Optical polarized properties related to the oxygen vacancy in the CaMoO4 crystal

The electronic structures, dielectric functions and absorption spectra for the CaMoO₄ (CMO) crystal with and without oxygen vacancy V_O²⁺ have been calculated using the CASTEP code with the lattice structure optimized. The calculated results indicate that the optical properties of the CMO crystal show anisotropy and its optical symmetry coincides with the lattice structure geometry of the CMO crystal. The calculated absorption spectra indicate that the perfect CMO crystal does not display absorption band in the visible and near-ultraviolet range. However, in this range, the absorption spectra of the CMO crystal containing V_O²⁺ exhibit one peak at about 1.84 eV (673 nm). It predicates that the 680 nm absorption band is related to the existence of V_O²⁺ in the CMO crystal.     

Correlation effects on optical properties of TbOFeAs

Based on local density approximation and Hubbard-U corrections (LDA+U), we study the influence of Coulomb interaction for Tb 4f states on the optical properties of the recently discovered superconductor, such as TbOFeAs. Within the incorporation of onsite Hubbard effect in TbOFeAs, we discuss the electronic structure, as well as the optical spectra and we compared them to LDA calculations. For non-magnetic (NM) configuration, the electronic structure exhibits high density of states, N(E _F ) in the proximity of Fermi level. With and without the electronic correlation effects, we carried out the calculations for the optical properties such as the optical conductivity, joint density of states (JDOS), optical absorption, the electron energy loss function and reflectivity of TbOFeAs in a large photon energy scale of 30 eV. Despite the absence of a Mott insulator transition, we infer that the electronic correlation effects are prominent in the recently discovered superconductor, like TbOFeAs. We also predict the in-plane anisotropy of plasma frequency that has been evaluated recently in the other ReOFeAs systems.     

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Optical and acoustic properties of TeO2/WO3 glasses with small amount of additive ZrO2

The optical and acoustic properties of tellurite glasses in the system TeO₂/ZrO₂/WO₃ have been investigated. The refractive index at different wavelengths and the optical spectra of the glasses have been measured. From the refractive index and absorption edge studies for prepared glasses, the optical parameter viz; optical band gap (E_opt), Urbach energy, (ΔE), dispersion energy, E_d, and the average oscillator energy, E₀, have been calculated. Sound velocities were measured by pulse echo technique. From these velocities and densities values, various elastic moduli were calculated. The variations in the refractive index, optical energy gap and elastic moduli with WO₃ content have been discussed in terms of the glass structure. Quantitatively, we used the bond compression model for analyzing the room temperature elastic moduli data. By calculating the number of bonds per unit volume, the average stretching force constant, and the average ring size we can extract valuable information about the structure of the present glasses.     

Polarization-modulated infrared reflection difference microspectroscopy: Experiment and model

Polarization-modulated infrared reflection difference microspectroscopy is a sensitive tool to spatially and spectrally resolve optical anisotropy of micrometric crystalline domains. However, intrinsic artifacts are associated with polarization modulation experiments, in particular due to the wavenumber-dependent efficiency of the photoelastic modulator. Here an account addressing this problem in our experimental approach is given. Comparing data from measurements on Ca_1.4Sr_0.6RuO₄ and Bi_0.3Ca_0.7MnO_3+δ with model calculations, it is shown that a simple model, which is based on reports in the literature, can be used to describe the experimental results.     

Polarization selection rules for the allowed optical transitions in europium-terbium double activated calcium tungstate phosphor

The paper is devoted to the problem of the optical anisotropy of the rare-earth ions occupying low-symmetry positions in crystals. The crystal field multiplets arising from LSJ terms of Eu³⁺ and Tb³⁺ ions in the crystal field of calcium tungstate scheelite (CaWO₄) are analyzed (S₄ point symmetry). The selection rules, in particular, polarization rules for the allowed electric dipole optical transitions in the electronic shells of the Eu³⁺ and Tb³⁺ in CaWO₄ host lattice are discussed. Special attention is paid to the study of the angular (polarization) dependence of the two-photon absorption that seems to be an effective tool for the understanding of the complicated optical pattern. The peculiarities of the anisotropy of the two-photon absorption prove to be specific for each allowed dipole transition in S₄ symmetry center.     

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.     

Optical properties of anatase and rutile titanium dioxide: Ab initio calculations for pure and anion-doped material

The optical properties of rutile and anatase titanium dioxide (TiO₂) are calculated from the imaginary part of the dielectric function using pseudopotential density functional method within its generalized gradient approximation (GGA) and a scissors approximation. The fundamental absorption edges calculated for the unit cell of both rutile and anatase are consistent with experimentally reported results of single crystal rutile and anatase TiO₂ and with previous theoretical calculations. A significant optical anisotropy is observed in the anatase structure which holds promise for investigating the band gap modification with better visible-light response and provides a reliable foundation for addressing the effect of impurities on the fundamental absorption edge/band gap of anatase TiO₂. Further calculations on the electronic structure and the optical properties of C-, N-, and S-doped anatase TiO₂ are performed. The results are analyzed and discussed in terms of optical anisotropy and scissors approximations.     

Optical anisotropy of InAs/GaSb broken-gap quantum wells

We investigate in detail the optical anisotropy of absorption of linearly polarized light in InAs/GaSb quantum wells grown on GaSb along the [001] direction, which can be used as an active region of different laser structures. The energy level positions, the wave functions, the optical matrix elements, and the absorption coefficients are calculated using the eight-band k · p model and the Burt-Foreman envelope function theory. In these calculations, the Schr?dinger and Poisson equations are solved self-consistently taking the lattice-mismatched strain into account. We find that a realistic Hamiltonian, which has the C _2v symmetry, results in considerable anisotropy of optical matrix elements for different directions of light polarization and different directions of the initial-state in-plane wave vector, including low-symmetry directions. We trace how the optical matrix elements and absorption are modified when spin-orbit interaction and important symmetry breaking mechanisms are taken into account (structural inversion asymmetry, bulk inversion asymmetry, and interface Hamiltonian). These mechanisms result in an almost 100% anisotropy of the absorption coefficients as the light polarization vector rotates in the plane of the structure and in a plane normal to the interfaces.     

Determination of the scattering coefficient of biological tissue considering the wavelength and absorption dependence of the anisotropy factor

The anisotropy factor g, one of the optical properties of biological tissues, has a strong influence on the calculation of the scattering coefficient μ _s in inverse Monte Carlo (iMC) simulations. It has been reported that g has the wavelength and absorption dependence; however, few attempts have been made to calculate μ _s using g values by taking the wavelength and absorption dependence into account. In this study, the angular distributions of scattered light for biological tissue phantoms containing hemoglobin as a light absorber were measured by a goniometric optical setup at strongly (405 nm) and weakly (664 nm) absorbing wavelengths to obtain g. Subsequently, the optical properties were calculated with the measured values of g by integrating sphere measurements and an iMC simulation, and compared with the results obtained with a conventional g value of 0.9. The μ _s values with measured g were overestimated at the strongly absorbing wavelength, but underestimated at the weakly absorbing wavelength if 0.9 was used in the iMC simulation.     

Polaritons in anisotropic semiconductors

We show how to compute the optical properties (reflection and absorption) of anisotropic semiconductors in the exciton energy region, taking into account polariton and electron-hole coherence effects. The method is applied to a GaAs/Ga_1–x Al _x As superlattice, and the modifications in the optical properties with respect to GaAs are related to the anisotropy.     

外应力场下双层铁磁薄膜中的铁磁共振性质

用铁磁共振方法得到了双层铁磁薄膜的色散关系解析表达式.发现共振场依赖于层间耦合强度和应力场.假定层间为反铁磁性耦合，且铁磁层Ａ有较强的平面内各向异性.随着外磁场的增强，铁磁层Ｂ中的磁化强度突然由最初的反平行转为平行，从而导致色散曲线的阶跃，并且发现光学模阶跃幅度比声学模大.随着应力场的增强，Ｂ层中磁化强度反转所需的外磁场减弱.此外，在不同的交换耦合强度和应力场下，光学模共振场对外磁场方向的依赖性较强. 关键词： 双层铁磁薄膜 界面相互作用 应力各向异性场 铁磁共振     

Electronic structure calculations of transparent conductor Cd3TeO6: Optical properties and the effect of In-substitution

The electronic structure of Cd₃TeO₆ has been studied in the terms of first-principles calculations based on the density functional theory in order to investigate their optical properties and In-substitution effects. It was found that the highly dispersed bottom of the conduction band formed from Cd-s orbitals is the origin of the high transparency and conductivity. Cd₃TeO₆ exhibited optical anisotropy in its main crystal axes, and the c-axis showed the most suitable crystal growth direction for obtaining a wide transparent region. A pronounced shift of the absorption edge was effectively observed by the In-substitution, reflecting the domination of the In-5s level in the conduction band near the Fermi level.     

The effect of scattering-medium parameters on signal magnitude under acousto-optic tomography

We have experimentally studied the influence of scattering anisotropy parameter g of a medium on the magnitude of signal S (visualization parameter) at an ultrasonic frequency that is registered upon acoustooptic tomography. Aqueous solutions of mixtures of cream and skimmed milk with different ratios between them were used as scattering media. The optical properties of media (absorption coefficient μ_a and reduced scattering coefficient μ′ _S ) have been measured on a spectrophotometer (Perkin-Elmer Lambda 950 UV-VIS-NIR) using the inverse adding-doubling technique. As a result of the investigation, we have found that there is a certain correlation between the value of the scattering anisotropy parameter g of aqueous solutions of investigated mixtures and the percentage of the mixture in the aqueous solution, which ensures the required small value of extinction coefficient μ of the scattering medium. An increase in signal S has been revealed with increasing anisotropy parameter g of the medium at a invariable value of extinction coefficient μ. We have concluded that, to solve an inverse problem on the acousto-optic tomography, it is necessary to take into account possible changes in the g factor in scattering media, including biological ones.     

Crystal field and exchange interactions in the SmFe3(BO3)4 multiferroic

The optical spectra of oriented SmFe₃(BO₃)₄ single crystals are studied in the region of the f-f transitions in the Sm³⁺ ion by Fourier spectroscopy. The energies, the symmetry properties, and the exchange splittings of the Stark sublevels of the ground and 17 excited multiplets of the Sm³⁺ ion in a crystal field of symmetry D ₃ are determined from the measured temperature dependences of polarized-radiation absorption spectra. The parameters of the crystal field acting on samarium ions and the parameters of the exchange interaction between Sm³⁺ and Fe³⁺ ions are found. The anisotropy of the effective exchange interaction is shown to be substantially stronger than the magnetic anisotropy, due to a strong crystal-field-induced mixing of the ground and excited multiplets.     

Spatial and temporal thermal analysis of acousto-optic deflectors using finite element analysis model

Thermal effects greatly influence the optical properties of the acousto-optic deflectors (AODs). Thermal analysis plays an important role in modern AOD design. However, the lack of an effective method of analysis limits the prediction in the thermal performance. In this paper, we propose a finite element analysis model to analyze the thermal effects of a TeO₂-based AOD. Both transducer heating and acoustic absorption are considered as thermal sources. The anisotropy of sound propagation is taken into account for determining the acoustic absorption. Based on this model, a transient thermal analysis is employed using ANSYS software. The spatial temperature distributions in the crystal and the temperature changes over time are acquired. The simulation results are validated by experimental results. The effect of heat source and heat convection on temperature distribution is discussed. This numerical model and analytical method of thermal analysis would be helpful in the thermal design and practical applications of AODs.     

Elasticity, electronic structure, chemical bonding and optical properties of monoclinic ZrO2 from first-principles

Structural parameters as well as elastic, electronic, bonding and optical properties of monoclinic ZrO₂ were investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The calculated structural properties and independent elastic constants of monoclinic ZrO₂ are in favorable agreement with previous work. We have derived the bulk and shear moduli, Young’s modulus and Poisson coefficients for monoclinic ZrO₂ and estimated the Debye temperature of monoclinic ZrO₂ from acoustic velocity. Electronic and bonding properties are studied from the calculation of band structure, densities of states and charge densities. Furthermore, in order to clarify the mechanism of optical transitions in monoclinic ZrO₂, the dielectric functions are calculated and analyzed by means of the electronic structure, which shows significant optical anisotropy in the components of polarization directions (1 0 0), (0 1 0) and (0 0 1).     

Properties of melt-grown single crystals of “YB68”

Single crystals of yttrium boride YB_n with n = 61 ± 3 were grown from the melt. Precision density and lattice parameter measurements indicate a congruent melting point at n = 61.7 and a stoichiometric composition at n = 68. Measurements of elastic constants, acoustic attenuation, electrical resistivity and optical absorption are presented. High resolution transmission electron microscopy reveals a complex crystal structure similar to that found by using X-rays. A comparison of the properties of YB_n with those of β-boron show that there are many similarities.     

Plasmons in double quantum wells in a parallel magnetic field

Collective intraband charge-density excitations in the quasi-two-dimensional electron system of double GaAs/AlGaAs quantum wells in an external parallel magnetic field B_∥ are studied by inelastic light scattering. It has been found that the energy of the excitations under study (acoustic and optical plasmons) exhibits anisotropy depending on the mutual orientation of B_∥ and the excitation quasi-momentum k. It is shown theoretically that, in a strong parallel magnetic field, the effects associated with the finite width of the quantum wells dominate over the effects associated with interlayer tunneling and determine the anisotropy of plasmons. The experimental data are compared with a theoretical calculation.     

Distinctive features of Raman-Nath scattering of light by sound in easy-plane antiferromagnets

This paper discusses the acoustic diffraction of light in the presence of an exchange-enhanced photoelastic interaction of aniferromagnetic origin resulting from acoustic modulation of the dielectric permittivity due to oscillations in the antiferromagnetism vector L. In the “easy-plane” type of antiferromagnet these oscillations arise from antiferromagnetoelastic interactions, and can be so large that the photoelastic interaction corresponding to them can be comparable in value to (or even exceed) the interaction in well-known nonmagnetic crystals actually used in acoustooptic devices. The advantage of antiferromagnets lies in the fact that both the diffraction angle and the amplitude of the diffracted light can depend on the magnitude and direction of a magnetic field in these materals. Here the Raman-Nath diffraction regime is discussed, which is probably more favorable from an experimental point of view for the antiferromagnets in question. It is shown that for these materials, the usual mechanism of photoelastic interaction associated with acoustic modulation of the index of refraction is accompanied by an additional mechanism arising from modulation of the polarization of the optical modes. Qualitative estimates are given for FeBO₃. Zh. éksp. Teor. Fiz. 112, 1464–1475 (October 1997)