Annotation of the results obtained in 2021
1. In the course of the research, highly impurity natural IaB diamonds from deposits in Mirny (Yakutia) and Arkhangelsk (up to 16 pieces in total), as well as more pure synthetic IaA diamonds produced by Diamond Technologies (St. Petersburg) were selected in the amount of three pieces. The primary spatial distribution of mechanical stresses related to the type and concentration of optical centers, as well as structural one-dimensional (dislocations) or two-dimensional (twins, packing defects) defects was estimated by polarimetric measurements (without immersion) of birefringence parameters by the phase run-up. For primary characterization of diamonds, a complex of methods based on registration of spectra of PL, absorption in visible range, infrared absorption, data of qualitative and quantitative estimation of content of various optical centers according to infrared spectroscopy were used. It is obvious that strongly luminescent natural diamonds also demonstrate high optical absorption of the centers in the bandgap zone, whereas IR spectroscopy shows a defect distribution not so uniquely related to the aforementioned optical characteristics of these diamonds. Mapping of the initial impurity-defect structure of the near-surface layers in the polished face of a natural diamond was carried out using the cathodoluminescence and cathodoluminescence topography method on the JEOL-7001F (JEOL) scanning electron microscope attachment, because a unique color luminescence portrait of the diamond face is of certain interest for tracing purposes. However, since this characteristic is related only to the micron surface layer, is easily disturbed by chipping a part of the facet or disturbing its polishing, and in this sense is uncompetitive for tracing compared to the bulk characteristics (IR absorption, PL-micromarking), cathode and X-ray luminescence studies were performed only for panoramic evaluation of impurity and defect abundance on polished diamond facets.. 2. In the experiments on photoexcitation, crystals of cubic shape were pumped by hard-focused radiation (NA = 0.25, 0.3, 0.4, 0.65) of USP with wavelength 0. 5 and 1 μm through one pair of planes, through another pair the dynamic PL emission was collected and the PL emission region was visualized with a camera, through the third pair the continuous PL was pumped in situ in the focal zone of the USP using continuous lasers at 405 nm, 532 nm or 633 nm. Passive broadband (0.2-2.4 μm) polarization-sensitive PL imaging of optical center spectra was performed using two spectrometers, ASP-150F (190-1100 nm) and ASP-IR (0.9-2.6 μm). As indicators of photoexcitation, we chose the dominant PL spectra of free excitons with a band energy near the width of the indirect gap (synthetic diamond) and the PL of the A-band of recombination origin. For an informative detailed comparison, we studied wide ranges of variation in the intensity I0 – from 0.1 to 100 TW/cm2 and a variable duration of 0.3–6.3 ps, and the analysis paid attention to where the critical self-focusing power for natural and synthetic diamond is exceeded. A continuous change in N from I0 was found, starting with values > 10 not previously observed in the literature and continuing to 1, and for synthetic diamond all curves (for different durations of ultrashort pulses) practically fall on one common curve, in contrast to natural diamond with its possible intracenter transitions. Further, the method of Raman light scattering (RLS) with pumping of USP of variable duration 0.3-12 ps (wavelength 0.5 μm) was used in studies to evaluate in situ dynamics and magnitude of mechanical stresses which are formed in the focal region of diamond during USP, since this parameter, together with the temperature, is important for the dynamics of structural transformations of optical centers. Diagnostics using spontaneous instantaneous RLS on the optical phonon of diamond shows both plasma-induced structural dynamics and dynamic isotropic and anisotropic stresses with an amplitude of about 10 GP in the focal region, which can locally affect the structure of optical centers during recording of photoluminescent micromarks in the same way as pressures in static high pressure apparatuses. Finally, the PL emission excited at wavelengths of 405 and 532 nm in micrometers recorded by hard-focused (NA= 0.65) USP with different energy, duration (0.3, 1 and 2.4 ps) and exposure was recorded in a confocal microscope circuit to characterize the effects of USP energy/intensity and exposure on micromarks formation. PL micromarks with a period of 20 μm were recorded at a depth of 200 μm in natural diamond №1 USP with wavelengths of 0.5 μm and durations of 0.3, 1.0, and 2.4 ps at focusing with NA=0.25 at varying exposures. Under these conditions, PL emission of diamond vacancies as well as local vibrational modes associated with the vacancies were previously recorded. Confocal RLS/PL microspectroscopy shows bands of characteristic optical centers-the N3(3NV)-center with phononless line around 415 nm and long-wavelength phonon repeats, the NV center with phononless line at 637 nm and unresolved long-wavelength phonon repeats. The enhancement of the PL emission compared to the background has a spectral contrast of 250 for NV centers around 700 nm versus 4-5 for N3 and other centers in the 400-550 nm region. Similar micro-QR codes are read by the PL-scanner we designed and built for PJSC ALROSA (https://youtu.be/X3Z_jcWowks). Many times greater enhancement of the PL emission for NV centers can be attributed to intensive detachment of nitrogen atoms from the available A- (2N), B1- (4NV) and B2- (N) centers in diamond №1 by photoinjected vacancies, with formation of NV centers. For a more detailed understanding of the PL-mark formation process, the peak intensities of the PL emission (637 nm, NV- phononless line) of NV centers and A-band (420 nm) in this diamond were compared depending on the duration and intensity of the USP as well as the exposure. For the first time, it is noted that the formation of NV-centers turns out to correlate well with the dynamic recombination of the electron-hole plasma represented by the A-band. Also, the intensity of the PL emission of NV centers increases linearly with increasing exposure at all durations and energies of USP, in contradiction to the previous results of other researchers. Thus, the studies of the PL output under the action of USP made it possible to investigate the fundamental multiscale processes of transport of the USP to the recording site, mechanisms of nonlinear photoexcitation of the crystal material with consideration of polarization effects and related electronic dynamics, ultrafast plasma transport and transfer of its energy to the lattice with generation of isotropic and anisotropic stresses, necessary for understanding the subsequent structural modification of optical centers in the diamond volume. PL studies of micrometrics recorded by USP in the volume of natural diamond showed the scalability of the process both downward, to the generation of single-photon sources, and upward, to increase the label contrast, as well as its possible mechanism related to redistribution of impurity nitrogen between the centers with regard to laser-induced vacancy injection. These fundamental studies were implemented by the project participants and the industrial partner LLC "Microlaser" for PJSC "ALROSA" in the form of photoluminescent scanner of micrometals in diamonds (http://eng.alrosa.ru/alrosa-introduces-revolutionary-nanomarking-technology-to-trace-diamonds/, https://www.bloomberg.com/news/articles/2021-07-06/from-mine-to-wedding-finger-russian-diamond-track-and-trace, https://russian.rt.com/science/article/937270-tehnologiya-markirovka-almazy-lazer), development of the technology will be continued within the project in the following years. 3. The polarization effects during photoexcitation of diamond were studied in the mode of surface single-pulse ablation (100) of natural diamond under the action of hard-focused (NA = 0.65) USP with durations of 0.3, 1 and 3 ps and wavelength of 0.5 and 1 μm. The polarization rotation within 2 relative to the crystallographic axes caused photoexcitation of diamond in the U,W points of the Brillouin zone with the direct slit width of 11.4 and 18.1 eV. As a result, for the first time in diamond studies, a change in the ablation threshold from 4 to 7-8 J/cm2 was observed for 0.3 ps USP, whereas picosecond pulses did not show such a contrast. This character of the dependences was confirmed for the facet (111) and bulk measurements of the PL output and the transmittance of the USP pumping through diamond as a function of polarization. The observed effect is very important because it demonstrates the need for proper orientation of the diamond or linear laser polarization to achieve stable recording, and also provides different means - maximizing the PL output or minimizing the transmittance of the recording USP to optimize the in situ orientation. 4. As immersion materials for diamond, polished plates of light-fusible chalcogenide glasses were investigated. From them, three glasses that had a refractive index close to that of diamond (2.38) and provided optical transmission in the near- and mid-IR range were preliminarily selected. These were two samples of immersion compositions based on germanium selenide and one sample based on arsenic and sulfur. The first sample of germanium selenide has low optical transmission and high refractive index dispersion, arsenic-based glass, although it transmits light in the red region of light, but contains poisonous arsenic - to work with it is dangerous. As a result, it was decided to use eutectic germanium selenide glass. It has good optical transmission in the range of 800-11500 cm^-1 (12.5-0.87 microns), no light scattering, and has insignificant dispersion of the refractive index. This glass is very promising for near-IR imaging of diamonds with a developed surface in order to detect hidden internal defects and for measurements in the mid-IR range. The softening temperature of glass is about 175 ° C, while it practically does not evaporate, it is not poisonous, it is easy to work with it. Fine adjustment of the refractive index of the immersion material is planned to be realized by a small change in the composition or by selecting the measurement temperature. When measuring the radiation resistance of immersion compositions, attention was paid to the nonlinear properties of materials that manifest themselves as absorption saturation or multiphoton processes of interaction between laser radiation and the target substance. USP at a wavelength of 1030 nm and a frequency of 10 kHz was focused on the sample in a spot with a diameter of ~60 μm. Under these focusing conditions, the surface and the target volume were not destroyed, the surface breakdown was observed on several samples at laser pulse energies close to the maximum. A ZnSe-based solid-state immersion press for high-temperature forming was designed, purchased and tested. 5. A mathematical model of the process of passage of optical radiation through the optical system was realized and a program in Python was created on its basis. The developed program makes it possible to reconstruct the phase value of the wave through the distribution of its intensity, which allowed us to study the peculiarities of formation of the structure of a two-dimensional wave front. The developed method for reconstruction of the complex field amplitude based on the solution of the intensity transfer equation relates the intensity I (the modulus of the square of the complex field amplitude) and its longitudinal derivative ∂I/∂z to the phase 𝜙 of the light wave. It is built around the calculation of the longitudinal derivative ∂I/∂z between two or more selected planes from the layer of space in which the intensity distributions have been obtained. This solution method is notable for its simplicity and efficiency in terms of speed and accuracy of the coherent light field modulus recovery. Preliminary data on phase imaging of the diamond plate surface of orientation (100) were obtained. The accuracy and reproducibility of the measurement result are evaluated. 6. On October 4-7, 2021, at the P.N. Lebedev Physical Institute of the Russian Academy of Sciences, the School of Young Scientists "Quantum Technologies for Laser Formation and Broadband Spectral Identification of Optically Active Point Defect Complexes in Natural Diamonds for Industrial Tracing" was held with inviting the world leading scientists on the project topics as lecturers. The event was attended by 51 participants, of which: 7 Russian and 9 foreign scientists-lecturers, as well as 31 listeners - Russian young scientists under the age of 35 inclusive, graduate students (residents, interns, adjuncts) and students. The School's website is https://laser-diamond-lab.ru/school/.

 

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Annotation of the results obtained in 2022
The results obtained in 2022 as part of the project can be divided into several areas: 1) deep characterization of diverse diamonds, 2) the interaction of ultrashort laser pulses with diamonds, laser recording of optical micromarks in diamonds and the study of atomistic processes of laser-induced structural transformation in them of defect-impurity centers, 3) development and testing of means of solid-state and liquid-phase immersion, 4) development of phase methods for analyzing the spatial distributions of defects in diamonds. 1. Deep characterization of diverse diamonds During the course of the project in 2022, for the first time in the world, it was formed and applied in research (Kudryashov et al. "Permanent optical bleaching in HPHT-diamond via aggregation of C-and NV-centers excited by visible-range femtosecond pulse lasers." Carbon 201 (2023): 399-407) a unique hardware complex of spectral methods (Fourier IR spectroscopy and micro-spectroscopy in the range of 400-8000 cm-1, optical spectroscopy and transmission microspectroscopy of the UV-near IR range - 200-1100 nm and 350-900 nm, respectively, as well as 3D scanning confocal microspectroscopy of Raman Raman scattering and PL photoluminescence excited at wavelengths of 405, 532, 785, and 1064 nm), which allows almost complete characterization of the spatial distributions of defect-impurity centers and microdefects in diamonds. The complex is supplemented by polarimetric micromapping of mechanical stresses and structural anisotropy in diamonds based on the birefringence induced by them. The complex was used to study depth-heterogeneous zonal-sectoral distributions of defect-impurity centers in pink and colorless natural diamonds from the Arkhangelsk deposit, natural diamond with a weakly radioactive inclusion (radiation-induced vacancies), as well as pseudo-heterogeneity in synthetic HPHT-diamond Imperial Diamond Red Due to the strong absorption of the input excitation and output Raman/PL radiation, a technique for normalizing the distributions of the PL output to the distribution of the Raman signal was developed taking into account the confocal effect of the focal volume of the exciting laser radiation. It is shown that at liquid nitrogen temperature the NV center is in a negative charge state, while at room temperature it passes into a neutral state due to thermal ionization. The stresses and inhomogeneity of the distribution of defect-impurity centers in periodic microplanes of plastic deformation of pink and brown natural diamonds are optically visualized and characterized by Raman/PL methods. Using IR microspectroscopy of nitrogen impurity centers on the surface of natural diamonds, the sensitivity of X-ray photoelectron microspectroscopy and its verification capabilities of structural studies for surface mapping of a wide range of impurity centers of various chemical elements in combination with IR spectroscopy were evaluated. 2) Interaction of ultrashort laser pulses with diamonds, laser recording of optical micromarks in diamonds, and investigation of atomistic processes of laser-induced structural transformation of defect-impurity centers in them For oriented HPHT diamond crystals, the azimuthal dependences of the dynamic PL yield in the prefilamentation mode of propagation of linearly polarized ultrashort femto-picosecond laser pulses were studied for the first time. As it turned out, they reflect the dispersion of the band structure of diamond upon multiphoton interband excitation and the absence of anisotropy upon excitation of a dense electron-hole plasma, which dominates the absorption of laser radiation. A similar effect for the azimuthal dependences of the PL yield and PL track length was demonstrated in the filamentation mode of propagation of linearly polarized ultrashort femto-picosecond laser pulses. Depending on the femto-picosecond duration of ultrashort laser pulses with a wavelength of 515 and 1030 nm for diamonds and other dielectric materials, for the first time, an inverse proportionality between the threshold power of the onset of filamentation and the duration of laser pulses was found, indicating the threshold energy as an invariant of the onset of filamentation, including in as a result of the delayed contribution to the nonlinear polarization of the medium from the coherent excitation of Raman-active optical phonons. In the regime of evolutionary relaxation of the input energy (through successive electronic, acoustic, thermal, and structural-phase stages) without cumulative heating, but with multipulse cumulative accumulation of structural changes depending on energy and exposure, ultrashort laser pulses were used to record PL micromarks in diamonds with low ( atomic N, diatomic 2N) and high (2NV, 3NV, 4NV, 4N2V, etc.) degree of aggregation of nitrogen impurity centers, the mechanisms of their structural modification in labels as a result of photodissociation and the generation of vacancy-interstitial Frenkel pairs were found, threshold laser pulse energies were found processes of transformation of centers. It is shown that, at a low concentration, nonequilibrium vacancies attach to impurity nitrogen centers, increasing the degree of aggregation, while at a high concentration, they detach nitrogen atoms in a concerted manner to form NV centers. At high energies and exposures, a significant yield of N3a and N3b-type centers (3NV-B2 complexes, including carbon interstitials) was noted, which can be associated with the local accumulation of carbon interstitials with the intensive consumption of carbon vacancies associated with them. As a result, it turned out to be possible to implement laser exposure modes, which, depending on the conditions, partially discolor or partially color natural and synthetic diamonds, including those with a local significant change in the refractive index, which has prospects for recording nano- and micro-optical diamonds in the volume. devices. Under stationary heating in the range of 25-450 0C in natural diamond, the recording of PL micromarks with a several-fold increase in the PL yield was realized, which will be further used to develop a method of temperature additions in the study of thermal mechanisms of laser recording of PL micromarks with cumulative multipulse laser heating. On the other hand, annealing in an oxygen-free environment of natural diamond with marks at temperatures up to 1200 C shows the preservation and even enhancement of the PL intensity of the optical contrast marking. 3) Development and testing of solid state and liquid phase immersion tools Solid-state immersion was developed on the basis of crystalline ZnSe (in the region of normal dispersion n = 2.3-2.5, transparency in the region > 600 nm), more refractory (melting point - 1525 0С), but having higher thermal stability and low toxicity compared to the other studied chalcogenides. With the help of the existing stand for pressing diamonds in an argon atmosphere at temperatures < 800 0C, a detachable optical contact ZnSe-diamond without traces of chalcogenide pyrohydrolysis was obtained, on which the surface graphite micromarking of diamond was clearly observed. Nevertheless, strong two-photon absorption at a wavelength of 1030 nm was found for ZnSe, the measured coefficient of which for pulses with a duration of 0.3 and 10 ps was 4 cm/TW. This excludes the possibility of using ZnSe as a solid-state immersion for micromarking using ultrashort laser pulses with available wavelengths of 515 and 1030 nm, but allows the use of diamonds in the optical and mid-IR ranges for viewing purposes. However, a range of more suitable available optical materials for this purpose has been outlined. For liquid-phase immersions, calculations within the framework of the effective medium theory for Ge nanoparticles (n = 4.4) in carbon tetrachloride (refractive index n = 1.5) with minimal absorption in the mid-IR range showed that matching of the refractive indices with diamond is achieved at an acceptable mass fraction of nanoparticles about 0.1. For the visible range, liquid-phase immersion based on TiO2 nanoparticles (n = 2.68) in water (n = 1.33) does not provide the necessary refractive index matching with diamond even in a paste form (mass fraction >0.9). Therefore, studies for liquid-phase immersions are promising and will be continued only for the mid-IR range. 4) Development of phase methods for analyzing the spatial distributions of defects in diamonds. A mathematical model was implemented and, on its basis, a Python program was created, which makes it possible to analyze and study wave fields, taking into account their polarization, to determine internal stresses and extended defects of any crystals, including natural diamonds. The proposed model is based on the relationship between the phase function of the complex field amplitude and the intensity distribution of this field through a second-order partial differential equation (intensity transfer equation). The quality of restoration of surface and bulk defects of a certain crystallographic orientation of the crystal was assessed based on the solution of this equation. The limitations and reproducibility of the method based on the intensity transfer equation, as well as its accuracy, were evaluated. An ablation crater obtained on the surface of a crystal of orientation (110) by hard focusing of laser radiation with a wavelength λ = 515 nm and a pulse duration τ = 300 fs was chosen as a model object of study, and a comparison was made with the data of atomic force microscopy. As a result, the average error in reconstructing the profile of the crater surface was ≈4.88%, while the error in determining the depth of the crater was ≈2.31%. At the same time, the process of determining the depth took less than 3 minutes, and on an atomic force microscope, a similar procedure takes about 60 minutes. In addition, the reconstruction error of the crater surface profile was calculated for all cases of boundary conditions (without boundary conditions, Neumann and Dirichlet) and several sizes of the study area, which varied from 4.875 μm to 15.6 μm with a step of 0.95 μm. In this case, the error was calculated only in the region where the ablation crater is located, regardless of the size of the region under study for the intensity transfer equation. The resulting dependences of the error on the size of the study area used for TIE for different boundary conditions have the same character and slight local differences. In this case, the smallest error in the calculation of the intensity transfer equation is achieved using the Neumann boundary conditions.

 

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Annotation of the results obtained in 2023
1. For the first time in the world, natural and synthetic diamonds in the form of plates have been fully locally characterized with respect to point impurity centers and areas of plastic deformation using panoramic photoluminescent visualization of the internal structure (Diamond Inspector, etc.), Fourier IR and optical spectral microspectroscopy (transmission in range 200-1100 nm, Raman scattering and photoluminescence pumped at wavelengths of 405 and 532 nm), as well as polarimetric mapping (633 nm), revealing various sectors and growth zones of crystals, as well as their inclusions before laser modification of the composition of point impurity centers and areas of plastic deformation. For the semi-quantitative analysis of the average concentrations of photoluminescent impurity centers N3, H3/H4 using standard diamonds, for the first time in the world, calibration relations have been developed using the spectra of the optical absorption coefficient of these centers near their zero-phonon line, as well as their measured absorption cross sections for femtosecond laser pulses. Electron paramagnetic resonance spectroscopy complemented the characterization of diamond impurity centers. The presented diamond characterization procedures have been used in all major papers published by the team. 2. Using a set of thin plates of natural diamonds of different types (IIa, IIa-IaA, IIa-IaAB, IIa-IaB, IaA, IaB) with widely varying concentrations of various types of nitrogen defects, a new technique for measuring the thickness and refractive index of diamonds, based on interferometric and spectrophotometric (infrared (IR) Fourier spectroscopy) methods. A reference absorption spectrum of diamond in the range from 400 to 5000 cm-1 was constructed based on the spectra of type IIa diamonds. A method is proposed for determining the baseline and calibrating IR spectra using an original set of reference points (400, 1838, 2493, 2666, 4000 cm-1) and the resulting reference spectrum in the two-phonon region. A new, more accurate procedure for calibrating IR spectra reduces the instrumental error in measuring the absorption coefficient and thickness of the sample, and, accordingly, measures many times lower threshold concentrations of IR-active impurity centers. 3. The study of the regime of local modification of the composition of point impurity centers and areas of plastic deformation of diamonds under the influence of low-energy femtosecond laser pulses (525 nm, 150 fs) with a repetition rate of 80 MHz in the pre-filamentation mode was carried out using dynamic in situ Raman scattering of pump radiation in the focal volume on optical phonons. It is shown that cumulative quasi-static heating of diamond does not occur as the exposure increases (the number of absorbed pulses) - the intensity, position and half-width of the Raman peak at 1332 cm-1 do not change), each pulse acts on its own and the thermal conductivity of diamond is sufficient to dissipate the released heat , and all changes in the spectrum, due to the inertia-free Raman effect, occur during a femtosecond laser pulse. However, the dependence of the intensity, position and half-width of the Raman peak at 1332 cm-1 on the pulse energy showed that with increasing energy, the electron-hole plasma generates pressure through the acoustic deformation potential, increasing the frequency of the optical phonon, and at higher energies the decay of optical phonons according to the Clemens scheme into acoustic phonon modes are accelerated already during the pulse, causing local quasi-heating of the lattice. In the latter mode, the modification of nitrogen impurity centers sharply increases, which we associate with the increased formation of the vacancy-interstitial Frenkel pairs necessary for this in the diamond lattice. Using the additive method (additional stationary heating to temperatures of the order of 400 C), the effective temperatures achieved in the quasi-heating mode of diamond at the end of each laser pulse were independently estimated. 4. For the first time in the world, multiphoton resonant absorption in diamond of femtosecond laser pulses with wavelengths of 4 and 5 μm (duration – 250 fs), corresponding to the bands of its two-phonon absorption for the optical branch (optical biphonon) and mixed (optical/acoustic phonons) was studied. At radiation intensities less than 1 TW/cm2, the z-scan and I-scan methods have established two-photon absorption of diamond with a coefficient of about 30 cm/TW. Irradiation of Imperial Diamond Red synthetic diamond at higher radiation intensities showed visual and spectral brightening of the plate in the irradiation zone, a decrease in the photoluminescence intensity for NV centers and an increase for H3 centers; in the IR spectra the intensity of H1a (NC4V) and H1b noticeably decreased (NC4VN) centers – intermediate products of the decomposition/formation of NV and H3 centers. The observed structural modification is associated with the direct thermal effect of IR laser vibrational excitation of diamond. A series of atomistic supercomputer calculations of the evolution of NV and H3 defects in diamond at finite temperatures in the presence of additional vacancies was carried out. 5. Residual stresses caused by laser optical breakdown and graphitization in the bulk of natural IaA-type diamond (with B2 defects) were studied in the subfilamentation mode by femtosecond laser pulses with different energies. Using crossed polarizing filters, compression and tension zones in the affected areas are visualized. Using Raman spectroscopy, spatial profiles of compression (up to 1 GPa) and rarefaction (up to 2 GPa) stresses in the breakdown region were obtained depending on the laser pulse energy. The data obtained showed that the increase in tensile stress occurs faster than compression with increasing pump energy, which indicates the role of compressive stress in the microfracture of diamond, accompanied by graphitization. It has been established that residual stresses are clearly expressed near the boundary of the graphitization region (pierced and unpierced zones). The induced voltages propagate beyond the visible breakdown zone and decrease to zero at a distance of about 10 μm. 6. Using the example of a model Ib-type synthetic diamond with a known concentration of atomic nitrogen C-centers for the filamentation mode of focusing femtosecond laser pulses of the visible range in the crystal volume based on the total local consumption of C-centers (IR and optical spectrophotometry data) and the corresponding saturation of the yield of irradiation products - NV centers – by attaching a free vacancy to a C center, the productivity of the processes of formation of NV centers (ηNV = 10^12–10^13 NV/cm3) and I-V pairs (ηIV = 10^13–10^14 IV/cm3) is considered ) per pulse, as well as the efficiency of formation of point defects ηIV/Peh = 10^−7–10^−8 I-V pair/e-h pair in laser electron-hole plasma with a near-critical density Peh = 10^21 cm−3. These estimates are consistent with the results of scaling irradiation modes that produce single-photon sources of NV centers, and also shed light on the processes of radiation damage to diamond and other non-conducting materials. 7. A model sample of synthetic diamond shows the possibility of its multiphoton femtosecond laser micromarking by nonlinear absorption in the laser focus (two-photon absorption coefficient at a wavelength of 525 nm - 13 cm/TW) - recording and reading photoluminescent marks - at a wavelength of 525 nm through a ZnS plate 3 mm thick (without pressing diamond) with a slight loss of contrast and intensity of the marks, as well as a slight nonlinear absorption of the immersion medium (two-photon absorption coefficient at a wavelength of 525 nm - 0.6 cm/TW). 8. Studies of volumetric defects in diamond crystals were carried out using the methods of optical image correlator, digital holography and intensity transfer based on a mathematical model developed earlier. The applicability of an image correlator based on grayscale and binary Fourier holograms for phase visualization of volumetric inhomogeneities in a diamond crystal is demonstrated.

 

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