Far-infrared optical properties of CrCl3 and CrBr3

We measured the far-infrared transmittance and reflectance spectra of the layered compounds CrBr₃ and CrCl₃ at normal and oblique incidences and at room temperature. The results are compared with the spectra from previous Raman measurements and are analyzed on the basis of the unit cell group method applied to the diperiodic crystalline structure of a single layer. A fit of the CrBr₃ reflectivity spectrum with damped Lorentzian oscillators is made which also allows us to evaluate the static dielectric constant ?₀.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

Far infrared optical properties of NbSe3

We have measured the far infrared reflectance of NbSe₃ and used models of the frequency dependent conductivity to fit the data. General arguments show that at 2 K a charge density wave (CDW) energy gap exists between 120 and 190 cm^?1, the relaxation time(s) of the free carriers and CDW pinned mode is >30×10^?12 s, and the ratio of the free carrier concentration to band mass is <2×10²⁰ cm^?3/m₀.     

Structural and optical properties of WO3-ZnO-PbO-B2O3 glasses

Glass samples of compositions 20PbO-80B₂O₃ and xWO₃—(20−x) ZnO-20PbO-60B₂O₃ with x varying from 0% to 10% mole fraction are prepared by the melt quench technique. Decrease in the band gap from 2.86 to 2.16 eV for ZnO-PbO-B₂O₃ glasses with an increase in the WO₃ content has been observed and discussed. The FTIR spectral studies have pointed out the conversion of structural units of BO₃ to BO₄ and WO₄ to WO₆ with the presence of W-O-W vibration of tungsten and incorporation of ZnO₄ structural units of zinc in these glasses. The increase in density from 2.75 to 4.03 gcm⁻³ for ZnO-PbO-B₂O₃ glasses is observed with an increase in WO₃ content. Due to the formation of WO₆, WO₄ and BO₄ units, changes in the atomic structure with WO₃ composition are observed and discussed.     

Electrical and optical properties of single crystal In2Te3 and Ga2Te3

Electrical conductivity and thermopower measurements are reported for the defect semiconductors p-In₂Te₃ and n-Ga₂Te₃. The hole mobility μ_p in the former varied as Tⁿwheren=+5.98 forT<350 K and n=-4.13 forT>350 K showing a maximum of 210 cm²V^-1 sec^-1 at 350 K. Electron mobility in n-Ga₂Te₃ also went through a maximum at 320 K. The optical band-gaps for both were found to be direct, with values of 1.01 and 1.08 eV for In₂Te₃ and Ga₂Te₃ respectively at 300 K. Pronounced effects of annealing on TEP and optical absorption gave evidence of defect ordering at low temperatures followed by defect creation at T>350 K.     

Linear and nonlinear optical properties of Ca3(VO4)2

Single crystals of Ca₃ (VO₄)₂ have been grown by the Czochralski method. The refractive indices show that the crystals are phasematchable between 1.72 μm and 2.94 μm with favourable values for acceptance angle and spectral width of the incident laser beam in the entire phasematching range. The nonlinear optical coefficients have been measured by the Maker fringe method at a fundamental wavelength of 1.064 μm. They are comparable to those of KDP. Thus the crystals are potentially useful for frequency conversion of broadband or tunable laser sources as semiconductor and F-center lasers. They may also be installed in high f-number IR image converters.     

Electronic energy gaps and optical properties of LaMnO3

We investigate electronic structure and optical properties of LaMnO₃ through density-functional-theory calculations with a recent improved exchange potential. Calculated gaps are consistent with recent experimental values, and calculated optical conductivities and dielectric constants as functions of photon energy are in excellent agreement with low-temperature experimental results. These lead to a satisfactory theoretical understanding of the electronic gaps and optical properties of LaMnO₃ without tuning atomic parameters and can help elucidate electronic structures and magnetic properties of other manganite materials.     

Crystallographic and magnetic investigations on cobalt gallium sulfides: α-CoGa2S4 and γ-CoGa2S4

Crystals of α- and γ-phase CoGa₂S₄ were grown by vapour-phase chemical-transport and fluxmethod, respectively. The α-phase crystallizes in the space group I4&#x0304;, with Co and Ga atoms almost ordered on special positions. The α-form was found to be antiferromagnetically ordered below T = 7 ± 0.5 K. The γ-phase crystallizes in the space group F4&#x0304;3m, showing cationic sites randomly occupied by metal atoms and vacancies. Its magnetic properties were completely determined by the occurrence of short-range magnetic order.     

Luminescence properties of SrSi2AlO2N3 doped with divalent rare-earth ions

The optical properties of SrSi₂AlO₂N₃ doped with Eu²⁺ and Yb²⁺ are investigated towards their applicability in LEDs. The Eu²⁺-doped material shows emission in the green, peaking around 500 nm. The emission is ascribed to the 4f⁶5d¹–4f⁷ transition on Eu²⁺. In view of the too low quantum efficiency and the considerable thermal quenching of the emission at the operation temperature of high power LED (>1W/mm²) this phosphor is only suitable for application in low power LEDs. The Yb²⁺ emission shows an anomalously red-shifted emission compared to Eu²⁺, which is characterized by a larger FWHM, a larger Stokes shift and lower thermal quenching temperature. The emission is ascribed to self-trapped exciton emission. The Yb²⁺ activated phosphor is found to be unsuitable for the use in any phosphor-converted LEDs.     

Electronic structure and optical properties of Sb2S3 crystal

The electronic and optical properties of Sb₂S₃ are studied using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in Wien2k. In this approach, the alternative form of the generalized gradient approximation (GGA) proposed by Engel and Vosko (EV-GGA) was used for the exchange correlation potential. The calculated band structure shows a direct band gap. The contribution of different bands was analyzed from total and partial density of states curves. Moreover, the optical properties, including the dielectric function, absorption spectrum, refractive index, extinction coefficient, reflectivity and energy-loss spectrum are all obtained and analyzed in detail.     

Elastic, electronic, and optical properties of hypothetical SnNNi3 and CuNNi3 in comparison with superconducting ZnNNi3

The elastic, electronic, and optical properties of MNNi₃ (M=Zn, Sn, and Cu) have been calculated using the plane-wave ultrasoft pseudopotential technique, which is based on the first-principle density functional theory (DFT) with generalized gradient approximation (GGA). The optimized lattice parameters, independent elastic constants (C₁₁, C₁₂, and C₄₄), bulk modulus B, compressibility K, shear modulus G, and Poisson's ratio υ, as well as the band structures, total and atom projected densities of states and finally the optical properties of MNNi₃ have been evaluated and discussed. The electronic band structures of the two hypothetical compounds show metallic behavior just like the superconducting ZnNNi₃. Using band structures, the origin of features that appear in different optical properties of all the three compounds has been discussed. The large reflectivity of the predicted compounds in the low energy region might be useful in good candidate materials for coating to avoid solar heating.     

Large nonlinear optical properties of Fe2O3 nanoparticles

Nonlinear optical properties of Fe₂O₃ nanoparticles were investigated by the signal-beam Z-scan technique with Ar⁺ and Ne–He lasers. The largest reported effective nonlinear coefficient, n₂=−8.07×10⁻⁷ cm²/W, was obtained. It is demonstrated that the nonlinear optical response originals from quantum confinement effect.     

Far infrared optical properties of proustite (Ag3AsS3)

The infrared reflection and transmission spectra of Ag₃AsS₃ single crystals have been between 20 and 650 cm^-1 at room temperature. Fourteen phonon modes were observable in reflectivity with the dominant restrahlen bands occuring at 360 cm^-1 and 35 cm^-1 for E? c and at 335 cm^-1 and 35 cm^-1 for E⊥ c. These measurements indicate that generation of far infrared radiation by mixing of two laser frequencies using proustite as the down-conversion crystal is not possible.     

Effect of WO3 on structural and optical properties of CeO2-PbO-B2O3 glasses

Pure and WO₃ doped CeO₂-PbO-B₂O₃ glasses are prepared by the melt-quench technique. The structural and optical analyses of glasses are carried out by XRD, FTIR, density and UV-vis spectroscopic measurement techniques. FTIR analysis indicates the transformation of structural units of BO₃ into BO₄ with W-O-W vibration and the presence of WO₄ and WO₆ units observed with increase in WO₃ contents. Decrease in band gap for CeO₂-PbO-B₂O₃ glasses from 2.89 to 2.30 eV and for WO₃ doped glasses from 2.89 to 1.95 eV has been observed and discussed. This decrease in band gap with WO₃ doping approaches to semiconductor behavior. It shows that the presence of WO₃ in the glass samples causes more compaction of the borate network due to the formation of BO₄ groups and the presence of WO₄ and WO₆ groups, which result in a decrease in the optical band gap energy and increase in the density.     

BiTiO3电子结构及光学性质的第一性原理研究

采用基于第一性原理的赝势平面波方法, 对BiTiO₃的多种结构进行了计算. 计算结果表明, C1C1结构最为稳定, 对应晶格参数为a=b=5.606 Å, c=9.954 Å; α=β=105.1°, γ=61.2°. 进一步对C1C1结构的BiTiO₃的能带结构、电子性质和光学性质进行了研究, 发现BiTiO₃是间接带隙半导体, 其费米面附近的能带主要由Ti-3d和O-2p层的电子态构成. 通过介电函数、复折射率和反射率等的研究, 发现BiTiO₃的光学性质为近各向同性.     

Role of WO3 in structural and optical properties of WO3-Al2O3-PbO-B2O3 glasses

Glass samples of composition xAl₂O₃-20PbO-(80−x)B₂O₃ and xWO₃-xAl₂O₃-20PbO-(80−2x)B₂O₃ with x varying from 0% to 10% mole fraction are prepared by melt quench technique. The optical band gap decreases (from 3.21 to 2.37 eV) more for WO₃-Al₂O₃-PbO-B₂O₃ glasses with an addition of WO₃ content. The FTIR spectral studies have pointed out the conversion of structural units of BO₃ to BO₄ and WO₄ to WO₆ in these glasses. The increase in density from 4.51 to 5.80 g cm⁻³ for WO₃-Al₂O₃-PbO-B₂O₃ glasses is observed with an increase in WO₃ content. This is observed that the atomic structure changes more with the incorporation of WO₃. This is due to the formation of WO₆, WO₄ and BO₄ units.     

Structural, electrical and optical properties of TiO2 doped WO3 thin films

TiO₂ doped WO₃ thin films were deposited onto glass substrates and fluorine doped tin oxide (FTO) coated conducting glass substrates, maintained at 500 °C by pyrolytic decomposition of adequate precursor solution. Equimolar ammonium tungstate ((NH₄)₂WO₄) and titanyl acetyl acetonate (TiAcAc) solutions were mixed together at pH 9 in volume proportions and used as a precursor solution for the deposition of TiO₂ doped WO₃ thin films. Doping concentrations were varied between 4 and 38%. The effect of TiO₂ doping concentration on structural, electrical and optical properties of TiO₂ doped WO₃ thin films were studied. Values of room temperature electrical resistivity, thermoelectric power and band gap energy (E_g) were estimated. The films with 38% TiO₂ doping in WO₃ exhibited lowest resistivity, n-type electrical conductivity and improved electrochromic performance among all the samples. The values of thermoelectric power (TEP) were in the range of 23-56 μV/K and the direct band gap energy varied between 2.72 and 2.86 eV.     

Synthesis of In-doped Ga2O3 zigzag-shaped nanowires and optical properties

In-doped Ga₂O₃ zigzag-shaped nanowires and undoped Ga₂O₃ nanowires have been synthesized on Si substrate by thermal evaporation of mixed powders of Ga, In₂O₃ and graphite at 1000 °C without using any catalyst via a vapor-solid growth mechanism. The morphologies and microstructures of the products were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and photoluminescence spectroscopy (PL). The nanowires range from 100 nm to several hundreds of nanometers in diameter and several tens of micrometers in length. A broad emission band from 400 to 700 nm is obtained in the PL spectrum of these nanowires at room temperature. There are two blue-emission peaks centering at 450 and 500 nm, which originate from the oxygen vacancies, gallium vacancies and gallium-oxygen vacancy pairs.     

Structural and optical properties of thermally evaporated Bi2Te3 films

Bi₂Te₃ films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 °C were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67–162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5–10 μm for Bi₂Te₃ films with different thicknesses (25–99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi₂Te₃ is a high refractive index material (n has values of 4.7–8.8 in the wavelength range 2.5–10 μm). The allowed optical transitions were found to be direct optical transitions with energy gap  eV. The optical conductivities σ₁ = ƒ(hν) and σ₂ = f(hν) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions.     

Nature of optical properties of GdFe3(BO3)4 and GdFe2.1Ga0.9(BO3)4 crystals and other 3d5 antiferromagnets

Influence of the partial substitution of paramagnetic Fe³⁺ ions by diamagnetic Ga³⁺ ions in the trigonal crystal GdFe₃ (BO₃)₄ on its optical and magnetic properties is studied and discussed in connection with problems common for all antiferromagnets containing 3d ⁵ ions. Polarized optical absorption spectra and linear birefringence of GdFe₃ (BO₃)₄ and GdFe_2.1Ga_0.9 (BO₃)₄ single crystals have been measured in the temperature range 85–293 K. Specific heat temperature dependence (2–300 K) and structure of GdFe_2.1Ga_0.9 (BO₃)₄ crystal have been also studied. As a result of substitution of 30% Fe to Ga the Neel temperature diminishes from 38 till 16 K, the strong absorption band edge shifts on 860 cm^-1 (0.11 eV) to higher energy and the d-d transitions intensity decreases substantially larger than the Fe concentration does. Strong absorption band edge is shown to be due to Mott-Hubbard transitions. Correlation between position of the strong absorption band edge and the Neel temperature of antiferromagnets has been revealed. Properties of the doubly forbidden d-d transitions in the studied crystals and in other antiferromagnets are explained within the framework of the model of the exchange-vibronic pair absorption, which is theoretically analyzed in detail. The model permitted us to determine the connection between parameters of d-d absorption bands (intensity, width and their temperature dependences), on the one hand, and the exchange, spin-orbit and electron-lattice interactions, on the other hand.