The possibility of the femtosecond laser structural micromodification (microlabeling) of diamond through a solid immersion layer made of Ge7Se93 chalcogenide glass at a wavelength of 1.55 μm has been investigated. The two-photon absorption coefficient of Ge7Se93 has been measured to be β2 = (0.09 ± 0.01) cm/GW, which allows the propagation of intense femtosecond laser radiation in this spectral range through realistically thin (<0.1–1 mm) immersion layers. Despite the nonlinear absorption and optical damage in the immersion volume, photoluminescent microlabels have been observed in the test writing modes in the volume of diamond, which become more uniform with decreasing exposure time and pulse energy.

Recently, rotation sensors utilizing the nuclear spins of nitrogen-vacancy color centers in diamond have been demonstrated. However, these devices are power-intensive and challenging to integrate into small chip-based radiofrequency antennas and circuits necessary for controlling nuclear spins or producing relatively high magnetic fields. To address this issue, the coherent manipulation of nuclear spins via coherent population trapping at moderate magnetic fields using microwave fields has been successfully demonstrated in isotopically pure diamond. In this work, we demonstrate that a similar technique can be applied to a diamond plate with a natural abundance of carbon-13, which holds significant potential for practical sensing applications. Although the forbidden resonances required for coherent control were only partially observed, coherent population trapping was successfully demonstrated at both visible and invisible transitions, with an apparent contrast of up to 98+-11% and a true contrast of approximately 35+-7%. This finding confirms the feasibility of coherent nuclear spin control even in diamond plates with naturally occurring carbon-13

The photoluminescence spectra of the dominant H3 and H4 centers in a natural diamond sample, which is preliminarily characterized by optical and infrared spectrophotometry, are excited by femtosecond laser pulses with a wavelength of 470 nm and varying intensity. Saturation of the photoluminescence intensity of the H3 and H4 centers normalized by the intensity of laser radiation is observed and attributed to the saturation of the resonance transition responsible for the excitation. This makes it possible for the first time to estimate the absorption cross sections of H3 and H4 centers, which can be compared with values known from the literature and those determined from photoluminescence kinetics. The total concentration of H3 and H4 centers can then be found. Taking into account the known absorption coefficient of the sample at a wavelength of 470 nm and the previously established ratio of the contributions from H3 and H4 centers, their concentrations have been estimated separately for the first time.

For the first time, we demonstrated the sensitivity of diamond H3 color centers to a magnetic field. The diamond H3 color centers represent the neutral charge state of the $\mathrm{N}-\mathrm{V}-\mathrm{N}$ centers, i.e. two donor electrons are localized in the vacancy from nitrogen atoms included in the center structure. This suggests the spin splitting of the electronic states of the H3 center into spin sublevels. Thus, H 3 (or $\mathrm{N}_{2} \mathrm{~V}^{0}$) diamond color centers may be used in quantum sensors and, possibly, in quantum computing.

The nitrogen-vacancy center in diamond attracts a lot of attention in sensing applications, mainly for temperature, magnetic field, and rotation measurements. Nuclear spins of carbon-13 surrounding the nitrogen-vacancy center can be used as a memory or sensing element. In the current work, a diamond plate with a relatively large concentration of carbon-13 was synthesized and examined. The spectrum of optically detected magnetic resonance was recorded and analyzed in a magnetic field range of 5-200 G. A strain-independent measurement technique of carbon-13 isotope concentration based on the analysis of magnetic resonance spectra was developed. Additionally, narrow features in the spectrum were detected and understood.

The paper presents the results of experimental observation of two types of stimulated emission (SE) under pulsed laser pumping at 532 nm in diamond with NV centers. A comprehensive spectroscopic characterization of multisectorial HPHT diamond plate was performed. At low pumping power, the stimulated emission from NV- centers was recorded as a broad (>= 80 nm wide) band with a maximum at 706 nm in the {111} and {311} sectors of the diamond plate. As the pump power increased in the {111} sector, narrow-band stimulated emission (<10 nm wide) was detected, with a maximum at 716 nm and a luminescence impulse duration of 1.5-3 ns. As the pump density increased, a fine structure in the spectrum of narrow-band stimulated emission was revealed for the first time. The concentration of NV- centers in the {111} and {311} growth sectors was approximate to 10 ppm. However, there were considerable differences in the concentrations of C (35 and 3.5 ppm) and C+ centers (6.1 and 3.2 ppm, respectively). It was demonstrated that the presence of a high concentration of NV- centers is not the only necessary condition for the initiation of narrow-band SE in the 710-720 nm range. In the {311} sector, lighting at 360, 405, and 488 nm reduced the concentration of NV- centers by 15 % while increasing the concentration of C+ centers in the {311} sector. This effect is weak in the {111} sector. The authors suggested a model for narrowband SE at the transition Valence Band -> C+ with charge-state conversion of C <-> C+ and NV0 <-> NV- centers. Further research on the dynamic processes is required in order to a detailed understanding of the operation of NV centers in diamond during SE generation.

The nonlinear absorption of ultrashort laser pulses with intensities of 0.17–1.7 TW/cm2 at an intrinsic two-photon absorption wavelength of 4673 nm in type IIb diamond has been studied experimentally. It has been shown that the main absorption mechanism in the studied sample is two-photon absorption with a coefficient of β2 = (72 ± 7) cm/TW. Transmission microspectroscopy, visible photoluminescence, and infrared Fourier-transform microspectroscopy have demonstrated the possibility of laser-induced transformation of nitrogen impurity centers in synthetic type Ib diamond at higher radiation intensities.

The photoluminescence spectra of two synthetic diamond samples containing 1-10 ppm NV centers and 100-200 ppm substitutional nitrogen were studied under optical pumping at 532, 560, and 575 nm. At pump intensities less than 0.5-1.0 MW/cm2, a vibronic photoluminescence band of NV- centers in the negative charge state was observed. With an increase in the pump intensity above 0.5-1.0 MW/cm2, a superluminescence band was observed on the long-wavelength shoulder of the phonon wing in the range 700-760 nm. The intensity of superluminescence band increased with increasing pump intensity. It is shown that the position of the superluminescence band with maxima in the range 715-720 nm is unchanged at all of optical pump wavelengths.

The effect of irradiation with 3 MeV electrons and subsequent high-temperature annealing on the thermal conductivity & kappa;(T) of synthetic HPHT diamond containing impurity nitrogen at a concentration of about 100 ppm was studied at temperatures from 5 to 410 K. At each stage of the experiment, the optical absorption and photoluminescence spectra, magnetization and thermal conductivity were measured on two plates from the same diamond single crystal. Color centers and their concentration in different locations in the samples were determined from the optical spectra. The concentration of paramagnetic impurities in the samples was determined from the temperature and field dependencies of the paramagnetic magnetization. Thermal conductivity is suppressed by two orders of magnitude at temperatures from 35 to 50 K after irradiation to a fluence of 5 x 1018 cm -2. Annealing at a temperature of 1800 degrees C restored the thermal conductivity of diamond at moderate and high temperatures, but not at temperatures below 120 K. The measured data on thermal conductivity were analyzed within the framework of the phenomenological Callaway model in order to understand the role of various phonon scattering processes and how it changes during irradiation and annealing.

Superluminescence of NV centres with a band peaking at λ = 718 nm in the phonon wing of the photoluminescence spectrum of a high-pressure high-temperature (HPHT) diamond sample under pulsed optical excitation at λ = 532 nm with an intensity of 2 – 46 MW cm −2 is demonstrated. Superluminescence is observed in the diamond crystal region containing 6 ppm NV centres and 150 ppm substituent nitrogen; it is absent in the crystal part with a lower nitrogen content. Superluminescence pulses are observed on the leading edge of the optical excitation pulse at λ = 532 nm and have an FWHM value of 4 ns. The enhancement of the photoluminescence of NV centres is suggested to be due to the total internal reflection in the diamond plate (waveguide effect).

In the context of the NV− diamond laser creation (Savvin and Dormidonov in Nat. Commun. 12:7118, 2021), an urgent task is to determine the characteristics of diamonds that can affect laser generation. This work is aimed at investigating the mechanisms of the creation of superluminescence in diamond under the action of optical pumping by the second harmonic of the Nd:YAG laser (λ = 532 nm). It was found that when the HPHT diamond is irradiated by 532 nm radiation with an intensity above ~ 2.0 MW/cm2, a nonlinear intensity increasing in the spectral region 700–750 nm is manifested against the background of the spontaneous photoluminescence spectrum, which, with a further pumping intensity increase turns into a pronounced peak of superluminescence with a maximum of about 718 nm. An increase in the pumping intensity from 2.7 to 46 MW/cm2 widened this peak at half-maximum from 13 to 19 nm. At high levels of pumping intensity, nonlinear pumping radiation absorption and accumulation of NV centers in the excited state were detected. The position of the photoluminescence band was calculated depending on different values of the population inversion density of the color centers, taking into account the diamond's own absorption spectrum. The calculation results are close to the experimental data.

The research was conducted to study the thermal conductivity of detonation nanodiamonds-based composites. Composite nanodiamond materials were obtained in the course of thermobaric sintering at the press-free high-pressure apparatus (BARS) under 5 GPa and at temperatures within the range of 1100 -1500 degrees C. It was ascertained that unlike diamond monocrystals with their thermal conductivity reaching up to 2100 W / (mK), the thermal conductivity of a nanodiamond composite is considerably lower and does not go beyond 18 W / (mK). Specifically, the temperature dependence of the thermal conductivity coefficient of a nanodiamond composite is anomalous as compared to a similar dependence in diamond monocrystals. The thermal conductivity coefficient in diamond monocrystals grows in compliance with the rising temperature, whereas it shows practically no changes in a nanodiamond composite in the temperature range of 50 - 300 degrees C. Such a temperature dependence of the thermal-conductivity coefficient is apparently related to the features of the phonon spectrum of diamond monocrystals. This feature is stipulated by the dependence of the phonon spectrum of nanocrystals on their size, represented by a set of phonon modes in the range of the wave vector 0 < q<1/L, i.e., the size of a diamond nanocrystal of 4.5 nm is alleged to limit the excitation of harmonics during nanodiamond composite heating, as opposed to macroscopic crystals that demonstrate the excitation of higher-frequency phonon modes during temperature growing.

Проведено исследование теплопроводности металлоалмазных композитов на основе алмазных порошков размером частиц 30-300 мкм. Композиционные металлоалмазные материалы получены в ходе термобарического спекания на беспрессовом аппарате высокого давления БАРС при температуре 1300 °С и давлении 5 ГПа. Разработанная методика проведения экспериментов позволила получить образцы объемом более 250 мм3. Установлено, что в отличие от монокристаллов алмаза, теплопроводность которых может достигать 2100 Вт/мК, теплопроводность металлоалмазного композита ниже и может достигать 490 Вт/мК. Характерно, что температурная зависимость коэффициента теплопроводности монокристаллов алмаза монотонно возрастающая, что соответствует фононному механизму теплопередачи. Теплопроводность металлоалмазных композитов является величиной эффективной и представляет собой комбинацию низкой теплопроводности металла связки и высокой теплопроводности микронных алмазных частиц с учетом весовых параметров. Очевидно, что теплопроводность зависит от наличия примесных атомов, особенно азота, в решетке алмазных монокристаллов, на которых осуществляется рассеяние фононов. Однако, существенное влияние на теплопроводность оказывают границы раздела, на которых также происходит рассеяние фононов. При наличии карбидообразующего элемента в исходной шихте теплопроводность композита возрастает в связи с образованием алмазного каркаса и хорошей смачиваемости карбида железа медью. Если в монокристаллах алмаза коэффициент теплопроводности растет при повышении температуры, то в металлоалмазном композите в интервале 50-300 °С он снижается. Такая зависимость коэффициента теплопроводности от температуры, очевидно, связана с конкурирующим вкладом фононного и электронного механизмов теплопроводности. Фононный механизм приводит к росту теплопроводности кристалла алмаза, напротив, электронный механизм теплопереноса при повышении температуры снижает теплопроводность в связи с увеличением сопротивления медной связки.

The dynamics of the formation of ensembles of nitrogen–vacancy color centers (NV centers) in single-crystal diamond plates after irradiation by high-energy (2 MeV) electrons and subsequent annealing is investigated. The concentration of the nitrogen impurity is determined from the infrared (IR) absorption spectrum to be 90 and 4 ppm for the diamond growth sectors 111 and 001, respectively. The transverse relaxation times T_2^* for the electron spin of NV centers are studied; they are found to amount to 200 and 900 ns for the 111 and 001 sectors, respectively, and are limited by the concentration of paramagnetic impurity defects in diamond. The sensitivity of quantum sensors based on the obtained ensembles of NV centers is estimated; the estimation shows feasibility of using denser ensembles in low-field magnetometry.

The nature of decoherence in a diamond plate that is most optimal for magnetometry (i.e., a plate with compromise values of concentration and coherence time of NV centres) is investigated in detail. The concentration of C centres, which serve as donors for the formation of NV centres and at the same time limit their coherence time, is measured in this plate. The ensemble of NV centres in diamond is used as a sensing element, which makes it possible to record the coherence dynamics and concentration of C centres. The recording is performed using the double electron - electron resonance technique. Its significant advantage over IR spectroscopy, which yields some averaged concentration of defects in diamond, is the possibility of measuring locally the concentration of C centres. The method proposed by us yields a value of 50.1 +/- 1.4 ppm for the C-centre concentration, which refines the IR spectroscopy data (57.5 +/- 4.8 ppm).

The spectra of pulsed cathodoluminescence and photoluminescence of synthetic diamonds subjected to radiation-thermal treatment were investigated. The luminescence spectra showed vibronic bands of NV0 centers, interstitials, nickel-nitrogen complexes, as well as previously unidentified bands at 370-380 nm and paired bands at 510 and 532 nm. On the basis of spectral analysis of peaks positions, the 510 and 532 nm bands are assigned to transitions in 100 split self-interstitials (vibronic system 3H) upon interband excitation.

The sensitivity of the nitrogen-vacancy (NV) color centers in diamond-based magnetometers strongly depends on the number of NV centers involved in the measurement. Unfortunately, an increasing concentration of NV centers leads to a decrease in their dephasing and coherence times if the nitrogen content exceeds a certain threshold level (approximately 1017cm-3 or 0.6 ppm). Here, we demonstrate that this increased dephasing can be efficiently compensated for by optimizing the electron irradiation dose in postprocessing procedures in the vicinity of the threshold concentration, thus extending the range of possible useful concentrations of NV centers in diamonds with a natural carbon content.