorption increases. The authors of [22] suggest that the observed changes in the optical properties of As-Se-Te are caused by the homopolarization of chemical bonds, namely, an increase in the proportion of As-As, Se-Se, and Te-Te bonds over As-Se and As-Te bonds. As a result, the band gap parameters change, which leads to a shift in the absorption edge. Such a rearrangement of chemical bonds leads to a change in the material polarizability and, consequently, to a change in the permittivity and refractive index. In this case, the material remains amorphous. The effect opposite to darkening--bleaching--is observed in the As-Se system. It leads to an absorption edge shift to the short-wavelength spectrum region and is associated with chemical bond heteropolarization. This effect was detected in the As40 Se44 Te16 film by increasing the annealing temperature and time. In combination with the darkening effect, local film heating can occur due to the laser radiation's absorption. If the film heating in the affected area reaches the softening or melting temperatures, both a structure change and a redistribution of chemical bonds in the film are possible, which will also lead to changes in the sample's optical properties. In the present study, the structure of PbSe chalcogenide films was modified using CW and pulsed laser radiation in the focused beam scanning mode. Laser treatment contributed to a change in the optical properties of the samples.