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In yttrium aluminum garnet (YAG) crystals containing terbium and cerium impurities, along with the main EPR signals of Tb3+ and Се3+ ions located in the dodecahedral sites of the YAG lattice in a regular environment, EPR lines with a lower (several percent) intensity were observed. They also belong to the paramagnetic centers of terbium and cerium, but are characterized by slightly altered parameters—the initial level splitting for non-Kramers Tb3+ ions and g-factors for Се3+ ions. It is shown that the nature of such centers and their number can be explained by the presence of YAl antisite defects, i.e. yttrium ions in the octahedral aluminum positions, in the environment of Tb3+ and Се3+.

The methods of high-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE) and optically detected magnetic resonance (ODMR) are used to study the unique properties of nitrogen-vacancy (NV) defects in diamond in strong magnetic fields. It is shown that in strong magnetic fields (∼3–5 T) there occurs an effective optically induced alignment of populations of spin levels resulting in filling the level MS = 0 and emptying of the levels MS = ±1, that makes it possible to record ODMR using the change in the intensity of photoluminescence which reaches 10% at resonance. It is demonstrated that the efficiency of the alignment has the same order as in zero and low magnetic fields. The samples were preliminary studied by the ODMR method in zero magnetic fields that allowed accurate determination of the main parameters of the fine structure and hyperfine interactions with nitrogen nuclei, as well as dipole-dipole interactions between the NV center and deep nitrogen donors in the form of a nitrogen atom replacing carbon, N0. Hyperfine interactions with the nearest carbon atoms (isotope 13C) were observed in the high-frequency ODMR spectra, that opens up opportunities for measuring the processes of dynamic polarization of carbon nuclei in strong magnetic fields using optical methods. It is assumed that narrow ODMR lines in strong magnetic fields can be used to measure these fields with submicron spatial resolution. A new method for recording ODMR of NV centers with microwave frequency modulation has been developed, which simplifies the technique of measuring high magnetic fields. A significant increase in the intensity of the ODMR signal was demonstrated when a strong magnetic field was oriented along the symmetry axis of the NV center.

A magnetic resonance spectrometer operating at several fixed frequencies in the millimeter range is designed on the basis of a new generation of microwave bridges and a cryogen-free magneto-optical cryogenic system. The spectrometer allows measurements of EPR in continuous wave and pulse (free-induction decay, FID and electron spin echo, ESE) modes, photo-EPR, and optically detected magnetic resonance (ODMR) in a wide range of temperatures (1.5–300 K) and magnetic fields (up to 7 T with the ability to invert the field). It is based on a line of unified microwave bridges with a powerful oscillator and a superheterodyne receiver. Currently, they operate in W and D bands (at 94 and 130 GHz, respectively), but the frequency bands can be extended. In addition to highly stable fixed-frequency oscillators with narrow spectrum (better than − 100 dBc at 10 kHz offset at 94 GHz), they have variable frequency oscillators, which simplify tuning of the microwave circuit with a resonator and allow operation with frequency modulation when using a non-resonant microwave system. In the latter, it is possible to study large samples and quickly change the operating frequency of the spectrometer, simply replacing the microwave bridge. For all frequencies, the spectrometer uses common hardware and original software. The performance of the spectrometer at 94 GHz and 130 GHz was tested in measurements of CW-EPR, ESE and ODMR of NV defects in diamond single crystals, Tb3+ and Ce3+ ions in yttrium aluminum garnet crystals, nitrogen donors, and V3+ ions in 6H-SiC.

A high-frequency electron paramagnetic resonance spectrometer with frequency modulation has been developed. Advantages of the applied method for recording spectra of paramagnetic centers with giant fine-structure splittings are demonstrated by the example of non-Kramers ions in a garnet crystal.

The advantage of the high-frequency spectroscopy of electron paramagnetic resonance (EPR) for the identification of nitrogen donors and a deep compensating vanadium impurity in various crystallographic positions of the silicon-carbide crystal is shown. Measurements are performed using a new generation EPR spectrometer operating in the continuous wave and pulsed modes at frequencies of 94 and 130 GHz in a wide range of magnetic fields (–7–7 T) and temperatures (1.5–300 K). A magneto-optical closed-cycle cryogenic system (Spectormag PT), highly stable generators (94 and 130 GHz), and a cavity-free system for supplying microwave power to the sample are used.

Electron paramagnetic resonance (EPR) spectra of Ce3+, Yb3+, Cr3+, and Gd3+ impurity ions in yttrium aluminum garnet Y3Al5O12 (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic g-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd3+ ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.

Mn2+ ions in yttrium positions have been studied by means of optically detected magnetic resonance (ODMR) via Mn2+ spin-dependent emission in manganese-doped yttrium aluminum garnet crystals. It was shown that the intensity of photoluminescence excited by circularly polarized light depends on the population of the spin sublevels of the Mn2+ ground state and therefore can be used to study the ODMR. EPR measurements have confirmed that Mn2+ ions in the crystals under study occupy preferentially dodecahedral positions in the YAG lattice. Observation of forbidden transitions in the ODMR spectra has proved that the observed ODMR signals belong to Mn2+. Thus the wavelength dependence of the ODMR amplitude reveals the emission band of Mn2+ ions at dodecahedral positions in YAG:Mn.

The electronic structure of non-Kramers ${\mathrm{Tb}}^{3+}$ centers in single crystals of yttrium aluminum garnet (YAG) was studied using a new-generation high-frequency magnetic resonance spectrometer that allows measurements of electron paramagnetic resonance (EPR), electron spin echo (ESE), the photon echo on hyperfine components of Stark levels in zero magnetic field, and optically detected magnetic resonance (ODMR). It was created on the basis of a highly stable microwave bridge operating in the near-terahertz region (0.094 or 0.130 THz), both in continuous-wave (cw) and pulse modes, a nonresonant system for supplying microwave power to a sample, and a cryogen-free magneto-optical cryostat. EPR, ESE, and cw measurements at two frequencies allowed us to reliably and with high accuracy determine the parameters of ${\mathrm{Tb}}^{3+}$ centers that occupy yttrium dodecahedral positions in YAG: ${g}_{||}=15.8\ifmmode\pm\else\textpm\fi{}0.2$, ${g}_{\ensuremath{\perp}}\ensuremath{\approx}0$ (|| corresponds to one of the $\ensuremath{\langle}100\ensuremath{\rangle}$ crystal axes), the zero-field splitting $\mathrm{\ensuremath{\Delta}}=2.705\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, and the hyperfine interaction constant $A=0.197\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. In addition to the ${\mathrm{Tb}}^{3+}$ centers in a regular environment, EPR spectra with lower intensity and a resolved hyperfine structure were found for at least three types of additional ${\mathrm{Tb}}^{3+}$ centers. They have symmetry, $g$ factors, and hyperfine interaction constants which are close to those of the main ${\mathrm{Tb}}^{3+}$ centers but differ in the zero-field splitting parameters \ensuremath{\Delta} that strongly depend on the crystal field. They were ascribed to ${\mathrm{Tb}}^{3+}$ centers with nearby antisite ${\mathrm{Y}}_{\mathrm{Al}}$ defects. Similarity with several types of ${\mathrm{Ce}}^{3+}$ centers in YAG crystals, the types, and relative concentrations of antisite defects are discussed. For one of the additional ${\mathrm{Tb}}^{3+}$ centers, splitting of the energy levels in the zero field turned out to be close to an energy of 94 GHz microwave quanta, and intense echo signals were observed in weak magnetic fields and in the zero field corresponding to the EPR transitions between the hyperfine components of the ${\mathrm{Tb}}^{3+}$ spin levels. ODMR spectra of ${\mathrm{Tb}}^{3+}$ centers in YAG crystals, containing Ce and Tb, were obtained by monitoring the intensity of the ${\mathrm{Ce}}^{3+}$ luminescence, which implies the transfer of energy from ${\mathrm{Tb}}^{3+}$ ions to ${\mathrm{Ce}}^{3+}$ ions with conservation of spin. This result seems to be important, since these systems are of interest for quantum communication and computations. Taking into account that cerium does not have isotopes with a nuclear spin and that terbium has 100% $^{159}\mathrm{Tb}$ having nuclear spin $I=3/2$ and a sufficiently large nuclear magnetic moment, the ${\mathrm{Tb}}^{3+}\ensuremath{-}{\mathrm{Ce}}^{3+}$ system in garnet crystals can be promising for coherent information processing. ${\mathrm{Tb}}^{3+}$ ions can play a role of qubits and ${\mathrm{Ce}}^{3+}$ ion with a lifetime of about 50 ns, and almost unity quantum efficiency of optical transitions can be used as a single-ion readout arrangement. The prospects for using this system as hardware for quantum communication and computations are discussed.

Electron paramagnetic resonance (EPR) in the ground state of Gd3+ ions in (LuGd)3 Al5O12 single crystals has been found by monitoring the Ce3+ photoluminescence intensity under circularly polarized excitation in the presence of the 35 GHz microwave field. Due to spin selection rules for optical transitions, the circularly polarized excitation allowed monitoring the populations of the Ce3+ ground-state spin sublevels, which can be changed at the EPR of Gd3+ due to Gd–Ce cross relaxation.

High-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE), electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) were applied for the investigation of the electronic properties of ZnO colloidal quantum dots (QDs) which consist of a ZnO nanocrystal core and Zn(OH)2 shell. Shallow donors (SDs) in the form of interstitial atoms of lithium and sodium, as well as substituent of the aluminum have been identified and the spatial distribution of their electronic wave functions has been determined in the regime of quantum confinement. Hyperfine interactions as monitored by ENDOR quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. We studied the process of charge separation in the quantum dot when excited by UV light resulting in the formation of shallow donors and deep acceptors in the paramagnetic state, which recombine when heated, accompanied by thermoluminescence go back again in the non-paramagnetic state. It was shown that in all studied quantum dots deep acceptors are present positioned near the interface and including one sodium atom. ODMR techniques, which is based on EPR detection via photoluminescence or via tunneling afterglow that can be observed after preliminary X-ray or UV irradiation proved to be very useful to study colloidal ZnO NCs. The higher sensitivity of ODMR via afterglow allowed characterization of ZnO QDs dispersed in transparent media, which is an important advantage, since these systems are more relevant for a number of practical applications. A direct evidence of Co (Mn) interaction with SD in the core and hyperfine coupling with 1H in the shell of QDs has been demonstrated in Mn (Co)-doped ZnO QDs, which are promising classes of diluted magnetic semiconductors.

An optical quantum thermometer with submicrometer resolution is proposed. Its operation is based on the physical phenomenon of the optical response in a system of spin centers in silicon carbide under conditions of anticrossing of spin sublevels of the excited quadruplet state (S = 3/2) with temperature-dependent fine-structure splitting.

We describe a series of high-frequency EPR/optically detected magnetic-resonance (ODMR) spectrometers operating in both continuous-wave and pulsed regimes in 2-, 3-, 4-, and 8-mm. The resonance signal can be monitored in both microwave and optical detection channels. The entire series of EPR/ODMR spectrometers has common design features.

The electron spin echo detected inversion recovery technique at the Q-band frequency was used to characterize spin diffusion effects in spin-lattice relaxation of compensating Fe3+ impurities in n-type doped GaN crystals. It was found that the selective saturation can be achieved in the GaN:Fe3+ system due to magnetization transfer based on the spin flip-flop cross-relaxation processes. The temperature dependence of 1/T1 can be explained by direct spin-phonon processes ( ∼ T) below 25 K and by Raman two-phonon processes ( ∼ T 9) at higher temperatures. Spin diffusion in this system is characterized by an additional cross-relaxation rate which is weakly temperature-dependent below 25 K. The transferred hyperfine interactions of Fe3+ centers with gallium and nitrogen neighbor nuclei were resolved using pulsed-electron nuclear double resonance. A comparative analysis of quadrupole interactions indicates the essential increase in the electric field gradients on the nearest nitrogen and gallium shells.

Exchange-coupled complexes consisting of Mn ions and holes have been revealed by optically detected magnetic resonance in the narrowest quantum wells of asymmetrical multiple-quantum-well structures (CdMn)Te/(CdMg)Te. Calculations have been performed to estimate the parameters of the complexes (exchange interactions and hole g-factors) and simulate the spectra. The formation of such complexes implies the directional electron tunneling from narrow to larger wells, i.e., an electron cascade, which results in the creation of the excess hole concentration in the narrowest and intermediate-width quantum wells.

Long-lasting afterglow of ZnO single crystals grown by hydrothermal method has been studied using high-frequency (94 GHz) optically detected magnetic resonance (ODMR) at helium temperatures. Giant (up to 30%) afterglow intensity-detected electron paramagnetic resonance signals of shallow donors and deep acceptors, which are lithium atoms substituting for zinc in the ZnO lattice, unambiguously indicate a spin-dependent character of the donor-acceptor recombination. The observed variations in the shape of the afterglow-detected ODMR spectra of lithium acceptors with time passed after irradiation are associated with the difference in the recombination rates of shallow donors with axial and non-axial LiZn centers and with a change in the recombination character with time.

The results of studies of shallow donors and deep-level color centers in bulk AlN crystals are presented. Two shallow donors (presumably oxygen located on the nitrogen site and carbon located on the aluminum site) are suggested to exhibit the DX-relaxation. Third shallow donor (presumably silicon on the Al site) shows the shallow donor behavior up to the room temperature and can be observed without light excitation at temperatures above 200 K. The values of the Bohr radius of the shallow donors are estimated. The structure of deep-level color centers (neutral nitrogen vacancy VN) in bulk AlN crystals is determined and analyzed by electron paramagnetic resonance, electron-nuclear double resonance, optical absorption and thermoluminescence induced by X-ray irradiation. Spin-dependent recombination processes in AlN crystals are studied by means of optically detected magnetic resonance.

The results of pulse electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are presented to show the electron structure of deep level defect color center in AlN single crystals such as a nitrogen vacancy in neutral charge state (VN0) as well as a new type of point defect such as exchange–coupled pair of the nitrogen vacancies (VN-VN). Analyzing spin density distribution of an unpaired VN0 electron on different ions of AlN crystalline lattice by means of high frequency ENDOR technique (94 GHz) give us a possibility to calculate hyperfine and quadrupole interactions (QI) with 27Al nuclei spins up to fourth coordinate sphere. Particularly data about quadrupole interactions allowed to estimate the fundamental parameter of AlN crystalline lattice in itself namely so called «crystalline field gradient» to be 4.9×1020V/m2 (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

To perform optical detection of high-frequency magnetic resonance (ODMR) and cyclotron resonance (ODCR) with a spatial resolution, we have proposed a noncavity scheme of an ODMR-ODCR spectrometer using a quasi-optical microwave channel. The efficiency of this scheme for obtaining information on the spin properties of ZnO quantum dots and CdMnTe quantum wells and on the effective masses of carriers in crystalline silicon films with a spatial resolution within a focused laser beam is illustrated.

EPR studies presented here reveal the presence of three different types of shallow donor (SD) impurities in AlN single crystals: one impurity is located at the N site (preferentially oxygen, labeled D1) and two impurities are located at the Al site (preferentially carbon and silicon, labeled D2 and D3) of the AlN lattice. We show that D1 and D2 centers exhibit a DX-like relaxation. The light-induced EPR signals of the D1 and D2 centers are characterized by slightly anisotropic g factors typical for SD in AlN but strongly different anisotropic EPR linewidths ΔB. The D3 center shows the SD behavior at the room temperature and can be observed without light excitation at the temperatures above 200 K. Combining EPR, optical absorption and thermoluminescence induced by the X-ray irradiation we establish the structure of deep level color centers in AlN - a neutral nitrogen vacancy VN. The an optical absorption band of the center is in the visible region with a maximum at ∼450 nm and the energy level at ∼0.75 eV relative to the bottom of AlN conduction band (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag(2+) ion inside of a (AgCl(6))(4-) complex as a result of the Jahn-Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn-Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra.

The presence of aluminum and gallium isotopes with large nuclear magnetic and quadrupole moments in the nearest environment of impurity ions Mn2+ and Ce3+ in garnets made it possible to use hyperfine and quadrupole interactions with these ions to determine the spatial distribution of the unpaired electron and the gradient of the electric field at the sites of aluminum and gallium in the garnet lattice. High-frequency (94 GHz) electron spin echo detected electron paramagnetic resonance and electron-nuclear double resonance measurements have been performed. Large difference in the electric field gradient and quadrupole splitting at octahedral and tetrahedral sites allowed identifying the positions of aluminum and gallium ions in the garnet lattice and proving that gallium first fills tetrahedral positions in mixed aluminum-gallium garnets. This should be taken into account in the development of garnet-based scintillators and lasers. It is shown that the electric field gradient at aluminum nuclei near Mn2+ possessing an excess negative charge in the garnet lattice is ca. 2.5 times larger than on aluminum nuclei near Ce3+.

Long-lasting (several hours) afterglow at helium temperatures was observed in ZnO nanocrystals, 36 nm in size, after switching off the UV excitation. A quenching of the afterglow intensity in the magnetic field was found, which appears because of the preferential orientation of the recombining spins along the field and the spin-dependence of the recombination. ODMR was detected as resonance increase in the afterglow intensity at saturation of the EPR transitions of defects, which take part in the recombination. Shallow donors and different types of acceptors responsible for the afterglow in Al doped ZnO nanocrystals were identified. ODMR of donors and Li acceptors was recorded via the afterglow of bulk ZnO:Li, which could last for about an hour. In the photoluminescence-detected ODMR of bulk ZnO:Li crystals exchange-coupled donor–acceptor pairs were observed in addition to the Li acceptor and donor signals. Exchange-coupled pairs dominated the ODMR spectra recorded on the photoluminescence of ZnO nanocrystals.

The centers involved in the spin-dependent recombination in KCl: AgCl insulator-semiconductor wide-band-gap crystal structures, bulk ZnO crystals, and quantum dots based on ZnO nanocrystals have been identified by detecting electron paramagnetic resonance from tunneling afterglow. Long tunneling after-glow has been excited by short-term UV irradiation of the sample at liquid-helium temperatures. The observed magnetic quenching of the afterglow at low temperatures results from the Boltzmann polarization of spins of recombination centers. The revealed giant increase in the afterglow intensity is induced by the reorientation of spins of these centers at electron paramagnetic resonance. A new technique for recording the spectra at a high frequency of 94 GHz has been developed and used with the aim of increasing the sensitivity and spectral resolution. This technique has made it possible to separate electron and hole centers in the KCl: AgCl structures and to demonstrate that ZnO: Al quantum dots contain two types of deep acceptors.

It is found that the energy released in the spin-dependent tunneling recombination of electron-hole pairs and self-trapped excitons in an ionic crystal matrix is directionally transferred to low-dimensional semiconductor structures embedded in the matrix as a result of self-assembled growth. The EPR spectra of electron and hole centers in the matrix crystal are detected by tunneling afterglow and photostimulated luminescence that are excited in the low-dimensional structure.

The chemical nature of donors unintentionally present in ZnO nanocrystals prepared by chemical methods has been studied by high frequency electron paramagnetic resonance and electron nuclear double resonance. The experiments show that H, which is present during the preparation process, and acts as a shallow donor in bulk ZnO, is located in the Zn(OH)2 shell covering the nanocrystals. We find that Li, probably in an interstitial position, is incorporated in the core of the nanocrystals. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optically-detected magnetic resonance (ODMR) and optically-detected cyclotron resonance (ODCR) were applied to study two types of nanostructures emitting around 1.3 μm: quantum wells with low nitrogen content and InAs / GaAs quantum dots (both isolated and vertically-coupled). ODMR with the axial symmetry (effective g factor |g‖|=3.61 and |g⊥|=0.7) was found in unannealed InGaAsN / GaAs multiple-quantum well structures and ascribed to electrons in the quantum wells; the sign of g factor is suggested to be negative. There is evidence that before annealing the InGaAsN / GaAs structure has properties, which are typical for quantum wells, but after annealing the structure is completely-changed and resembles a quantum-dot-like structure. The effect of cyclotron resonance on the luminescence of InAs quantum dots was found. The observed ODCR seems to belong to a two-dimensional system, such as a heterointerface InAs / GaAs .

The spectroscopic properties of V-doped Bi4Ge3O12 have been investigated in detail via several complementary techniques bringing a series of consistent results. The optical absorption spectrum of annealed samples is strongly modified under UV-illumination and the initial state can be restored optically with visible light. Optical absorption and magnetic circular dichroism (MCD) demonstrate that a diamagnetic defect is partly destroyed during UV-illumination while a paramagnetic one is created. The latter shows a very characteristic S-shaped MCD pattern in the near-IR, which is readily assigned to the 2 E → 2 T 2 internal transition of tetragonal V4+ centers at the Ge sites. This assignment was further confirmed by optically detected magnetic resonance (ODMR), via the change of the MCD under microwaves at 35 GHz. The g tensor of V4+ was found to be anisotropic with principal values g||=1.81±0.03 and g⊥=1.94±0.02. Two additional MCD bands in the visible spectral region could be attributed to V4+ via ODMR measurements. Ionization thresholds for holes (V5+) and electrons (V4+) were determined by optical absorption experiments and the V4+/5+ donor level was positioned 1.9 eV below the conduction band. The presence of the V4+/3+ acceptor state in the forbidden band is also suggested.

The phenomenon of spatial confinement of the electron-hole recombination in exchange-coupled donor-acceptor pairs was observed by optically detected magnetic resonance in AgBr nanocrystals formed as a result of the self-organized growth in an ionic KBr crystal matrix. The effect is manifested by the maximum distance between recombining donors and acceptors being restricted to the nanocrystal size and by a change in the g value of shallow electron donor centers. Based on an analysis of the exchange interactions, the distribution of distances in the donor-acceptor pairs is determined and the dimensions of nanocrystals are estimated.

Self-organized microcrystals and nanocrystals (quantum dots) of AgCl and AgBr embedded in KCl and KBr crystalline matrices and retaining the orientation of the host lattice were studied by optically detected magnetic resonance. It was unambiguously shown that self-organized microcrystalline silver halides can be grown inside alkali halide crystals with the properties of bulk crystals since the optically detected magnetic resonance spectra of the embedded microcrystals were practically the same or close to those in bulk AgCl and AgBr and could be used as a `fingerprint' for AgCl and AgBr. For AgCl nanocrystals in a KCl matrix the anisotropy of the g-factor both for isolated self-trapped holes and for self-trapped holes forming self-trapped excitons was found to be substantially reduced compared with those of bulk AgCl crystals. This implies a considerable suppression of the Jahn-Teller?(JT) effect in nanoparticles. A rather general mechanism of the suppression of the JT effect in nanocrystals is proposed, taking into account the additional deformation field appearing because of the strong vibronic interaction at the interface. It was concluded that the distribution of exchange interactions for electron-hole pairs and triplet excitons in the KBr:AgBr system is due to a distribution of AgBr crystal sizes. The holes seem to be self-trapped in the AgBr because of the dynamical JT effect. The exchange splitting increases for distant electron-hole pairs with a decrease of AgBr size. The spectra with exchange splitting larger than that in bulk AgBr (1.9?cm-1) seem to belong to AgBr nanocrystals. In contrast to AgCl the wavelength of the luminescence in AgBr micro- and nanocrystals embedded in a KBr matrix decreases with the decrease of AgBr crystal size.

Optically detected magnetic resonance (ODMR) and level anticrossing spectroscopy were applied to study g-factors and exchange splittings of localized excitons and separately localized electrons and holes at the Xz-Γ crossover of the conduction band states in a GaAs/AlAs superlattice with a composition gradient. g-factors, exchange splittings and the order of the exciton radiative levels were determined. In the transition region we clearly observed the disappearance of type II excitons created by the Xz electron in AlAs and a heavy hole in GaAs layers. Appearance of type I excitons with an order of magnitude larger exchange was verified by LAC spectroscopy. Intermediate `type II-like' and `type I-like' excitons were found in the transition region by ODMR and LAC with different exchange splittings and luminescence decay times. Besides ODMR of excitons with a definite value of exchange splitting, ODMR ascribed to separately localized electrons and holes with a distribution of exchange splittings was detected.

The nature of the green emission of PbWO 4 is still under discussion, models ranging from WO 3 groups over lead deficient phases to local distortions towards a wolframite structure. In CaWO 4 and PbMoO 4 optically detected magnetic resonance proved, using resolved Mo hyperfine structure, that tetrahedral MoO 4 complexes give rise to a green luminescence. Based on this, we have performed such experiments on a series of PWO samples from various sources and with different dopants. In all cases, we find resonances typical for MoO 4 groups, however, with strongly varying intensity depending on the Mo content of the crystals. These ODMR signals are connected with a luminescence band with a half width of 0.43 eV peaking at 508 nm.

GaAs/AlAs and GaAs/AlGaAs quantum wells and superlattices were grown by molecularbeam epitaxy and self-organized oriented silver halide micro- and nanocrystals were embedded in crystalline alkali halide matrix. We report the results of the application of optically detected magnetic resonance (ODMR) and level anticrossing (LAC) spectroscopy for the investigations of these two classes of nanostructures. ODMR and LAC spectroscopy are shown to be very suitable for obtaining important physical parameters of low-dimensional systems and for a local diagnostics of nanostructures.

Self-trapped holes (STH), self-trapped excitons (STE) and shallow electron centres in small AgCl crystals embedded in a KCl crystalline matrix have been observed by means of optically detected magnetic resonance (ODMR). The existence of the impurity clusters in heavily doped KCl:AgCl single crystals, ranging from single and paired Ag ions to AgCl nanometre- and micrometre-size crystals (nanocrystals and microcrystals) retaining the orientation of the matrix, was confirmed. ODMR spectra were used as a fingerprint of the embedded AgCl microcrystals and a signature of their crystallinity. For AgCl nanocrystals the anisotropy of the g-factor both for isolated STH and for STH forming STE was found to be substantially reduced compared with those of bulk AgCl crystals and AgCl microcrystals embedded in KCl. This implies a considerable suppression of the Jahn-Teller effect in nanoparticles. A rather general mechanism of the suppression of the Jahn-Teller effect in nanocrystals is developed, taking into account the additional deformation field appearing because of the strong vibronic interaction at the interface. It allows evaluation of the critical size of the embedded AgCl nanocrystals, at which the suppression of the Jahn-Teller effect has its onset; the value obtained is ≈10 nm, in agreement with experiment.

Fine structure of isolated Mn 2+ ions in CdMnSe/ZnSe quantum wells (QWs) is revealed by optically detected magnetic resonance (ODMR) recorded by monitoring both exciton emission and intra-Mn luminescence in the presence of simultaneous microwave irradiation. A large decrease of photoluminescence (PL) intensity of excitons and an increase of PL intensity of Mn 2+ ions is observed when an applied magnetic field satisfies the Mn 2+ electron paramagnetic resonance (EPR) condition. This suggests that a spin-dependent energy transfer from excitons to intra-Mn excitations occurs at the EPR condition.

This paper presents the first unambiguous optically detected magnetic-resonance (ODMR) evidence that AgCl crystals embedded in the KCl lattice and retaining the host orientation are formed in KCl crystals grown with a 2–3 mol % silver impurity. ODMR spectra were obtained of self-trapped holes, shallow electronic centers, and self-trapped excitons, which are typical of AgCl, and a number of substantially different ODMR spectra were also obtained. The differences between the ODMR spectra observed in samples cleaved from different parts of a KCl: AgCl crystal are probably accounted for by embedded AgCl crystals varying in size from large micro-to nanocrystals.

A strong decrease in the g-factor anisotropy was revealed by optically detected magnetic resonance for self-trapped Jahn-Teller holes (both isolated and forming self-trapped excitons) in AgCl nanocrystals embedded into the KCl crystal lattice. This is evidence for considerable suppression of the Jahn-Teller effect in nanoobjects. The suggested mechanism of suppression of the Jahn-Teller effect in nanocrystals is associated with an additional deformation field arising in nanocrystals owing to a strong vibronic interaction at the interface.

The infrared luminescence of Er3+ ions has been studied in bulk crystals of silicon carbide 6H-SiC doped with erbium in the process of their growth. The erbium centers of different symmetry in the crystals are revealed by the EPR technique. A number of intense luminescence bands of erbium ions are observed at a wavelength of about 1.54 µm. The luminescence can be excited by the light with quantum energies above and below the band gap of SiC. It is found that the luminescence exhibits unusual temperature behavior: as the temperature increases, the luminescence intensity abruptly rises starting with 77 K, passes through a maximum at ∼240 K, and, in the vicinity of ∼400 K, decreases down to the values observed at 77 K. The activation energies for the flare-up and quenching of the Er3+ luminescence are estimated at E A ≈130 and ≈350 meV, respectively. The mechanisms of the flare-up and quenching of the Er3+ luminescence in SiC are discussed.

Two types of excitons, localized at opposite interfaces and characterized by different magnitudes of the exchange interactions at the same radiation energies, are simultaneously in type-II GaAs/AlAs superlattices. It is shown that the additional long-wavelength luminescence line in superlattices grown with growth interruptions after the GaAs layers is due to the recombination of an exciton localized at an inverted interface in regions where the quantum-well width is increased by one monolayer.

We report on the optical detection of electron paramagnetic resonance at 36 GHz, 67 GHz and 70 GHz for all the absorption bands of Bi12GeO20 (BGO) in the visible spectral range. This completes our previous studies and demonstrates clearly that the three major magnetic circular dichroism features do belong to the same isotropic centre, i.e., a Bi3+Ge+h defect with the hole being delocalized onto the four surrounding oxygen atoms. We provide a simple explanation for the unusual sign of ODMR and the transient behaviour of the magnetic circular dichroism at resonance.

Electron paramagnetic resonance (EPR) was detected in the ground state of Ce3+ ions in YAG:Ce single crystals by monitoring the intensity of photoluminescence under nonresonant excitation. The photoluminescence intensity was found to increase strongly in magnetic field at liquid helium temperature. The optically detected magnetic resonance (ODMR) effect was of the order of percents and corresponded to resonance decrease in the luminescence intensity. ODMR correlated with the conventional EPR spectra of Ce3+ ions, which occupy the dodecahedral Y3+ sites in the YAG crystal lattice. The increase of the PL intensity in magnetic field was concluded to be caused by Boltzmann distribution between spin sublevels of the ground state of Ce3+ ions. The ODMR tend to equalize the populations of levels in resonance and results to a decrease of PL intensity. Different types of Ce3+ centres characterized by the presence of nearby antisite defects were observed, which should be taken into account in applications of single ion spectroscopy based applications of YAG:Ce.

Strong signals corresponding to electron paramagnetic resonance (EPR) transitions with the absorption of two, three and so on microwave quanta have been observed in optically detected magnetic resonance (ODMR) spectra of bulk semiconductors (A2B6, SiC), type-II GaAs/AlAs superlattices and silver chloride crystals. Multiquantum ODMR spectroscopy was applied to analyse the energy levels of Al, Ga and Sc accepters in silicon carbide and showed a nonlinearity of the energy levels of Sc acceptors in a magnetic field. In ASCI crystals, multiquantum ODMR of self-trapped holes, electrons and self-trapped excitons was found. For the first time, multiquantum ODMR was observed between fine-structure energy levels, i.e. for a system with electron spin S>1/2. Singlet-to-triplet multiquantum EPR transitions with absorption of up to seven microwave quanta were detected for self-trapped excitons and unambiguously identified by frequency shift measurements. The singlet-to-triplet splitting has been determined with a very high accuracy: J=-161.0+or-0.1 GHz.

Nuclear magnetic resonance of thallium impurities has been detected by monitoring the magnetic circular dichroism of the absorption of F centres produced by room temperature X-irradiation under optical pumping conditions in Tl-doped KCl and RbCl crystals. In addition, electron nuclear double resonance signals of the F centre ligands were detected without microwaves. The observed effects are caused by hyperfine coupling of the F centres to the surrounding nuclei. F centres are probably produced with a spatial correlation to the thallium impurities.

By monitoring the magnetic circular dichroism (MCD) of F-centers in irradiated alkali halides EPR of impurity centers was detected without microwaves due to a cross-relaxation of the impurities with optically pumped F-centers. Cross-relaxation effects were also observed in magnetic resonance detected via MCD in absorption and the intensity of spin-dependent recombination emission in ionic crystals doped with transition and rare earth ions.

The paper presents the results of optically detected magnetic resonance (ODMR) study of tunneling and photostimulated recombination processes in irradiated ionic crystals, observation of resonance effects using optically pumped F-centres, and investigation of spin-dependent recombination processes in silicon carbide doped with boron, aluminium, gallium and scandium.