Recent advancements in ab initio time-dependent Hartree-Fock (TDHF) theory have made it a technique of choice for modeling nanoscale nonlinear optical (NLO) materials from first-principles. We have used this method to study structure-NLO property relationships of GaN, GaP and GaAs clusters. The geometry of the clusters used in the study was optimized by ab initio Hartree Fock (HF) calculations with the use of even tempered Gaussian (ETG) basis set. The clusters used in this study are of the type Gam Xn (M = 1,3,4,7 and n = 1,3,4,7) where X=N, P, and As. The GamXn clusters are in a charge neutral (q = 0) state for m = n and in appropriately charged state for m ∦ n. The magnitude of the calculated (hyper)polarizabilities appears to strongly depend on the composition of the cluster. For the same composition of heteroatoms, the hyperpolarizability depends on the size as well as the geometry of the cluster. The cluster size-dependence of calculated (hyper)polarizabilities is more pronounced for the first-hyperpolarizability. β than for the polarizability, α The calculated β(–ωμ,ωl,ω2) corresponding to various second order effects shows the following trend β(–2ω; ω,ω) > β(–ω; 0, ω) >β(0;0,0).

The cascade of microscopic processes relevant to semiconductor laser operation is outlined. An integrated laser simulator which encapsulates these processes is applied to illustrate the connection between an accurate model of the optical gain in the quantum wells and measured characteristics of representative 1.3 μm InGaAsP/InP lasers. These results highlight the impact of carrier transport effects on the observed optical gain and the modulation response of semiconductor lasers.

We studied theoretically the electron spectrum and infrared transitions in a superlattice with a unit cell allowing for quasi-localised carrier states. The dispersion relation and the band structure of such a system have been found. We also calculated the dipole matrix element for inter-subband carrier infrared transitions. The wave functions and the electron spectrum in this superlattice show a peculiarity when the energy of a band state approaches the energy of the quasi-localised state in the single cell. In particular, the absorption strength peaks up at the respective frequencies.

The authors present calculations of quaternary III–V semiconductor alloy optical properties and the comparison of the theoretical data with available experimental results for AlGaAsSb, AlGaInP, AlGaInAs, and GaInAsP alloys. The investigation includes material's energy bandgap and refractive index calculations as a function of the incident wavelength in the transparent region, as well as the composition of the alloy. Optimization of the quaternary alloy refractive indices was obtained from a semi-empirical dielectric function calculations based on the interband transition contributions.

In order to calculate optical properties i.e. dielectric function, refractive index and absorption coefficient, the evaluation of integration including ground and excited wavefunctions is required over entire k-space. The contour of energy dispersion was proposed to form the criteria to select and limit the value of k in the integration. With the contour approach the integration can be determined to truncate at certain k value where the weight factor of Fermi-Dirac distribution function become very small and the fraction of integration can be ignored. The approach was applied to AIGaAs/AlAs/GaAs double barrier quantum well structures with 14-band k.P Hamiltonian. By systematically modifing this quantum well structure the dependence of absorption peak width was investigated in bound-to-bound and bound-to-quasibound intersubband transitions. The energy dispersion contours of each involved state were illustrated in two dimensional k-space including the compositions in perpendicular and parallel directions to the interface. The calculated refractive index and absorption as a function of wavelength can be simply extracted from the contour characteristic especially at the constant Fermi energy surface.

Semiconducting metal silicides are potential candidates of silicon-based light emitting materials. In order to carry out screening of the candidates, we calculated the oscillator strength between the valence and excited states near the band gap for various silicides. The electronic states were obtained by the full-potential linear augmented-plane-wave method (FLAPW) based on the local density approximation (LDA). The results show Ru2Si3 and Ca2Si have direct gap at τ point, but the values of the oscillator strength across the gap are evaluated to be zero. Among the indirect gap semiconductors, β-FeSi2, OsSi, and OsSi2 have several peaks and valleys facing each other near the band gap. Among the combinations, we obtained the biggest value of oscillator strength 0.3 at X point for OsSi with the transition energy of 0.42 eV.

Rare earth doped semiconductors are known to be efficient luminescent materials. In particular, SrS and SrxS and SrxGa2S4 doped with Ce+3 have recently been explored as efficient blue cathodoluminsecent and electroluminescent materials for flat panel display applications. The blue emission in these materials is due to Ce+3 5d-4f transitions, and the transition energies are sensitive to the local chemistry of the Ce+3 ion. To understand the effect that the local chemistry has on the Ce+3 4f–5d emission spectrum, we used a self-consistent-field configuration interaction (SCF/CI) model to calculate the electronic spectrum of the Ce−3 ion embedded in a cluster representation of the semiconductor lattice. The effects of changing nearest neighbor anions and cations have been modeled and are in excellent agreement with the experimental spectroscopy. While vibronic transitions are not calculated in this model, the calculated line transitions were fit to Gaussian peaks to generate absorption and emission spectra. Calculated and experimental Ce3+ 4f–5d emission spectra are compared for different materials, and the observed spectral changes are correlated to an analysis of the magnitude of the ligand field splitting and the nephelauxetic effect in various host lattices. Computed atomic orbital electron populations support these arguments.

We present the results for the imaginary part of the dielectric function of porous silicon, which were obtained with the tight-binding 128–atom supercell model for different porosities. The supercells have been chosen to allow the interconnection of the Si skeleton. We have analyzed also the effects of pore morphology. We have found that, at a fixed porosity, the developing of the surface, resulting in the increase of saturating hydrogen atoms, leads to a noticeable blueshift of the absorption edge.

Synthesis of polymeric carbon dioxide has long been of interest to many chemists and materials scientists. Very recently we discovered the polymeric phase of carbon dioxide (called CO2-V) at high pressures and temperatures. Our optical and x-ray results indicate that CO2-V is optically non-linear, generating the second harmonic of Nd: YLF laser at 527 nm and is also likely superhard similar to cubic-boron nitride or diamond. COz-V is made of CO4 tetrahedra, analogous to SiO2 polymorphs, and is quenchable at ambient temperature at pressures above 1 GPa. In this paper, we describe the pressure-induced polymerization of carbon dioxide together with the stability, structure, and mechanical and optical properties of polymeric CO2-V. We also present some implications of polymeric CO2 for high-pressure chemistry and new materials synthesis.

We present first-principles approaches based on density functional theory for calculating positron states and annihilation characteristics in condensed matter. The treatment of the electron-positron correlation effects (the enhancement of the electron density at the positron with respect to mean-field density) is shown to play a crucial role when calculating the annihilation rates. A generalized gradient approximation (GGA) takes the strong inhomogeneities of the electron density in the ion core region into account and reproduces well the experimental total annihilation rates (inverses of the positron lifetimes) by suppressing the rates given by a local density approximation (LDA). The GGA combined with an electron-state-dependent enhancement scheme gives a good description for the momentum distributions of the annihilating positron-electron pairs reproducing accurately the trends observed in the angular correlation (ACAR) or Doppler broadening measurements of the annihilation radiation. The combination of the present positron lifetime and momentum density calculations with the corresponding measurements yields a unique tool for defect identification. Especially, the investigation of various vacancy-type defects in semiconductors able to trap positrons will be an important field for these methods. We will show that the identification of vacancy-impurity complexes in highly n-Type Si and the study of the SiO2/Si interface are particularly interesting applications.

Fluorinated silica has a dielectric constant E in the range of 3—3.5, lower than that of F-free SiO2 (ω=4). The reasons behind this reduction are controversial. It is not known whether the electronic or ionic contributions to the overall screening are being diminished upon F doping. To shed more light on this phenomenon we have studied F-doped SiO2 with ab-initio modeling and various characterization techniques. FTIR transmission and spectroscopic ellipsometry give us information about the ionic and electronic contributions to ω Nuclear reaction analysis and Auger spectrometry measure F composition. XPS and FTIR provide information on the atomic structure and stability of the film. We use a large cell of cristobalite to model fluorinated silica theoretically. The ground state geometry is obtained via energy minimization. We calculate the vibrational density of states and find a localized mode (Si-F stretch), in good agreement with FTIR transmission. We analyze the effects of F incorporation on the dielectric properties.

Si KB X-ray emission spectra of SiO2—CaO binary glasses simulated with the DV-X α molecular orbital calculation were compared with ones measured by EPMA. The composition of the glass has been incorporated in the calculation of the Si 3p partial density of states (PDOS). The Si 3p PDOS, as a function of the CaO content, agreed well with the measured Si Kβ X-ray emission spectra of the 62.4mol%SiO2 – 37.6mol%CaO and 43.3mol%SiO2 – 56.7mol%CaO binary glasses. The present method was found to provide a very useful means of clarifying the change in the ionic configuration of the glass with the metal oxide content.

To investigate the origin of vacancies in SiC nanopowders, complex theoretical and experimental investigations of SiC nanopowders of different geometry sizes were carried out. Our theoretical approach included ab initio band energy calculations and molecular dynamics simulations with dynamic boundary conditions. Experimental methods included EPR spectra. The significant influence of nanocrystallite and grain sizes on the concentration and type of vacancies is demonstrated. On the other hand, our comparison of experimental data and theoretical simulations show that vacancies influence the optical properties of SiC nanocrystallites.

Fiber Bragg gratings are playing an important role in the field of optical telecommunications and sensors. We have simulated the growth of gratings, longitudinally written in the core of germanosilicate optical fibers by an Ar+ laser. Following a Kramer-Kronig model, absorption in the Germanium Oxygen deficient centers band (∼240 nm) is considered to be responsible for the fiber photosensitivity and a two photon absorption process models the dynamics of the refractive index variation.

The spatial propagation of light along the grating is described by the coupled wave theory and is calculated by using an inverse scattering matrix technique. The predicted physical properties of the gratings, reflectivity and temporal dispersion of propagating light as a function of wavelength and input power, show that a sustained growth is possible. A good agreement with previous simulations and experiments is found.

This paper reports a finite-element-method formulation, which is based on the Helmholtz vector equations in terms of transverse electrical field components to characterize two-dimensional dielectric waveguides with an arbitrary continuous permittivity profile. A virtual exterior element iteration technique is associated with the finite-element analysis to process the remote boundary condition. This technique achieves both excellent computational efficiency and accuracy when studying the dielectric propagation constants and cutoff frequencies. The spurious-infection-free modal electrical field can be obtained directly. Numerical results of circular optical fibers with the parabolic permitivity profile are presented to show the validity of this technique.

It has been demonstrated that porous silicon can be used to enhance the sensitivity, spectral bandwidth, and light acceptance angle of photodiodes. In an attempt to better understand light trapping effects in porous silicon, a three-dimensional model was used to approximate the surface geometry of porous silicon. It was found that the main contributing factor to the observed high efficiencies is the shape of the pores. The general shape resembles cylindrical shaped sections, whose radii are in the micron range. The results of the computer simulation were compared to actual experimental results where porous silicon was tested under variable conditions of lighting and porosity. The comparison shows that the simulation gives a good approximation of the behavior of porous silicon. Although the simulation does not perfectly match experimental values, it does provide the insight that the surface geometry of porous silicon is the main contributing factor to its efficiency in trapping light.

We have simulated the scattering of UV light in a photo-formable ceramic tape and experimentally measured the transport mean free path (random walk step) between scattering events. After processing the tape we have achieved features size of 70 μm in a 150 μm-thick tape. We were able to predict and control the undercut due to scattering of the light by alumina grains. The experimental verification of the model utilizes a modified version of diffuse-Transmission Spectroscopy (DTS) using the material development characteristics as a sensor. The technique involves the measurement of absorption coefficient by measuring the sample thickness as a function of energy. We also simulated the effect of changing the number density of scatter centers in side-wall morphology.