Superhyperfine Structure of the EPR Spectra of Nd<Superscript>3+</Superscript> Impurity Ions in Fluorite CaF<Subscript>2</Subscript>

EPR spectra of a CaF₂ single crystal that was grown from melt containing a small addition of NdF₃ were studied. Signals corresponding to tetragonal centers of Nd³⁺ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd³⁺ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd³⁺ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd³⁺ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.     

Nonlinear magnetic resonance in the (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript> crystal

The nonlinear microwave absorption in the (CH₃NH₃)₂CuBr₄ antiferromagnetic crystal is investigated experimentally. The temperature and angular dependences of the parameters of nonlinear resonance and the dependences of these parameters on the microwave pump power are analyzed. It is found that the nonlinear properties deteriorate with decreasing temperature and the linear and nonlinear contributions are competitive in character.     

Polarization-independent low-pass spatial filters based on one-dimensional photonic crystals containing negative-index materials

A new application of one-dimensional photonic crystals containing negative-index materials is proposed as low-pass spatial filters. Through optimizing the parameters of defect layer, a series of polarization-independent defect modes in the zero-average-index gap of the photonic crystals are obtained with the increase of the incident angle. Based on these defect modes, polarization-independent low-pass spatial filters are designed. The spatial-frequency bandwidth of the spatial filters can be adjusted by changing the period number of the defective photonic crystal structures. In addition, the effect of the losses of negative-index materials on the spatial filters is considered.     

Spectroscopic characteristics of Pr<Superscript>3+</Superscript> in biaxial La<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> crystal

A Pr³⁺-doped La₂(WO₄)₃ crystal grown by the Czochralski method has been investigated as a promising laser material. The principal axes of the optical indicatrix and Pr³⁺ concentration of the crystal were determined. The polarized absorption, fluorescence spectra and fluorescence decay curves of the main emission multiplets of the crystal were measured at room temperature. The spectroscopic parameters were obtained by the modified Judd–Ofelt theory combined with the normalized method. The peak stimulated emission cross-sections of the major emission lines were estimated. The good spectroscopic properties imply that the Pr³⁺:La₂(WO₄)₃ crystal is a potential laser gain medium for solid-state laser and self-stimulated Raman laser applications. PACS 78.20.-e; 42.70.Hj     

Vibration spectra and the valence force field of the LiCuVO<Subscript>4</Subscript> crystal

Polarized Raman spectra of single-crystal LiCuVO₄ containing quasi-one-dimensional cuprate chains are measured. The force constants of the valence force field are calculated for the phonon modes at the center of the Brillouin zone with an accuracy sufficient to describe the experiment. From the results, it follows that the crystal contains rigid Cu-O-V-Cu-O-V structural rings with a charge distributed along their bonds.     

Li-impurity effect in optical spectra of KTaO<Subscript>3</Subscript>:Er<Superscript>3+</Superscript> crystals

The optical properties of one-dimensional photonic crystals based on porous anodic aluminum oxide films have been studied by measuring transmittance and specular reflectance spectra in the visible and UV spectral regions. Angular dependences of the spectral positions of optical stop bands are obtained. It is shown that the reflectance within the first stop band varies from point to point on the sample surface, reaching a level of 98–99% at some points. The dispersion relation for electromagnetic waves in the model of infinite periodic structure is calculated for the samples under study. The possibility of using models with an infinite or finite number of layers to calculate reflectance spectra near the first optical stop band is discussed.     

Multiphonon relaxation of optical excitations in CsCdBr<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Nd<Superscript>3+</Superscript> crystals

The technique of calculation of the n-phonon transition rates between electronic sublevels of impurity rare earth ions in dielectric crystals is developed in the case when n>2. The n-phonon transition probabilities are calculated according to the 1st and 2nd orders of perturbation theory. The Hamiltonian of the electron-phonon interaction is constructed in the framework of the exchange charge model and developed as series in relative displacements of the rare earth ion and ligands. The contribution of the lattice anharmonicity on the probabilities of n-phonon transitions is taken into account. On the basis of the developed technique, the nonradiative relaxation rates of ⁴ G _7/2 multiplet of Nd³⁺ ions in LiYF₄:Nd³⁺crystal and ³P₁ multiplet of Pr³⁺ ions in CsCdBr₃:Pr³⁺ crystal were computed. The results of our calculations show that the 2nd order terms in the expressions for the probabilities studied here are comparable with, and in some cases prevail over the 1st order terms. An account of lattice anharmonicity in case of LiYF₄:Nd³⁺ crystal substantially modifies the corresponding multiphonon relaxation rates. The calculated nonradiative relaxation rates for both crystals agree well with the experimental data.     

Spectral properties of Nd<Superscript>3+</Superscript> ions in samples of transparent Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramics

The spectral and luminescent characteristics of samples of Y₂O₃:Nd³⁺ ceramics obtained from different precursors under different preparation conditions (the concentration of an HfO₂ compacting additive, the temperature and time of synthesis) are studied at 300 and 77 K. It is shown that the spectral positions of absorption and luminescence lines of ceramics correspond to those of a Y₂O₃:Nd³⁺ single crystal. At the same time, the absorption and luminescence spectra show an inhomogeneous broadening, characteristic of disordered crystals and glass. The energies of the ⁴ I _9/2 and ⁴ F _3/2 Stark states of the Nd³⁺ ion are calculated. The calculation results nearly coincide with the data from the literature for the Y₂O₃:Nd³⁺ single crystal and transparent ceramics. Samples containing the compacting additive show additional lines, whose intensities correlate with its concentration and the method of preparation of Y₂O₃:Nd³⁺ ultradispersed powders. It is assumed that these lines are related to the fact that either Nd³⁺ ions enter the composition of the HfO₂ compacting additive or Hf⁴⁺ ions are present in the nearest environment of Nd³⁺ ions at the boundaries of granules enriched with HfO₂.     

光脉冲在一维负折射材料光子晶体中的传播

讨论了一维负折射光子晶体对光脉冲传播的影响.通过对光脉冲的透射场强的数值计算发现,频谱处于一维负折射率材料光子晶体禁带中的短脉冲通过光子晶体后基本保持形状不变,而对同样的短脉冲通过传统的一维光子晶体的透射脉冲则出现严重畸变.当考虑负折射率材料的色散时,讨论了位相时间随光脉冲载波频率的变化情况.     

Geometry of the internal dynamics of C<Subscript>2</Subscript>H<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> carbocation

Using the symmetry group chain methods, the internal dynamics of the simplest carbocation, C₂H ₃ ⁺ , is analyzed under the traditional assumptions that the equilibrium structures of the carbocation are planar and that the nonrigid motion between them is in-plane. This geometry of the internal dynamics is shown to agree with the data of the microwave spectroscopy on the splittings of rotational energy levels caused by the nonrigid motion. Previously, this statement was based on the model that violated the requirement of self-adjointness of operators of physical quantities.     

Influence of configuration interaction on splitting of multiplets of TmF<Stack><Subscript>6</Subscript><Superscript>3−</Superscript></Stack> and TmCl<Stack><Subscript>6</Subscript><Superscript>3−</Superscript></Stack> molecular complexes

The Stark structure of elpasolites activated by Tm³⁺ ions is analyzed with the help of standard and modified crystal field theories. Application of the modified approach reduces considerably the root-mean-square deviation between theoretical and experimental energy values. Crystal field and covalence parameters are determined, the latter being in good agreement with the covalence parameters calculated using microscopic models.     

Fabrication of polaritonic structures in LiNbO<Subscript>3</Subscript> and LiTaO<Subscript>3</Subscript> using femtosecond laser machining

Fabrication of patterned materials in ferroelectric LiNbO₃ and LiTaO₃ crystals using femtosecond laser micromachining is presented and discussed. Damage feature sizes in the 10–100 μm range were achieved using 800-nm, 50-fs (FWHM) ultra-fast laser pulses with energies ranging from 10 μJ up to 350 μJ. Fabrication of polaritonic devices such as waveguides, resonators, focusing reflectors, diffractive and dispersive elements, photonic band gap materials, and other microstructures is demonstrated. PACS 77.84.Dy; 42.62.Cf; 71.36.+c     

IR photoluminescence of Bi<Superscript>+</Superscript> impurity centers in the RbY<Subscript>2</Subscript>Cl<Subscript>7</Subscript> ternary chloride

Single-crystal samples of RbY₂Cl₇ ternary chloride containing an Bi⁺ isomorphic impurity are prepared for the first time. The optical absorption spectra in the visible region and photoluminescence spectra in the near-IR associated with the presence of Bi⁺ impurity center are studied. The kinetics of the photoluminescence decay is investigated. The influence of the symmetry of the crystal field on the arrangement of the energy levels and on the characteristics of the absorption and photoluminescence spectra of Bi⁺ is demonstrated.     

Diode-pumped continuous-wave Nd<Superscript>3+</Superscript>:YAlO<Subscript>3</Subscript> intracavity-doubled red laser

It is reported that efficient continuous-wave (CW) red laser generation at 670 nm in a LBO crystal at type-I phase matching direction performed with a diode-pumped Nd³⁺:YAlO₃ (Nd:YAP) laser. With incident pump power of 15.6 W, output power of 273 mW at 670 nm has been obtained using a 10 mm-long LBO crystal. At the output power level of 273 mW, the output stability is better than 3.7%.     

Tunable emission and efficient energy-transfer properties of Ce<Superscript>3+</Superscript> and Mn<Superscript>2+</Superscript> co-doped Ba<Subscript>3</Subscript>Gd(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> phosphors

MgO powders subjected to Q-switched laser pulses in water were characterized by X-ray/electron diffraction and optical spectroscopy to have a significant optical property change with accompanied transformation into the phase assemblages of periclase, brucite and liquid-crystalline lamella. The periclase nanoparticles tended to have {111} habit planes parallel to the basal layers of lamella and brucite flakes more or less rolled as fibers, ribbons or tubes. A significant internal compressive stress was built up for periclase and brucite but not the more flexible lamellar phase. The colloidal suspension containing the densified periclase nanoparticles within the rolled brucite/lamellae flakes showed UV–visible absorption corresponding to a minimum band gap of ca. 5 eV.     

Electron paramagnetic resonance of Mn<Superscript>2+</Superscript> ions in single crystals of yttrium aluminum borate YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Single crystals of yttrium aluminum borate YAl₃(BO₃)₄ doped with manganese ions are studied using electron paramagnetic resonance spectroscopy. It is shown that manganese ions introduced at low concentrations into the sample predominantly occupy yttrium ion sites in the crystal structure. The shape of the electron paramagnetic resonance spectrum unambiguously indicates that the valence of manganese ions in this case is equal to 2+. The parameters of the spin Hamiltonian of Mn²⁺ ions in the YAl₃(BO₃)₄ matrix are determined at room temperature. The magnitude and sign of the fine structure parameter D allow the conclusion that the YAl₃(BO₃)₄ single crystals doped with manganese ions have a strong crystal field at the yttrium ion sites and easy-axis anisotropy.     

High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

Influence of magnetic ordering on electronic structure of Tb<Superscript>3+</Superscript> ion in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal

Optical absorption spectra of trigonal crystal TbFe₃(BO₃)₄ have been studied in the region of ⁷F₆ → ⁵D₄ transition in Tb³⁺ ion depending on temperature (2–220 K) and on magnetic field (0–60 kOe). Splitting of the Tb³⁺ excited states, both under the influence of the external magnetic field and effective exchange field of the Fe-sublattice, have been determined. Landé factors of the excited states have been found. Stepwise splitting of one of the absorption lines has been discovered in the region of the Fe-sublattice magnetic ordering temperature. This is shown to be due to the abrupt change of equilibrium geometry of the local Tb³⁺ ion environment only in the excited state of the Tb³⁺ ion. In general, the magnetic ordering is accompanied by temperature variations of the Tb³⁺ local environment in the excited states. The crystal field splitting components have been identified. In particular, it has been shown that the ground state (in D ₃ symmetry approximation) consists of two close singlet states of A ₁ and A ₂ type, which are split and magnetized by effective exchange field of the Fe-sublattice. Orientations of magnetic moments of the excited electronic states relative to that of the ground state have been experimentally determined in the magnetically ordered state of the crystal. A pronounced shift of one of absorption lines has been observed in the vicinity of the TbFe₃(BO₃)₄ structural phase transition. The temperature interval of coexistence of the phases is about 3 K.     

Angular dependence of the relaxation times of <Superscript>139</Superscript>La nuclei in La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript>

Relaxation of the magnetization of ¹³⁹La nuclei is considered in lanthanum manganites, which are materials with anisotropic interactions of localized electronic spins, namely, the Dzyaloshinski?-Moriya interactionand the interaction with a crystal field. Expressions are derived for the relaxation times of the longitudinal and transverse components of the nuclear magnetization, and the angular dependences of these relaxation times are found for the La_0.95Sr_0.05MnO₃ compound. In contrast to electronic relaxation, the anisotropy of nuclear relaxation contains a contribution from the shift in the electron Zeeman frequency. The theoretically calculated numerical values of the nuclear relaxation times and their ratios correspond to the range of experimental values in the compounds studied. The results can be of importance for designing devices based on these materials and for further investigation.     

LiNbO<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript> crystals for optical temperature sensors

Possibilities of using lithium niobate crystals, doped with ytterbium (LN:Yb³⁺) as materials for optical temperature sensor (OTS), are discussed. We consider both the radiative and absorptive characteristics of the crystals, assuming that their temperature dependences are caused by the Boltzmann factor of the initial population of Stark sublevels of the ion. It is shown that the crystals can be used as materials for OTS in the temperature range 150–400 K.