


Том 121, № 4 (2016)
- Жылы: 2016
- Мақалалар: 24
- URL: https://journal-vniispk.ru/0030-400X/issue/view/10068
XVI International Feofilov Symposium
DFT, L(S)DA, LDA+U, LDA+DMFT, …, whether we do approach to a proper description of optical response for strongly correlated systems?
Аннотация
I present a critical overview of so-called “ab initio” DFT (density fuctional theory) based calculation schemes for the description of the electronic structure, energy spectrum, and optical response for strongly correlated 3d oxides, in particular, crystal-field and charge transfer transitions as compared with an “old” cluster model that does generalize crystal-field and ligand-field theory. As a most instructive illustration of validity of numerous calculation techniques I address the prototypical 3d insulator NiO predicted to be a metal in frames of a standard LDA (local density approximation) band theory.



Spectroscopy of charge transfer states in Mg1 – xNixO
Аннотация
Photoluminescence and photoluminescence excitation spectra of solid solution Mg1–xNixO (x = 0.008) have been analyzed. The contributions of charge transfer electronic states and nonradiative Auger relaxation to the formation of the photoluminescence spectrum are discussed.



Radiolysis of LaF3 crystals with rare-earth impurities
Аннотация
The absorption spectra of LaF3 crystals, both pure and doped with rare-earth fluorides (YF3, CeF3, NdF3, PrF3, SmF3, EuF3, GdF3, TbF3, DyF3, HoF3, ErF3, TmF3, YbF3, and LuF3) have been investigated. All these impurities can be separated into two groups with respect to the shape of the absorption spectra of irradiated crystals. The spectra of the crystals doped with Nd, Sm, Tm, and Yb exhibit, along with 200-nm hole band F3-, weak bands due to RE2+-anion vacancy centers. The spectra of LaF3 crystals with Y, Ce, Pr, Gd, Tb, Dy, Ho, Er, and Lu impurities exhibit, along with the hole-center bands (F3- at 200 nm and VkA at 320 nm), bands of comparable intensity, which can be attributed to RE3+–F centers. This conclusion is confirmed by preliminary quantum-chemical calculations and the estimation of the levels location in the energy-band diagram.



Luminescence decay kinetics of Eu3+ ions in phosphate glasses of different composition under photo- and electron excitations
Аннотация
The influence of matrix composition, coactivators, and excitation method on the luminescence kinetics of Eu3+ ions in lithium–phosphate and lithium–phosphate–borate glasses activated by Eu, Eu/Tb, and Eu/Dy is studied. Luminescence is excited by a high current electron beam and a xenon lamp. It is found that, under photoexcitation, the europium luminescence decays more slowly than under electronic excitation. Depending on the content of cation modifiers ZnO and Li2O, the decay time decreases with increasing amount of ZnO. The decay time weakly depends on the europium concentration. The decay of the luminescence of europium ions is well described by the Inokuti–Hirayama model.



The effect of codopants on spectral-kinetic characteristics of luminescence of scintillation glasses doped with terbium ions
Аннотация
The effect of Ce3+ and Pr3+ ions on spectral-kinetic characteristics of luminescence of lithium–phosphate–borate glasses is studied. It is shown that terbium ion luminescence caused by transitions from 5D3 and 5D4 multiplets to the ground 7FJ term is detected in samples containing Tb3+/Ce3+ and Tb3+/Pr3+. It has been found that an increase in the concentration of cerium ions from 0.2 to 1 wt % leads to an increase in the intensity of main luminescence bands of terbium ions. In Tb3+/Pr3+ glasses, a decrease in the relative light yield is observed with an increase in the concentration of Pr3+ ions. Processes of energy transfer between Tb3+/Ce3+ and Tb3+/Pr3+ ions are discussed.



Optical properties of transparent cobalt-containing magnesium aluminosilicate glass-ceramics doped with gallium oxide for saturable absorbers
Аннотация
Transparent glass-ceramic materials based on glasses of the MgO–Al2O3–SiO2–TiO2 system doped with CoO and Ga2O3 are synthesized. The secondary heat treatment of the initial glasses at temperatures of 800–950°C leads to precipitation of nanosized (6–7 nm) crystals of magnesium aluminogallium spinel doped with cobalt ions and magnesium aluminotitanate solid solutions. The optical absorption spectra of the initial glass and glass-ceramic materials are studied. It is shown that the absorption band caused by the 4A2(4F)→ 4T1(4F) transitions of tetrahedrally coordinated Co2+ ions in glass-ceramics with nanosized Co:Mg(Al,Ga)2O4 crystals is shifted to longer wavelengths (up to ~1.67 µm) compared to the position of this band in materials with Co:MgAl2O4 crystals. The synthesized glass-ceramics are characterized by a relatively low saturation fluence FS ~ 0.5 ± 0.1 J/cm2 at a wavelength of 1.54 µm, as well as by a high radiation resistance to nanosecond laser pulses, which is no lower than ~15 ± 2 J/cm2. This explains their attractiveness as materials for saturable absorbers for erbium lasers emitting in the spectral range 1.5–1.7 µm.



PbWO4 crystal for laser energy accumulation and transformation
Аннотация
The picosecond interband two-photon laser excitation of PbWO4 crystals at a temperature of 10 K leads to electronic excitation energy accumulation, which results in almost 100% induced absorption in the 450–750 nm spectral range. The relaxation time of this induced absorption exceeds 100 min. The electronic excitation energy accumulated in the PbWO4 crystal at T = 10 K excites the intrinsic luminescence with a decay time longer than 45 min. The decay kinetics and the spectra of the intrinsic luminescence of the PbWO4 crystal at a temperature of 10 K were measured under two-photon and single-photon excitation. The luminescence under two-photon and single-photon excitation revealed a difference in the structure of the spectra.



Temperature quenching of intracenter luminescence of Mn2+ ions in diluted magnetic semiconductors
Аннотация
The temperature dependence of the spectral composition and intensity of intracenter luminescence of the Mn2+ 3d shell and the temperature dependence of the luminescence excitation spectra in an epitaxial layer of Cd0.2Mn0.8Te at different levels of optical excitation are investigated. It is found that in the crystals of type II1–xMnxVI with a high concentration of manganese, the characteristics of intracenter luminescence of Mn2+ depend on the efficiency of excitation migration over manganese ions. Under migration conditions, the luminescence quantum yield is determined by the cooperative effect (up-conversion).



Infrared luminescence of CaLa2 – xNdxGe3O10:Ho3+, Er3+
Аннотация
Solid solutions CaLa2–xNdxGe3O10:Ho3+, Er3+ have been synthesized, their optical characteristics have been studied, and the dependences of the Stokes luminescence in the 0.9–3.0 µm region on the concentration of neodymium ions under 808 nm laser diode excitation have been determined. Based on analysis of the obtained dependences, the optimal composition of the phosphor with the maximum efficiency of the energy conversion is established.



Two-step photoconductivity in LiYxLu1 – xF4:Ce,Yb crystals
Аннотация
Photoconductivity of LiYxLu1–xF4:Ce,Yb (x = 0–1) crystals is measured under one- and two-step excitation. It is established that the photoconductivity is due to intra-center transitions from excited states of Ce3+ ions. The position of the ground 4 f-state of Ce3+ ion relative to the bottom of the conduction band is determined. The choice of pumping conditions to obtain the lasing on the 5d–4f transitions of trivalent cerium in these active media is substantiated.



Analysis of excitation mechanisms of Ho3+ upconversion luminescence in Ho3+:LiYbF4 (0.2 at %) crystal via photographs of its longitudinal cross sections and via spectral and kinetic characteristics
Аннотация
The results of a complex analysis of the excitation mechanisms of the up conversion luminescence of Ho3+:LiYbF4 (0.2 at %) crystal are presented. The spatial distribution of the upconversion luminescence intensity is studied by the photographs of longitudinal cross sections at different positions of the laser beam waist with respect to the sample. The surface power density of the pump laser diode radiation (0.755 W, λ = 933 nm) was changed by focusing the beam (similar to Z-scanning). The dependences of the longitudinal luminescence cross sections, as well as of the spectral and kinetic characteristics of Ho3+ and Yb3+ luminescence, on the position of the laser beam waist are determined. It is found that there exist two different mechanisms of the population of the energy levels of Ho3+ ions from which green and red luminescence occur, namely, cooperative sensitization of luminescence and absorption of induced photon groups (JETP Letters, 102 (5), 279 (2015)). It is shown that the contributions of these mechanisms vary both in time and over the crystal volume. All the observed spatial, spectral, and temporal specific features of the upconversion luminescence of Ho3+:LiYbF4 (0.2 at %) crystal are qualitatively explained.



Li-impurity effect in optical spectra of KTaO3:Er3+ crystals
Аннотация
The f–f absorption and emission spectra of Er3+ impurities were observed and identified in KTaO3 and K1 – xLixTaO3 single crystals. It was shown that Li off-centers related random fields markedly influence intensity, width and position of zero-phonon lines. The detail analysis of the absorption spectra obtained allowed one to determine energies of the Stark sublevels of excited states for Er3+ dominant centers of noncubic symmetry.



Photoinduced toxicity of PrF3 and LaF3 nanoparticles
Аннотация
PrF3 and LaF3 nanoparticles were synthesized by the hydrothermal method. The size distribution of these nanoparticles in the colloidal solution produced was studied by photon correlation spectroscopy. The mean diameter of the nanoparticles was 42 ± 1 nm. During the study of the toxicity of the nanoparticles, the mixture of a colloidal solution of the nanoparticles with cells to be studied was irradiated by 30-mW continuous lasers at wavelengths of 532 and 473 nm. The concentration of salmonella cells in normal saline was 106 cell/mL, while that of nanoparticles was 0.1 g/L. The cell survival percentage was 39, 34, and 20% for the irradiation times of 5, 10, and 15 min, respectively, at an optimal laser radiation power density of 0.4 W/cm at a wavelength of 532 nm. It was ascertained that LaF3 nanoparticles do not possess the property of photoinduced toxicity and the apoptosing effect. Moreover, the property of photoinduced toxicity is not shared by microparticles, in contrast to nanoparticles.



Structure, lattice dynamics, and exchange interaction in Lu2V2O7, Y2V2O7: an ab initio approach
Аннотация
Ab initio calculations of the crystal structure and fundamental vibrations of vanadium pyrochlores Lu2V2O7 and Y2V2O7 are performed. The calculations are performed in the framework of the density functional theory (DFT) with the use of hybrid functionals. The ions involved in the vibrations are determined by the isotope substitution method. Values of the isotropic exchange interaction constant were calculated. Theoretical results for the crystal structure parameters, the vibrational frequencies, and the isotropic exchange interaction parameter are compared with the experimental data.






Modelling of the luminescent properties of nanophosphor coatings with different porosity
Аннотация
Coatings of Y2O3:Eu nanophosphor with the effective refractive index of 1.02 were obtained by flame aerosol deposition (FAD). High-pressure cold compaction decreased the layer porosity from 97.3 to 40 vol % and brought about dramatic changes in the photoluminescent performance. Modelling of interdependence between the quantum yield, decay time of luminescence, and porosity of the nanophosphor films required a few basic simplifying assumptions. We confirmed that the properties of porous nanostructured coatings are most appropriately described by the nanocrystal cavity model of the radiative decay. All known effective medium equations resulted in seemingly underestimated values of the effective refractive index. While the best fit was obtained with the linear permittivity mixing rule, the influence of further effects, previously not accounted for, could not be excluded. We discuss the peculiarities in optical response of nanophopshors and suggest the directions for future research.



Manifestation of the Hanle effect in submillimeter EPR spectroscopy of thulium impurity ions in synthetic forsterite
Аннотация
A signal related to the spin level crossing in a zero magnetic field—the Hanle effect—has been registered for the first time in the EPR spectrum. It has been shown that, in the general case, the shape of the signal is determined by two qualitatively different mechanisms: (i) the interference of unsteady-state contributions to the dynamics of atomic coherences (electric or magnetic quantum transition moments with certain phases) with close frequencies (“beats at a zero frequency”) and (ii) the summation of resonant signals determined by the steady-state dynamics of the same atomic coherences. The relaxation time of spin coherences has been determined for the EPR transition of Tm3+ ions in synthetic forsterite.



Condensed-Matter Spectroscopy
The influence of deuteration of complex organic molecules on their fluorescence quantum yield (a review)
Аннотация
Data on the influence of deuteration of organic molecules and/or solvent on their fluorescence quantum yield are summarized. It is demonstrated that the effect of deuteration is normal, i.e., deuteration increases the fluorescence quantum yield due to decrease in the internal conversion probability, for organic molecules exhibiting fluorescence in the red region of spectrum, in which the internal conversion competes with the emission of fluorescence. In the case in which the probability of internal conversion is low, i.e., energy of level S1 ≥ 16000–20000 cm–1, the intersystem crossing probability from the S1 state to the set of triplet levels depends on the relative position of S1 and the nearest triplet level. In so doing, the effect of deuteration can be either normal or anomalous, depending on whether the resonance between the S1 level and the level nearest to it, the Tn level, improves or deteriorates when these levels shift as a result of deuteration. In dilute vapors, cooled supersonic jets, and crystalline matrices, including Shpolskii matrices, the effect of deuteration at helium temperatures depends on exact resonance between the interacting levels. In the case of dilute vapors and cooled jets, the effect also depends on the vibronic level being excited. Deuteration of OH and NH groups, as a rule, slows proton transfer in the S1 state of the molecule that occurs with their involvement, leading to normal effect of deuteration.



Specific features of luminescence quenching in a nematic liquid crystal doped with nanoparticles
Аннотация
The change in the intensity of the photoluminescence (PL) spectra of nematic liquid crystal (NLC) composites as a function of the concentration of CdSe/ZnS semiconductor quantum dots (QDs) and TiO2 and ZrO2 nanoparticles ~5 nm in diameter has been investigated. It is shown that the PL-quenching intensity in composites with CdSe/ZnS QDs exceeds that in composites with TiO2 and ZrO2 nanoparticles. The lowfrequency spectra of these composites with a concentration of 0.1 wt %, recorded in the range of 102–103 Hz, and the content of mobile ions in them have been investigated. It is found that the dielectric loss in the composite with CdSe/ZnS QDs is much higher and the content of mobile ions is larger by a factor of 3 than in the composites with TiO2 and ZrO2 nanoparticles. It is shown that an increase in the CdSe/ZnS QD concentration in NLC composites leads to an increase in the dielectric loss and a decrease in the PL intensity. Possible mechanisms of the interaction between NLC molecules and CdSe/ZnS QDs are discussed.



The effect of oxygen on sensitization of AgBrI crystals with anionic dye
Аннотация
A mechanism is suggested for the effect of oxygen on spectral sensitization of AgBrI microcrystals (MCs) with anionic dye adsorbed on the surface, which adequately describes the changes of both luminescent and spectral-sensitometric properties of this system. It is shown that the effect of oxygen on the spectral sensitization of AgBrI MC with anionic dye cannot be explained only by desensitization of the AgBrI MC sensitivity as has been suggested in the literature, but rather its effect on the spectral sensitization of AgBrI MC was not so unambiguous. In the framework of the suggested mechanism such ambiguity is due to the fact that the Ag+ and Ag2+ silver centers that participate in the generation recombination processes and are different in structure, interact on the surface of the AgBrI MC with different aggregate forms of the dye, which are in turn differently affected by oxygen. Understanding physicochemical processes occurring in the considered system allows using the obtained results in holographic and spectrozonal photographic materials, gas-phase sensors, and in solar-energy systems.



A theoretical investigation of the effect of water on the structure and fluorescence spectra of L-tryptophan
Аннотация
Fluorescence spectra of two long-wavelength electron transitions S0 ← 1Lb and S0 ← 1La in uncharged and zwitterionic forms of L-tryptophan (Trp) in aqueous solution and in the complex of Trp with water molecule were calculated using the Frank–Condon approximation. Geometric parameters of Trp in electronically excited states were determined, and the vibrational structure of vibronic spectra was analyzed. It was shown that the relative position of structural fragments of alanine (R-Ala) and indole (R-In) could have a determining effect on the fluorescence and formation of the vibrational structure of electronic spectra. The increase of the rotation angle between the R-Ala and R-In, which depends on the Trp environment, results in the Trp fluorescence originating only from the singlet excited state 1La.



Nonlinear and Quantum Optics
Excitation of photon echo by noise pulses
Аннотация
The excitation of photon echo by noise pulses that are formed by modulation of the carrying frequency with Gauss noise is modeled. The modeling is based on optical Bloch equations the solution of which for noise pulse realizations is constructed by their stepwise approximation. In terms of the formalism of state transfer matrices, the two- and three-pulse excitation modes are analyzed. The complex envelopes of the primary and stimulated echo responses are determined. In the linear (low-level-signal) mode, the shape of the two-pulse echo corresponds to that of the time delayed and inverted noise pulse. The boundary of the linear mode, upon exceeding of which distortions of the shape of the noise pulse become noticeable, is determined. The shape of the stimulated (three-pulse) echo in the linear mode corresponds to that of the autocorrelation function of the noise pulse realization. Upon passage beyond the boundary of the linear mode, the shape of the three-pulse echo corresponds either to the cross-correlation function of distorted noise pulses (with different intensities) or to the autocorrelation function of distorted pulses (with the same intensities). The modeled photon echo excitation modes can be used in photon echo processors to process signals in the light range.



Quasi-phase-matching of harmonic waves in plasmas: Calculations, new schemes, and applications
Аннотация
We review recent studies of the coherent phase matched conversion of ultrashort pulses in the modulated plasmas. Particularly, we discuss the influence of ablated and tunneled electrons on the quasiphase- matched high-order harmonic generation in laser-produced plasma and application of mid-infrared pulses for the quasi-phase-matching of harmonics in silver plasma. These studies demonstrate the perspectives of frequency conversion in properly modulated laser-produced plasmas.



Surface waves at the interface between a dielectric and a topological insulator
Аннотация
Surface waves that propagate along the interface between an isotropic linear or nonlinear (of the Kerr type) dielectric and a topological insulator have been studied theoretically. A dispersion relation for surface waves, which are represented by superpositions of TE and TM waves, has been obtained. This hybridization occurs because, upon passage through the interface, the polarization of a surface wave changes, which is caused by an induced surface current (which is transverse to the electric field vector of the wave). The surface current of this kind is characteristic of topological insulators. Expressions for the energy flux transferred by a surface wave have been given.


