Kinetics of electron tunneling transfer in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals with hydrogen bonds

A model of electron transfer by tunneling between trapped electron and hole centers in crystals with hydrogen bonds under the conditions of thermostimulated mobility of one carrier type in the recombination process has been developed. The proposed model describes all features in the kinetics of induced optical density relaxation observed in nonlinear optical crystals of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) on a wide temporal scale (10⁻⁸–10 s) under pulsed irradiation. The results of model calculations have been compared with experimental data on the photoinduced transient optical absorption (TOA) in KDP and ADP crystals in the visible and UV ranges. The nature of the radiation-induced defects, which account for the TOA, and the dependence of the TOA decay kinetics on the temperature, excitation power, and other experimental conditions have been considered.     

Electronic structure,ferroelectricity and optical properties of CaBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

Using first-principles calculations based on density-functional theory in its local-density approximation, we investigated the Electronic structure, ferroelectricity and optical properties of CaBi₂Ta₂O₉ (CBT) for the first time. It is found that CBT compound has an indirect band gap of 3.114 eV and the O 2s and 2p states are strongly hybridized with the 6s states of Bi which belong to the (Bi₂O₂)²⁺ planes. The quite strong Ta–O and Bi–O hybridization is the primary source for ferroelectricity. Our results imply that the interaction between Bi and O is highly covalent. The anisotropy occurs mainly above 4 eV in the optical properties. The different optical properties have been discussed.     

Optical second harmonic generation and its photoinduced dynamics in ferroelectric semiconductor Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript>

By means of optical pump–probe technique, the ultrafast dynamics of nonlinear optical response of the ferroelectric semiconductor Sn₂P₂S₆ crystal excited with a femtosecond laser pulse has been investigated. It has been shown that, under the action of femtosecond pulses, change in optical second harmonic generation occurs in the sample, which can be due to screening of existing electric polarization.     

Photoacoustic spectroscopy of thin films of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, As<Subscript>2</Subscript>Se<Subscript>3</Subscript> and GeSe<Subscript>2</Subscript>

Photoacoustic spectroscopy (PAS) is one of the important branches of spectroscopy, which enables one to detect light-induced heat production following the absorption of pulsed radiation by the sample. As₂S₃, As₂Se₃ and GeSe₂ exhibit a wide variety of photo-induced phenomena that enable them to be used as optical imaging or storage medium and various electronic devices, including electro-optic information storage devices and optical mass memories. Therefore, accurate measurement of thermal properties of semiconducting films is necessary to study the memory density. The thermal conductivity of thin films of As₂S₃ (thickness 100 μm and 80 μm), As₂Se₃ (thickness 100 μm and 80 μm) and GeSe₂ (thickness 120 μm and 100 μm) has been measured using PAS technique. Our result shows that the thermal conductivity of thicker films is larger than the thinner films. This can be explained by the thermal resistance effect between the film and the surface of the substrate.      

Kinetics of the transient optical absorption in Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> and LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> lithium borate crystals

This paper reports on a study of the kinetics of electron tunneling transport between electron and hole centers in Li₂B₄O₇ and LiB₃O₅ lithium borate crystals under the conditions where the mobility of one of the partners in the recombination process is thermally stimulated. A mathematical model has been proposed to describe all specific features in the relaxation kinetics of the induced optical density observed in Li₂B₄O₇ (LTB) and LiB₃O₅ (LBO) nonlinear optical crystals within a broad time interval of 10⁻⁸−1 s after a radiation pulse. The results of calculations have been compared with experimental data on transient optical absorption (TOA) of LTB and LBO crystals in the visible and ultraviolet spectral regions. The nature of the radiation defects responsible for TOA and the dependence of the TOA decay kinetics on temperature, excitation power, and other experimental conditions have been discussed.     

Exciton absorption spectra of Cs<Subscript>2</Subscript>CdI<Subscript>4</Subscript> and Rb<Subscript>2</Subscript>CdI<Subscript>4</Subscript> ferroelastic solid solutions

The exciton absorption spectra of thin films of (Cs_{1 − x} Rb _x )₂CdI₄ solid solutions have been investigated and the refractive index n(λ) in their transparency window in the concentration range of 0 ≤ x ≤ 1 has been measured. The exciton-band parameters and optical permittivity ɛ_∞(x) have been found to linearly depend on the concentration. It is established that excitons are incorporated into the CdI₂ sublattice of the solid solutions and belong to intermediate-coupling ones. The characteristics of excitons in ferroelastics are compared with the corresponding parameters for CdI₂, RbI, and CsI, which are used as components to synthesize ternary compounds.     

Absorption and luminescence spectra of KHo(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> and KEr(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystals

The optical absorption and luminescence properties of potassium-holmium and potassium-erbium double tungstates have been studied. It is established that the potassium-holmium double tungstate belongs to a group of biaxial and pleochromic crystals. The emission and excitation spectra of the KHo(WO₄)₂ and KEr(WO₄)₂ single crystals were measured at room temperature.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 810–814, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Theoretical Study of the Structural,Electronic, Chemical Bonding and Optical Properties of the A2<Subscript>1</Subscript><Emphasis Type="Italic">am</Emphasis> Orthorhombic SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

The structural parameters, density of states, electronic band structure, charge density, and optical properties of orthorhombic SrBi₂Ta₂O₉ have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principle density functional theory (DFT). The calculated structural parameters were in agreement with the previous theoretical and experimental data. The band structure showed an indirect (S to Γ) band gap with 2.071 eV. The chemical bonding along with population analysis has been studied. The complex dielectric function, refractive index, and extinction coefficient were calculated to understand the optical properties of this compound, which showed an optical anisotropy in the components of polarization directions (100), (010), and (001).     

Spectroscopic Properties and Energy Transfer Analysis of Tm<Superscript>3+</Superscript>-Doped BaF<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass

This paper reports on the spectroscopic properties and energy transfer analysis of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂-La₂O₃ glasses with different Tm₂O₃ doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm³⁺, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm₂O₃ doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm₂O₃-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm³⁺ in this sample is about 0.48 × 10⁻²⁰ cm² at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂- La₂O₃ glass for 2.0-μm optical fiber laser.     

Third-order optical nonlinearity of a novel material: [(C<Subscript>4</Subscript>H<Subscript>9</Subscript>)<Subscript>4</Subscript>N][Co(dmit)<Subscript>2</Subscript>]

A novel dmit organometallic complex—[(C₄H₉)₄N][Co(dmit)₂] (dmit^2? = 4,5-dithiolate-1,3-dithiole-2-thione), abbreviated as BuCo, is synthesized and its third-order optical nonlinearity of the acetone solution is characterized using the Z-scan technique with a 40-ps pulse width at 1064 nm. Z-scan curves show that BuCo sample solution possesses the negative nonlinear refraction, exhibiting a self-defocusing effect. In addition, the two-photon absorption has been observed in BuCo. The effective molecular second-order hyperpolarizability γ_eff of the BuCo molecule was estimated to be as large as 5.60 × 10^?31 esu, suggesting BuCo is a potential material for optical device applications.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

An Ellipsometric Investigation of the Optical Properties of Ru<Subscript>2</Subscript>Ge<Subscript>3</Subscript> and Ru<Subscript>2</Subscript>Sn<Subscript>3</Subscript> Compounds

Ellipsometric investigations of the optical properties of Ru₂Ge₃ and Ru₂Sn₃ intermetallic compounds are carried out in the wavelength range from 0.22 to 15 μm. The nature of interband light absorption is analyzed based on a comparative analysis of the experimental and theoretical frequency dependences of an optical conductivity. The obtained results confirm the existence of energy gaps at the Fermi level in the electronic spectra of these materials predicted earlier by the band-structure calculations.     

Phonon spectrum of lead oxychloride Pb<Subscript>3</Subscript>O<Subscript>2</Subscript>Cl<Subscript>2</Subscript>: Ab initio calculation and experiment

IR and Raman spectra of Pb₃O₂Cl₂ in the range of 50–600 cm^–1 have been detected for the first time. Ab initio calculations of the crystal structure and the phonon spectrum of Pb₃O₂Cl₂ in the framework of LCAO approach have been performed by the Hartree–Fock method and in the framework of the density functional theory with the use of hybrid functionals. The results of calculations have made it possible to interpret the experimental vibration spectra and reveal silent modes, which do not manifest themselves in these spectra but influence the optical properties of the crystal.     

Optical transmission coefficient of crystalline [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>MeCl<Subscript>4</Subscript> in the incommensurate phase

This a study of the optical transmission of [N(CH₃)₄]₂ZnCl₄ and [N(CH₃)₄]₂CuCl₄ crystals in the incommensurate phase when a defect density wave is present. It is found that an anomalous reduction in the transmission coefficient is caused by scattering of light owing to a realignment of the superstructure during transitions between metastable states. When a defect density wave is present, the anomalous optical transmission of the crystal is related to the scattering of light on superstructure inhomogeneities produced by a superposition of existing modulation waves.     

The Optical Gain of a Si-Based Lattice-Matched Si<Subscript>0.15</Subscript>Ge<Subscript>0.621</Subscript>Sn<Subscript>0.229</Subscript>/Si<Subscript>0.637</Subscript>Ge<Subscript>0.018</Subscript>Sn<Subscript>0.345</Subscript> MQW Laser

We study the optical-gain characteristics of a Si-based MQW laser, in which the active region has 20 Si_0.15Ge_0.621Sn_0.229 quantum wells separated by 20 Si_0.637Ge_0.018Sn_0.345 barriers. We reach a maximum optical gain of 2300 cm^?1 with an estimated carrier concentration of 5·10¹⁸ cm^?3, which is equivalent to the transparent current density equal to 0.5 kA/cm². Furthermore, we discuss the optical confinement factor and modal gain. The modal gain depends sensitively on the number of the quantum wells (QWs), and this fact restricts the optical confinement factor. The modal gain of the model we proposed can reach 1500 cm^?1 at the injection current density equal to 3 kA/cm². We hope that our results show the possibility to obtain a Si-based near-infrared laser.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.     

Polarization modes in the Ba<Subscript>2</Subscript>Mg<Subscript>2</Subscript>Fe1<Subscript>2</Subscript>O<Subscript>22</Subscript> multiferroic

The spectra of complex permittivity of a Ba₂Mg₂Fe₁₂O₂₂ single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm⁻¹). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of E _u symmetry, which are allowed by the symmetry of the Ba₂Mg₂Fe₁₂O₂₂ crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response exhibits new absorption lines due to magnetic excitations.     

Electron energy structure of defect chalcopyrite CdGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>

The CdGa₂Se₄ compound, with a crystalline lattice of defect chalcopyrite, shows birefringence. However, in contrast to the similar CdGa₂S₄ compound, CdGa₂Se₄ has no experimentally observed isotropic points in the optical spectral range. In this study, the electron energy structure and optical spectra of CdGa₂Se₄ are calculated for three different experimentally obtained sets of crystalline parameters. Only one of these sets shows the absence of isotropic points.     

Optical and magnetooptical properties of CoFeB/SiO<Subscript>2</Subscript> and CoFeZr/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> granular magnetic nanostructures

The optical and magnetooptical properties of the new granular nanocomposites (CoFeB)/(SiO₂) and (CoFeZr)/(Al₂O₃), which are grains of amorphous ferromagnetic alloys embedded in dielectric matrices, have been studied. The dependence of the optical, magnetooptical, and magnetic properties of the nanocomposites on their qualitative and quantitative composition, as well as on the conditions of their preparation, was investigated. Spectra of the dielectric functions ε = ε₁ ? iε₂ were obtained by the ellipsometric method in the range 0.6–5.4 eV. Above 4.2 eV, the absorption coefficient of the (CoFeB)/(SiO₂) composites was found to be close to zero for all magnetic-grain concentrations. The polar Kerr effect measured at a photon energy of 1.96 eV in dc magnetic fields of up to 15 kOe reaches values as high as 0.25°–0.3° for these nanocomposites and depends only weakly on the conditions of preparation. On the other hand, the (CoFeZr)/(Al₂O₃) nanostructures reveal a considerable difference in the concentration dependences of the Kerr effect between samples prepared in a dc magnetic field and in zero field.     

Magnetism and optical properties of nanocrystalline Cu<Subscript>2</Subscript>O and TiO<Subscript>2</Subscript> powders

The optical spectra of Cu₂O and TiO₂ nanopowders have been studied, which contain information about structural defects and are of interest in the search for optimum regimes providing the synthesis of ferromagnetic nanocrystals with Curie temperatures above room temperature.