## 1 Introduction  
A number of unique properties of nanodiamonds make them attractive for a variety of industrial and biomedical applications [1; 2; 3; 4]. The mass production of nanodiamond particles is currently carried out by two methods: (i) detonation of high explosives with high carbon content [5; 6] and (ii) milling micrometer-sized diamond particles [7] produced while using hydraulic presses at high pressure and high temperature [8]. Detonation nanodiamonds and high-pressure high-temperature (HP-HT) nanodiamonds both have a wide variety of impurities [5; 9; 10; 11]. More than 60 elements are known to be present in diamonds according to analytical data, but only a few of them, namely nitrogen, boron, nickel, silicon, and hydrogen, can form intrinsic structural defects [12]. These impurities and vacancies affect the optical properties of nanodiamonds, such as absorption and luminescence.  
HP-HT nanodiamonds contain substitutional nitrogen impurities with a concentration of about 100-300 ppm [11; 13]. These substitutional nitrogen impurity atoms form stable complexes with vacancies (or other impurity atoms), leading to the formation of optically active defect centers. Among these are the well-studied nitrogen-vacancy [N-V] (having both neutral [N-V]\({}^{0}\) and negatively charged [N-V]\({}^{-}\) forms), consisting of one nitrogen atom [N] in a substitutional position and one vacancy [V] [14]. These complexes have a high probability of electronic transitions [15]. HP-HT nanodiamonds grown while using nickel or a nickel alloy as a catalyst contain a number of nickel related optical centers [16,17]. The absorption and luminescence spectra of the HP-HT nanodiamonds grown in the Fe-Ni-C system exhibit a large number of absorption lines in a wide spectral region (300-900 nm) belonging to [Ni]- [16,18] and [Ni-N]- [19,20] centers. The defect centers listed above, as well as other impurity centers formed during the synthesis of HP-HT nanodiamonds, can affect the material response to laser irradiation.  
It is known that focused laser radiation can initiate processes that lead to laser modification of nanodiamonds. A large number of studies have demonstrated laser modification of the surface of single and polycrystalline diamonds, nanocrystalline diamond films, ultra-thin diamond-like carbon, amorphous diamond-like films, and tetrahedral amorphous carbon while using powerful nanosecond, picosecond, and femtosecond lasers operating in the ultraviolet, visible, and infrared ranges [21, 22, 23, 24, 25, 26, 27, 28]. In these studies, laser microstructuring occurs as a result of local graphitization of the surface due to single- or multi-photon absorption (depending on the laser quantum energy) of powerful laser radiation, which causes the ionization of the sp\({}^{3}\)-carbon material.  
Recently, we have demonstrated the possibility of blackening and microstructuring of semitransparent detonation nanodiamond films upon low-power focused He-Ne laser irradiation at 633 nm [29]. Such modification of the nanodiamond films can be used in order to fabricate optical structures designed on a micro-scale. Briefly, we have also demonstrated that cw He-Ne laser at a wavelength of 633 nm causes the transformation of HP-HT nanodiamonds into sp\({}^{2}\)-carbon (blackening) [30]. In the current paper, we have studied this phenomenon accompanied by green luminescence in more detail, which allows for us to establish a link between film blackening and the local presence of Ni-related photoluminescence centers.