Far-infrared absorption due to impurities in CsBr crystals

The far-infrared absorption in the phonon band of CsBr due to Na⁺, K⁺, Rb⁺, In⁺, and Tl⁺ impurities has been measured and compared with the calculated density of phonon states of the host lattice.     

Specific features of waves in magnetic crystals

On the basis of the asymmetric velocity tensor introduced we develop an orthogonal representation of wave propagation in anisotropic magnetic dielectrics. Simple expressions are obtained for the direct dependence of polarization vectors, refractive indices, and phase and group velocities of isonormal waves in a magnetic dielectric on the vectors of principal velocities. We show that the presence of magnetic properties is manifested in the optical range in the fact that the velocities of wave propagation in uniaxial crystals are determined by three main refractive indices, and therefore, both waves except for specific directions become extraordinary, whereas in biaxial crystals (also because of a large number of parameters) the cones of internal and external refraction become elliptic. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 597–600, September–October, 1999     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

Influence of the interband transitions on the absorption spectrum of CsBr crystals containing colloids

The contribution of interband transitions to the optical absorption of Cs colloids in a CsBr crystal has been calculated by introducing the expression of Wilson and Butcher for the imaginary part of the dielectric constant of cesium into the standard Mie theory. The resulting absorption bands are smaller and much broader than those corresponding to the free-electron model. The dependence of the calculated band half-width on colloid radius is compared with experimental data. The agreement obtained is considerably better than that corresponding to the sole Drude contribution     

Specific features of the temperature quenching of luminescence of self-trapped excitons in alkali halide crystals under low-temperature deformation

The activation energy of temperature quenching of luminescence of self-trapped excitons in alkali halide crystals subjected to low-temperature uniaxial deformation is evaluated experimentally. It is found that an increase in the activation energy is observed in the following series of crystals: KBr → NaCl → KI → NaBr → CsBr → RbI. The effect of enhancement of intrinsic luminescence of alkali halide crystals due to the lowering of the symmetry of the crystal lattice under low-temperature uniaxial deformation is interpreted by analyzing the observed increase in the activation energy that characterizes the height of the potential barrier separating channels of radiative and nonradiative decay (with the formation of radiation defects) of self-trapped excitons.     

Specific features in the kinetics of thermally stimulated luminescence of α-Al2O3 crystals containing defects

A study of specific features in the kinetics of thermoluminescence (TL) of defective α-Al₂O₃ crystals is reported. The TL properties are experimentally shown to be related to the presence of oxygen vacancies of thermochemical or radiation origin. It is established that the differences in the TL kinetics in α-Al₂O₃ samples grown under different conditions can be accounted for by the hypothesis that the energy spectra of the trapping levels have different widths. As is demonstrated with a titanium impurity, this broadening of the energy spectrum can originate from the presence of traces of native impurities in the α-Al₂O₃ lattice. Fiz. Tverd. Tela (St. Petersburg) 39, 1538–1543 (September 1997)     

Specific features of luminescence properties of nanostructured aluminum oxide

Aluminum oxide nanopowders are prepared by the gas phase method and characterized according to the particle sizes and the phase composition. Samples of the nanostructured ceramic material are produced by pressing and annealing in air. The photoluminescence and cathodoluminescence spectra of the Al₂O₃ nanostructured ceramic material and α-Al₂O₃ anion-defect single crystals are investigated under comparable conditions. The luminescence bands of centers formed by oxygen vacancies are revealed in the spectra of two types of samples. The nanostructured ceramic material is characterized by the appearance of a new luminescence band at 3.4 eV and a decrease in the luminescence decay time. The inference is made that the characteristic features of the luminescence of the nanostructured ceramic material can be associated with the presence of non-equilibrium phases and the specific features of relaxation processes.     

On temperature dependence of impurity absorption and luminescence bands in dielectric crystals

In terms of the adiabatic approach to determining the states of an impurity dielectric crystal, the temperature dependence of the first moment (maximum) of the optical band that arises upon vibronic transitions in a local center is studied. It is shown that the frequency of the band maximum and absolute temperature are linked with each other by a relation that is analogous to the Wien’s displacement law for equilibrium radiation. This result is consistent with experimental data obtained, in particular for F absorption and luminescence bands in alkali halide crystals.     

Specific features of optical diffusion in nematic liquid crystals

Methods of computer simulation are used to study multiple light scattering in the ordered phase of a nematic liquid crystal. The development of the diffusion regime is studied in detail. It is demonstrated that the transient time depends on the external magnetic field and the direction of the incident radiation. The diffusion coefficients along and across the director are calculated. The validity of the diffusion approximation is controlled using the moments of the distribution function. Analysis of the diffusion coefficients versus the external field shows that the diffusion coefficients remain almost unchanged in the experimental range of the magnetic fields. In the presence of relatively strong fields, the diffusion coefficients increase due to a variation in the shape of the scattering indicatrix and the free-path length of photons. The dependence of the diffusion coefficients on the radiation wavelength is also calculated. Comparison of the simulated results and the experimental data shows that the calculated diffusion rate across the director virtually coincides with the experimental rate. The calculated diffusion coefficient along the director is substantially greater than the corresponding experimental rate.     

Specific features of luminescence of oxygen-deficient centres in nanostructured silicon dioxide

Specific features of radiative and non-radiative relaxations of oxygen-deficient centres (ODC's) in nanostructured silicon dioxide were studied using optically stimulated electron emission and time-resolved photoluminescence. It was found that modifications of oxygen-deficient centres in the form of surface analogues can exist in nanostructured samples prepared by thermal decomposition of polysilazane in air. Photoluminescence of these centres was efficiently excited in the optical absorption bands of surface $E_{\mathrm{s}}^{\prime }$-centres and silicon clusters $\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/tbnd"></mglyph>}\mathrm{SiSiSi}\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/tbnd"></mglyph>}$ and could be associated with the intercentre energy transfer during their nonradiative relaxation. Specific features of thermally induced changes in the luminescence characteristics of the defects due to transformation of the structure of silica samples from amorphous to partially crystalline modification were established from analysis of the spectral composition and the decay kinetics of the photoluminescence.     

Peculiarities of absorption and luminescence spectra of nitrogen-vacancy color centers in diamond crystals

We have studied photoluminescence spectra of nitrogen-vacancy (NV) centers upon their pumping by short-wavelength visible and near-UV radiation. We have shown that NV⁰ centers, as distinct from NV^? centers have an absorption line in the UV range. Inversion of the zero-phonon line of NV^? centers has been shown and interpreted.     

Specific features of emission of chiral photonic crystals with an anisotropic defect: II. Specific features of emission

Specific features of emission of chiral photonic crystals with an anisotropic defect are studied. Under certain conditions, an anomalously strong emission of defect modes is observed. A low-threshold laser generation as well as the effect of suppression of the emission of these modes is possible. Threshold values of the gain are determined numerically.     

Additional light waves in crystals near the region of exciton absorption and luminescence

The theory of additional light waves formulated in 1957 is compared to the latest experiments. Traditional birefringence theory fails to account for experimental results even qualitatively, while the additional waves theory explains them quantitatively. New boundary conditions are presented which are required for calculation of the reflection coefficient.     

Specific features of phase transitions in molecular crystals of diols

A comparative analysis of the phase transitions in molecular crystals of diols with different chain lengths [1,15-pentadecanediol (CH₂)₁₄(OH)₂ and 1,16-hexadecanediol (CH₂)₁₅(OH)₂] has been performed using differential scanning calorimetry. A quantitative analysis of the temperature dependence of the heat capacity within the theory of diffuse (Λ-shaped) first-order phase transitions and the use of the temperature hysteresis have revealed a number of new effects associated with the specific features of the phase transitions in molecular crystals of diols.     

Luminescence of aggregate centers in CsBr:Eu2+ single crystals

We have used the Bridgman method to grow CsBr:Eu²⁺ single crystals, adding an activator to the mix in the form of Eu₂O₃ in amounts of 0.0125, 0.0250, and 0.0500 mole %. At T = 300 K, we studied the absorption spectra, the photoluminescence (PL) spectra, and the photostimulated luminescence (PSL) spectra of the grown crystals. We have established that the structure of the photoluminescence and photostimulated luminescence centers in crystals grown from the CsBr:Eu₂O₃ mix includes isolated dipole centers Eu²⁺-V_Cs, emitting in bands with maxima at 432 nm and 455 nm respectively, and in crystals grown at activator concentrations of 0.025 and 0.050 mole % they also include aggregate centers (AC) based on CsEuBr₃ nanocrystals with emission bands at 515 m and 523 nm. We have shown that the maximum concentration of aggregate centers of the CsEuBr₃ nanocrystal type in CsBr:Eu²⁺ crystals is achieved for an activator content in the mix within the range 0.01–0.05 mole %. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 359–362, May–June, 2006.     

Optical absorption spectra of H center in CsBr

H center in CsBr is studied and found to have two σ-polarized optical absorption bands at 437 and 490 nm and one π-polarized band at 1062 nm. Oscillator strengths of these three bands are found to be in the ratio of 1:0.2:0.003.     

Specific features in reflectance and absorbance spectra of one-dimensional resonant photonic crystals

The optical spectra of resonant Bragg and quasi-Bragg quantum-well structures are studied theoretically. The existence of special frequencies in the reflectance and absorbance spectra at which the reflectance or absorbance does not depend on the number of quantum wells in a structure is explained analytically. It is shown that the reflectance and absorbance spectra of structures in which the quantum-well width substantially exceeds the exciton Bohr radius also contain special frequencies and that the smooth spectral component is determined primarily by the interaction of the light wave with the quantum-well ground-state exciton.