## 2 Materials and Methods
Figure 1: (**a**) Image of high-pressure high-temperature (HP-HT) nanodiamond film with a thickness of 20 \(\upmu\)m on a quartz substrate before laser irradiation; (**b**) image of 20-\(\upmu\)m-thick HP-HT nanodiamond film with a thickness of 20 \(\upmu\)m on a quartz substrate after laser irradiation where a laser beam scanned the entire area of the film. Note that a portion of the film remained non-blackened  
Figure 2: X-ray diffraction pattern of HP-HT nanodiamond powder and X-ray diffraction pattern of diamond (Powder Diffraction File 00-006-0675). The luminescence spectra of an aqueous suspension of HP-HT nanodiamonds were studied while using the LS55 luminescence spectrophotometer (Perkin Elmer). It allows for changing the wavelength of the exciting radiation using the excitation monochromator in the range from 200 to 800 nm. The spectral range of the emission monochromator spans from 200 to 900 nm. The luminescence intensity of the suspension was recorded, depending on the wavelength of the exciting radiation using a photomultiplier installed at the output of the emission monochromator. When recording the luminescence spectra, the slit widths of both monochromators were equal and set to 5 nm. At the input of the emission monochromator, various optical filters significantly suppressing the exciting radiation were installed in order to reduce its influence on the luminescence spectra. Figure 4: **(a)** Extinction spectrum of the HP-HT nanodiamond film with a thickness of 0.5 \(\upmu\)m; **(b)** the luminescence spectra of the aqueous HP-HT nanodiamond suspension with concentration of 0.02 wt.% when excited by radiation at wavelengths of **(b)** 220 and 320 nm, **(c)** 400 and 420 nm; and, **(d)** the up-conversion luminescence spectra of aqueous HP-HT nanodiamond suspension with a concentration of 0.02 wt% when excited by radiation at wavelengths of 633 and 650 nm. Figure 3: Scanning electron microscopy (SEM **(a)** and atomic force microscopy (AFM) **(b)** images of the HP-HT nanodiamond film surface. Figure 4b,c illustrate the luminescence spectra recorded in the Stokes region at excitation radiation wavelengths \(\lambda_{\text{ex}}\) = 220, 320, 400, and 420 nm. It can be seen that when the wavelength \(\lambda_{\text{ex}}\) is changed, the luminescence spectrum changes too. However, in all recorded spectra, there are peaks that are common to all excitation wavelengths \(\lambda_{\text{ex}}\). These are the peaks that correspond to the wavelengths of 461, 485, and 529 nm. Figure 4d shows the luminescence spectra recorded in the anti-Stokes region when excited by radiation at wavelengths \(\lambda_{\text{ex}}\) = 633 and 650 nm. This spectrum clearly shows up-conversion luminescence bands in the blue (461-490 nm) and green (510-535 nm) wavelength regions. It is noteworthy that, in the blue and green regions of the optical spectrum, there are peaks at 485 and 528 nm, respectively, which were observed when luminescence was excited by radiation at \(\lambda_{\text{ex}}\) = 220, 300, and 360 nm. Note that all of the spectra in Figure 4b-d are identical to that of the HP-HT nanodiamond film presented in Figure 1a. Experiments on the laser modification of HP-HT nanodiamond films and the study of their Raman spectra were carried out at the wavelength of 633 nm while using Horiba HR800 Raman spectrometer with a He-Ne excitation source. In this case, the laser radiation could be focused on the surface of the film using interchangeable objectives with a magnification of 10 (10\(\times\)), 50 (50\(\times\)), or 100 (100\(\times\)). The laser beam diameters (at a level of 1/e\({}^{2}\)) in the focal plane of the objectives were measured by scanning laser beam profilometer BC 106-VIS (ThorLabs) along the laser beam axis [31]. The diameters of the waist of a laser beam focused with 10\(\times\), 50\(\times\), and 100\(\times\) objectives were 20.7, 7.4, and 4.1 \(\upmu\)m, respectively.