Optical absorption and electronic structure of intermetallic compound RuIn3

The optical properties of intermetallide RuIn₃ are investigated by ellipsometry in the spectral range of 0.22–10 μm. The experimental data point to the existence of an energy gap of about 0.5 eV in the electronic spectrum of the compound. The density of the electron states and interband optical conductivity are calculated in terms of the density functional theory. The experimental and theoretical spectra of the optical conductivity are compared. It is found that the formation of basic absorption bands is caused by interband transitions of electrons of the d-band of Ru and p-band of In.     

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.     

Optical and spectral tunability of multilayer spherical and cylindrical nanoshells

This theoretical work presents a comparative study of the optical properties and spectral tunability of hybrid multilayer spherical and cylindrical nanoshells based on the quasi-static approximation of classical electrodynamics. The interband transitions have been considered using the Drude–Lorentz model for the complex dielectric function of metallic layers because the optical properties of metals arise from both the optical excitation of interband transitions and the free-electron response. A general formula for N-ayer concentric nanoshells is arranged, and numerical calculations are performed for the four-layer nanoshells as an example. We have analyzed in detail different configurations of nanoshells such as dielectric-metal-dielectric-metal with dielectric core, metal-dielectric-metal-dielectric with metal core and semiconductor-metal-dielectric-metal with semiconductor core because composition of nanoshells have dramatic influence on their optical properties. The absorbance spectrum behavior of the shell thicknesses, surrounding medium, shape and composition of each layer of the nanoshell is numerically investigated.     

Structural Features of Boron-Doped Si（113） Surfaces Simulated by ab initio Calculations

Based on ab initio calculations, boron-doped Si（113） surfaces have been simulated and atomic structures of the surfaces have been proposed. It has been determined that surface features of empty and filled states that are separately localized at pentamers and adatoms indicates a low surface density of B atoms, while it is attributed to heavy doping of B atoms at the second layer that pentamers and adatoms are both present in an image of scanning tunnelling microscopy. B doping at the second layer should be balanced by adsorbed B or Si atoms beside the adatoms and inserted B interstitials below the adatoms.     

Adsorption structure of germanium on the Ru(0 0 0 1) surface

Coverage-dependent adsorption energy of the Ge/Ru(0 0 0 1) growth system and the geometrical distortions of the most stable adsorption structure are investigated through first-principles calculations within density functional theory. A local minimum in adsorption energy is found to be at a Ge coverage of 1/7 monolayer with a Ru(0 0 0 1)- symmetry. Based on this stale superstructure, the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) images are simulated by means of surface local-density of states (LDOS). The results are consistent well with the STM measurements on the phase for Ge overlayer on Ru(0 0 0 1). From this stimulation, the relations between the STM images and the lattice distortion are also clarified.     

Spectra of optical functions and transitions in diamond

Spectra of a complete set of optical functions are obtained for three different diamond samples in the region from 0 to 32 eV. The calculations were performed by the Kramers-Kronig method using experimental reflection spectra. Special features and differences of the spectra of optical functions of the three samples are analyzed. Based on the method of joined Argand diagrams, the spectra of the dielectric constant are decomposed into elementary components for the first time, and their energies and oscillator strengths are determined. The component structure is in good agreement with the theoretical spectrum of the dielectric constant and the expected spectrum of the interband transitions.     

Adsorption on nanostructured chiral surfaces studied by the Monte Carlo method

A Monte Carlo (MC) lattice gas model of adsorption of a racemic mixture of enantiomers of 1,2-dimethylcyclopropane on a chiral surface with different spatial distribution of active sites was proposed. The calculations were performed on a square lattice for both stepped chiral surfaces and smooth surfaces with chiral patterns of active sites. The adsorbing molecules were assumed to be rigid structures of two types being mirror images one of another. Regardless of the enantiomer type, each molecule was composed of four segments occupying four lattice sites. The chiral surfaces were exposed to equimolar mixture of enantiomers whose individual equilibrium adsorption isotherms were calculated using standard Grand Canonical MC technique. The major purpose of the simulation was to examine how the structure of the surface affects separation of enantiomers, that is, to determine enantioselectivity defined as the ratio of their adsorbed amounts. Additionally, comparison of the enantioselectivities corresponding to the stepped and smooth surfaces was made.     

Electron energy loss spectroscopy of TiC,ZrC and HfC

The dielectric properties of commercial TiC, ZrC and HfC powders were determined by analyzing the low loss region of the EELS spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) were obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions across the energy gap, these spectra can be interpreted on the basis of existing energy-band calculations. Comparison between experimental results and theory shows good agreement.     

Optical Nonlinear Properties of CdSeS/ZnS Core/Shell Quantum Dots

The optical nonlinear properties of CdSeS/ZnS quantum dots (QDs) areinvestigated by Z-scan technique using fundamental harmonic generation(1064nm) of mode-locked Nd:YAG laser for the first time. The experimental results show that two photon absorptions (TPA) occur at input intensity up to 12.5GW/cm². CdSeS/ZnS QDs have an average TPA cross section of 13710GM and large nonlinear refractive index on order of 10^-7esu. The large optical nonlinearities perhaps allow the CdSeS/ZnS QDs to be one kind of candidate material for bioimaging and fluorescence label, optical limiting and all-optical switching.     

Dielectric and optical properties of nanometric nickel silicides from valence electrons energy-loss spectroscopy experiments

Valence and Core Electron Energy Loss Spectroscopy (VEELS and CEELS) experiments are performed from nanocrystallized nickel silicide thin films. Three different silicide compounds are identified in the films. Their chemical compositions are determined from Ni-L(2,3) to Si-K core edges quantification. The results obtained are coherent within less than 2% error with the pure Ni2Si, NiSi and NiSi2 phases. The analysis of the shape and energy position of Ni-L(2,3) near edge structures and volume plasmon peaks indicates that both are reliable signatures to identify unambiguously each compound. Nickel silicides low-loss spectra have been submitted as references to the EELS database (www.cemes.fr~eelsdb). Low-loss spectra are processed to extract single scattering spectra and determine the dielectric function. The results show that nickel silicides dielectric functions deduced from VEELS are in quite good agreement with epsilon1 and epsilon2 deduced from ellipsometry experiments. The optical properties (refractive index (n), absorption coefficient (k), reflectivity (R%) and resistivity (rho(opt))), calculated from VEELS dielectric function are then compared in details with the data resulting from others techniques available in the literature. We show that, except some minor divergences, the nickel silicides optical properties are generally well reproduced. This indicates that VEELS is a relevant technique for accessing reliably to physical properties and can be a successful alternative to conventional techniques when high spatial resolution is needed.     

Band transitions in wurtzite GaN and InN determined by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in wurtzite InN, deposited by molecular beam epitaxy on (0001) sapphire substrates or GaN buffer layers. The GaN buffer layer was used as VEELS reference. At room temperature a band transition for wurtzite InN was found at (1.7±0.2 eV) and for wurtzite GaN at (3.3±0.2 eV) that are ascribed to the fundamental bandgap. Additional band transitions could be identified at higher and lower energy losses. The latter may be related to transitions involving defect bands. In InN, neither oxygen related crystal phases nor indium metal clusters were observed in the areas of the epilayers investigated by VEELS. Consequently, the obtained results mainly describe the properties of the InN host crystal.     

Spectral properties of RuAl<Subscript>2</Subscript> and RuGa<Subscript>2</Subscript> compounds: Ellipsometric analysis

The optical properties of RuAl₂ and RuGa₂ intermetallic compounds have been investigated in the spectral range of 0.22–14 μm. The nature of interband light absorption has been interpreted based on a comparative analysis of calculated and experimental frequency dependences of optical conductivity. The data obtained confirm the existence of pseudogaps with a width of ~0.8 eV localized at the Fermi level in the electron densities of states of these materials, which was predicted in previous energy-band calculations.     

Theoretical investigation on near-infrared and visible absorption spectra of nanometallic aluminium with oxide coating in nanoenergetic materials: size and shape effects

The effects of metal core dimension, oxide shell thickness and ellipsoid aspect ratio of Al-Al2O3 core-shell nanoparticles on the near-infrared and visible absorption spectra of nanocomposite Al-Al2O3/nitrocellulose（NC） film are investigated by numerical calculations. Both the size-dependent interband transitions and frequency-dependent free electron damping of the nanometallic aluminium are taken into account in the calculations. Oxidation effect of nanoaluminium is also analysed. It is shown that oxidation may enhance but may also reduce the optical absorption, depending on the excited light energy and initial dimension of nanoparticle. Metal core size and excited light energy dominate the absorption characteristic. The absorption ability of ellipsoidal nanoparticles is larger than that of spheroidal nanoparticles and increases by the square index as the aspect ratio increases. These calculations will provide some significant theoretical guidance for the preparation and laser ignition of nanoenergetic materials.     

An He-implanted optical planar waveguide in an Nd:YGG laser crystal preserving fluorescence properties

We report the formation of a planar waveguide in an Nd:YGG laser crystal by low-energy He-ion implantation at liquid nitrogen temperature (77 K). The optical properties are measured by the prism coupling and end-face coupling methods, the absorption properties the waveguide and Nd:YGG substrate are obtained. The fluorescence spectrums are investigated by confocal methods. The experimental results revealed that the planar waveguide preserved the absorption and fluorescence properties of the Nd:YGG laser crystal. Thus, the planar waveguide formed by the ion implantation method is a promising candidate in waveguide lasers.     

Study of structural stabilities and optical properties of HgTe under high pressure

A theoretical investigation on the structural stabilities and electronic properties of HgTe under high pressure was conducted using first principles based on density functional theory. Our results demonstrate that the sequence of the pressure-induced phase transitions of HgTe is from the zinc blende, to cinnabar, rocksalt, orthorhombic, and CsCl-type structures. The pressure effects on the optical properties were discussed and compared with previous calculations and experimental data whenever available.     

Photodesorption of rubidium atoms from a sapphire surface

The optical properties of the GdRhGe compound have been investigated in a wide spectral range by ellipsometry. Self-consistent calculations of the electronic structure have been performed within the local electron spin density approximation with a correction to strong electron interactions in the 4f shell of gadolinium ions (LSDA+U method). The experimental dispersion relation of the optical conductivity in the region of interband light absorption is interpreted based on the results of calculating the electron densities of states.     

Electronic structure and optical properties of the HoCoSi and ErNiSi compounds

The electronic structure and the optical properties of the HoCoSi and ErNiSi compounds are studied. Spin-polarized band calculations are performed in the local electron density approximation corrected for the strong electron–electron interactions in the 4f shell of a rare-earth ion (LSDA + U method [11]). The optical constants are measured by ellipsometry in a wide wavelength range, and the frequency dependences of a number of spectral parameters are determined. The calculated densities of states are used to interpret the structural features of the interband optical conductivities of the intermetallic compounds.     

Size dependent effective band gap,optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn_1−xMg_xS/ZnS/Zn_1−xMg_xS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn_1−xMg_xS quantum wells.     

Effects of doping on the surface energies of nanocrystals and evidence from studies at high pressure

Zn_1−XMn_XS (X=0.85% and 1.26%) nanoparticles have been synthesized using a specially designed equipment and we have studied the influence of doping Mn²⁺ on the surface energy of ZnS. The high pressure behaviors of ZnS nanocrystals with different dopant contents have been investigated using angle-dispersive synchrotron X-ray powder diffraction up to 45.1 GPa. Theoretical calculations show that doping with Mn²⁺ increases the surface energy of the nanocrystals. The theoretical result has been further corroborated by our experimental observation of an increase in the phase transition pressure of Mn²⁺ doped ZnS nanocrystals in diamond-anvil-cell studies.