## 3 Results and Discussion
### Optical Characterization
The parameters of laser processing, in which the effects of bleaching and darkening were observed, are shown in Tables 1 and 2. First, a processing mode was discovered in which the material is bleached with subsequent destruction (Figure 4). This mode can be observed when the scanning speed is reduced to 17 mm/s, which leads to an increase in acting energy. With a subsequent decrease in the scanning rate, the PbSe film begins to break down, starting from the track's central part to its edges, which is associated with the radiation intensity's Gaussian distribution over the spot. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Darkening** & **Bleaching** \\ \hline Scanning speed, mm/s & 1 & 1 \\ \hline Power density, kW/cm\({}^{2}\) & 0.74 & 1.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Scanning modes with continuous NUV radiation. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Darkening** & **Bleaching** \\ \hline Scanning speed, mm/s & 120 & 13 \\ \hline Pulse frequency, kHz & 5 & 50 \\ \hline Pulse duration, ns & 4 & 20 \\ \hline Power density, kW/cm\({}^{2}\) & 1.45 & 0.39 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Scanning modes with pulsed NIR radiation. Figure 4: Images of a PbSe film obtained using optical microscopy: in a bright field of reflected light, (**a**) darkening mode when modified by pulsed radiation, (**b**) bleaching mode when modified by pulsed radiation; in the transmitted light field (**g**), the bleaching regime upon modification by pulsed radiation. Pictures of the PbSe film obtained with the SEM secondary electron detector: (**c**,**e**) darkening mode when modified by pulsed radiation, (**d**,**f**) bleaching mode when modified by pulsed radiation. Pictures of the PbSe film obtained using an X-ray diffractometer: (**h**) darkening mode when modified by pulsed radiation, (**i**) bleaching mode when modified by pulsed radiation. The PbSe film's sharp heating due to short pulses (<20 ns) exposure led to the occurrence of excess stress, which caused the crack formation in the laser exposure areas (Figure 4c). When the sample is modified, the material is redistributed, and part of it shifts towards the laser spot center with a moving local heat source and a high temperature gradient. As a result of the samples' treatment in the bleaching mode, the absorption coefficient decreases with increasing band gap, which can be observed in the case of treatment with short pulses and in the authors' previous work [23] for the PbSe films modified by continuous radiation with a wavelength = 405 nm. Additionally, the sample bleaching is characterized by a transmission edge shift towards higher energies in the IR range. With darkening, an increase in absorption and a transmission edge shift towards lower energies are observed. Due to the absorption of laser radiation, the material is heated to a softening state, while the sample structure and the band gap undergo changes, the chemical bonds in the sample are redistributed, and the band gap edge shifts. After exposure to pulsed laser radiation on the film in the darkening mode, a decrease in the crystal size to 37 nm for the PbSe phase can be observed in the X-ray image (Figure 4h) compared with the raw film. However, after modification of the sample by pulsed radiation in the bleaching mode (Figure 4i), the PbSe phase had crystal sizes of about 65 nm. Optical characterization was carried out, and the PbSe film structure subjected to laser treatment in the darkening mode was studied. As a result of the laser exposure, periodic structures were formed in the track's central part, which were local hills, since during the melt pool formation, material was periodically drawn into the heat source center, limited by the laser spot. The sample modification did not entail the damage formation in the scanning area and also did not change the original film properties located between the tracks, but it caused melting and contributed to the molten material redistribution.