## 2 Materials and Methods  
The HP-HT nanodiamond particles with an average size of 150 nm were sourced from the vendor (Diamond Innovations, Worthington, OH, USA) and then dispersed in deionized water [30], 20-\(\upmu\)m-thick (see Figure 1a) and 0.5 \(\upmu\)m-thick films were obtained by dropping the HP-HT nanodiamond aqueous suspension on a quartz substrate and then drying it at room temperature. The 0.5-\(\upmu\)m-thick films were used to study the extinction spectrum. The main experiments on laser modification were carried out with the 20-\(\upmu\)m-thick films. X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the films and HP-HT nanodiamonds under study. HP-HT nanodiamond films are insulating; therefore, to eliminate the effect of charge accumulation on the surface in the course of examination with a scanning electron microscope, the surfaces of the films were sputtered with a thin layer of gold with a thickness of about 20 nm. A D2 PHASER diffractometer from Bruker was used to study the XRD patterns of nanodiamonds. A copper-based X-ray tube that generates radiation at wavelengths \(\lambda_{1}\) (K\(\alpha\)1 line) = 0.1541 nm, \(\lambda_{2}\) (K\(\alpha\)2 line) = 0.1544 nm, and \(\lambda_{3}\) (K\(\beta\) line) = 0.1392 nm is used in the diffractometer. For the spectral separation of the K\(\alpha\) line, an X-ray filter from a nickel plate that attenuates radiation at a wavelength of \(\lambda_{3}\) is used. The K\(\alpha\)1 and K\(\alpha\)2 emission lines are a doublet; therefore, their spectral separation from each other using a nickel filter is not possible. However, the intensity of the K\(\alpha\)2 spectral line is approximately two times lower than the intensity of the K\(\alpha\)1 line. Thus, the diffraction lines of K\(\alpha\)1 copper radiation should be noticeable in the X-ray diffraction pattern of the material under study. The HP-HT nanodiamond powder obtained from the suspension was placed on a glass substrate to record XRD patterns. The XRD pattern of the substrate without powder was recorded separately. Then, to obtain a true diffraction pattern of the powder, the corresponding subtraction of the data of two experiments was carried out. Figure 2 shows the diffraction pattern of the HP-HT nanodiamonds. One can see that, in the range of the angle 20 from 25 to 100\({}^{\circ}\), the pattern contains three large peaks and one small peak. Large peaks at 20 angles of 43.9, 75.3 and 91.5\({}^{\circ}\) correspond to X-ray diffraction K\(\alpha\) on the [11], [220], [311] planes of diamond crystallites with interplanar distances \(d_{111}\) = 0.2060 nm, \(d_{220}\) = 0.1261 nm, and  
\(d_{311}=0.1075\) nm, respectively, where the subscripts denote the crystallographic plane, according to the Powder Diffraction File (PDF) 00-006-0675 (see Figure 2 lower part). The weak peak at an angle of 20 = 39.5\({}^{\circ}\) is due to the diffraction of the weak CuK\(\beta\) radiation line on the \(\{111\}\) planes. Thus, X-ray diffraction confirms that the sample under study is a high-quality diamond material. Figure 3a presents the scanning electron microscope Inspect S50 image of the film under study. Figure 3b illustrates the typical morphology of the same film, measured with an atomic force microscope SOLVER 47 PRO. The images show that the film surface has a granular structure, with a characteristic size of the observed particles of about 150 nm. The extinction spectrum of the HP-HT nanodiamond film with a thickness of 0.5 \(\upmu\)m shows the maximum at the wavelength of 208 nm, corresponding to the photon energy of 5.96 eV (Figure 4a). This absorption band corresponds to the transition from the valence band to the conduction band of a diamond. Figure 4a also shows monotonically decreasing extinction of the film due to decreasing Rayleigh scattering by nanoparticles and film inhomogeneities with the increase in the wavelength.