## 3 Results and Discussion
### Optical Characterization
In the laser exposure zone, one can observe the presence of dark and light ridges, which are characterized by different material densities. More dense areas--light ridges--are formed due to the material redistribution and the neighboring dark area depletion of the track, respectively. As in the case of heat treatment in a furnace, a crystal structure is formed at the track periphery due to laser action. However, when laser radiation is used, the formation of an oxide phase is absent. This feature is an advantage since the presence of oxide can contribute to the film's degradation and reduce its service life. Moreover, when the film was rescanned in the mode in which darkening was observed, the film was clarified (Figure 5b). After exposure to continuous laser radiation on the film both in the darkening mode (Figure 5h) and in the bleaching mode (Figure 5i), an increase in crystal size to 63 nm and 72 nm, respectively, for the PbSe phase can be observed on an X-ray relative to the raw film. The FESEM image of two films before and after heat treatment is shown in Figure 6a,b. The pictures clearly show the formation of oxide after heat treatment. The presence of constituent elements, such as O, Se, and Pb, in two films is shown in Figure 6a,b. FESEM images modified with pulsed (Figure 7) and continuous radiation (Figure 8) in darkening and bleaching modes were studied. The composition of the films changes drastically depending on the laser irradiation mode. In the darkening mode, no oxide is formed on the surface (Figures 7a and 8a), which is an important difference in comparison with heat treatment. This may be due to a high temperature gradient and a short laser exposure time. In the bleaching mode, the presence of oxide is observed (Figures 7b and 8b), as well as elements characteristic of the cover glass substrate. Chemical elements, except Pb, Se, and in some cases O, that is, elements such as Si, Na, Ca, etc. (Figures 7 and 8) belong to the elementary composition of the cover glass--the film substrate. The reason for the detection of these elements is their destruction, including the cracking of the film under the action of laser radiation and the further removal of these elements to the surface of the samples. Figure 5: Images of a PbSe film obtained using optical microscopy in a bright field of reflected light: (**a**) darkening mode when modified by continuous radiation; (**b**) bleaching mode when modified by continuous radiation; in the field of transmitted light (**g**), the bleaching regime when modified by continuous radiation. Pictures of the PbSe film obtained with the SEM secondary electron detector: (**c,e**) darkening mode when modified by continuous radiation; (**d,f**) bleaching mode when modified by continuous radiation. Pictures of the PbSe film obtained using an X-ray diffractometer: (**h**) darkening mode when modified by continuous radiation; (**i**) bleaching mode when modified by continuous radiation. Figure 6: FESEM-EDX image: (**a**) original film, (**b**) film after heat treatment. Figure 7: FESEM-EDX image: (**a**) darkening mode with pulsed radiation modification, (**b**) bleaching mode with pulsed radiation modification. Figure 9 shows the reflectance spectra for samples that have undergone various processing types. The smallest reflection, almost over the entire measured wavelength range, was observed for the samples subjected to thermal treatment in the furnace, which is associated with the formation of a surface-transparent oxide. For PbSe films modified in sections using short laser pulses in the darkening mode, a sharp decrease in reflection was observed relative to the initial film but more so relative to heat treatment. However, in the exposure case to continuous laser radiation at a wavelength of 405 nm in the darkening mode, the reflection was less important than during heat treatment, and the modified regions approached a black body in their optical properties [23]. Figure 8: FESEM-EDX image: (**a**) darkening mode with CW modification, (**b**) bleaching mode with CW modification.