The properties of silicon dioxide have been studied extensively over the years. However, there still remain major unanswered questions regarding the nature of the optical spectrum and the role of excitonic effects in this technologically important material. In this work, we present an ab initio study of the optical absorption spectrum of alpha-quartz, using a newly developed first-principles method which includes self-energy and electron-hole interaction effects. The quasiparticle band structure is computed within the GW approximation to obtain a quantitative description of the single-particle excitations. The Bethe-Salpeter equation for the electron-hole excitations is solved to obtain the optical spectrum and to understand the spatial extent and physical properties of the excitons. The theoretical absorption spectrum is found to be in excellent agreement with the measured spectrum. We show that excitonic effects are crucial in the frequency range up to 5 eV above the absorption threshold.

We discuss the current status of a computational approach which allows to evaluate the dielectric matrix, and hence electronic excitations like optical properties, including local field and excitonic effects. We introduce a recent numerical development which greatly reduces the use of memory in such type of calculations, and hence eliminates one of the bottlenecks for the application to complex systems. We present recent applications of the method, focusing our interest on insulating oxides.

We present an ab initio computational scheme to calculate ε(ω) including the screened electron-hole interaction. Results are presented for GaP and optically-pumped GaAs. We also discuss a time-dependent formulation which has some conceptual advantages.

First-principles calculations of the inelastic lifetime of low-energy electrons in Al. Cu and Au are reported. Quasiparticle damping rates are evaluated from the knowledge of the electron self-energy, which we compute within the GW approximation. Inelastic lifetimes are then obtained along various directions of the electron wave vector, with full inclusion of the band structure of the solid. Average lifetimes are also reported, as a function of the electron energy. In Al splitting of the band structure over the Fermi level yields electron lifetimes that are smaller than those of electrons in a free-electron gas. In Cu and Au, a major contribution from d electrons participating in the screening of electron-electron interactions yields electron lifetimes which are well above those of electrons in a free-electron gas with the electron density equal to that of valence (4s1 and 6s1 respectively) electrons.

We investigate the formation of excitons in low-dimensional semiconducting systems. To this end a recently developed ab initio approach is employed to determine the excitation energies and optical transition matrix elements of bound and unbound excitonic states. From these results the optical spectrum can be evaluated. We discuss two classes of low- dimensional prototype materials: conjugated polymers and a semiconductor surface. Due to the reduced dimensionality, the excitonic binding energies are much larger than in con- ventional semiconductors.

We present the first ab initio calculations of the optical properties of the (110) clean surface of Cu for which the Anisotropic Surface Reflectivity (RAS) has been recently measured. This technique exploits the anisotropy of the reflectivity along the directions [110], [001] on the surface. Calculations are performed with a Full Potential Linear Muffin Tin Orbital method, within LDA, in the slab geometry. The comparison of our calculated Surface Dielectric Anisotropy (SDA) ω[1110] – [001] with the one obtained from the RAS spectrum is almost satisfactory for a slab of 11 atomic layers.

We have calculated the reflectance anisotropy for the GaAs (110) surface using the discrete cellular method. This method extends the range of application of standard discrete dipole calculations by incorporating nonlocal polarizabilitites. The method adds a second quantum mechanical channel of nonlocality, which turns out to be necessary and yields very good agreement between theory and experiment.

We study tile optical absorption spectra of Na clusters using the time-dependent density-functional theory with gradient correction. A jellium-sphere background model, which is free from basis-set incompleteness error and is suitable for the comparison of various theoretical methods, is adopted. For energies of surface-plasinon excitations governing profiles of photoabsorption spectra with huge oscillator strengths., the gradient correction by van Leeiiwen and Baerends with correct asymptotic behavior of the effective potential is found to show considerable improvement over the time-dependent local-density approximation.

Ab initio quasiparticle gaps. self-energy corrections. exciton Coulomb energies. and optical gaps of Si nanocrystals are calculated using the higher-order finite difference psendopotential method. The calculations are performed in real space on hydrogen-passivated Si clusters with diameters up to 30 A (> 1000 atoins). The size-dependent self-enerkgy correction is enhanced substantially compared to bulk. and quantum confinement and reduced electronic screening result in appreciable excitonic Coulomb energies. Calculated optical gaps are in very good agreement with absorption data from Si nanocrystals.

The study of electronic excitations in hydrogen terminated silicon clusters is important for understanding the optical properties of confined systems such as quantum dots and porous silicon. Here we calculate the excitation energies and absorption spectra for SinHm clusters using linear response theory within the time-dependent local density approximation (TDLDA). We find the computed excitation energies and photoabsorption gaps agree with available experimental data. The TDLDA optical absorption spectra exhibit a substantial blue shift with respect to the spectra calculated within the time-independent local density approximation. This trend is consistent with other theoretical calculations for excited state properties, such as the Bethe-Salpeter technique.

We describe our progress in developing an ab initio computational scheme for the calculation of the dielectric response function of solids, with special emphasis here on Si and C clusters. All calculations are carried out employing a basis of localized atomic-like orbitals and include the evaluation of quasiparticle corrections. The self-energy operator is evaluated in the GW approximation, with a full frequency dependence for the dielectric matrix. The approach is convenient and computationally optimal for the calculation of optical properties of complex systems lacking full periodicity, such as surfaces and clusters. We present here the dielectric response functions of clusters with structures found after full equilibration via molecular dynamical simulations, and discuss the sensitivity of the optical properties to quasiparticle corrections.

We simulate α - Si21 –N2: H that contains 6-atom boat-type rings and a – Si17–iNi: H that contains 5-atom planar rings, where i = 0, 1 or 4. The simulations were carried out using the DFT-LDA approximation contained in the DMol code of MSI. We report calculations of impurity levels and relate these to the size of the gap and to the optical absorption curves for each cluster. A comparison is made between the two α – SiN clusters studied to analyze the effect of the ring topology.

In nanoscale quantum dots, subpicosecond laser pulses can induce and probe strong time-dependent Coulomb correlations between confined electrons and holes. Correlation dynamics for one or two electron-hole pairs driven by both interband and intraband lasers can be simulated by numerical solution of the time-dependent Schrddinger equation within a configuration-interaction description. For example, Coulomb correlations of two electrons and two light holes in a 5×25×25 nm3 GaAs quantum dot yield strong oscillations in the luminescence. Pure correlation effects are revealed by a carefully chosen sequence of three circularly polarized subpicosecond laser pulses. For this case, the Coulomb and electron-laser matrix elements were calculated within the effective-mass approximation with infinite potential walls. For a quantum dot with an internal tunneling barrier that splits the energy levels on the 10 meV scale, correlation effects couple the interband and intraband optical response. Work in progress aims at more realistic geometries, finite outer potential walls, and better description of the band structure, using real-space methods, multi-band models, and tight-binding Hamiltonians. With the help of 'dynamic state selection,' simulation times can be reduced by a factor of 5–10. Dynamic state selection allows the computer, by generic selection criteria, to use only those determinants that are momentarily most important. This approach is especially useful in multi-pulse simulations where the coupled determinants belong to different classes at different times.

We describe computational methods for the theoretical study of explicit correlations beyond the mean field in excitons confined in semiconductor quantum dots in terms of the Auxiliary-Field Monte Carlo (AFMC) method [1]. Using AFMC, the many-body problem is formulated as a Feynman path integral at finite temperatures and evaluated to numerical precision. This approach is ideally suited for implementation on high-performance parallel computers. Our strategy is to generate a set of mean-field states via the Hartree-Fock method for use as a basis for the AFMC calculations. We present preliminary results.

A density-matrix description has been developed to treat relaxation (decoherence) phenomena during resonant and non-resonant radiative transitions of quantized electronic systems, including many-electron atoms and quantum-confinement systems (e. g., semiconductor microstructures). Radiative and collisional relaxation phenomena have been treated using Liouville-space proj ecti on-operator techniques. Both time-independent (resolvent-operator) and time-dependent (equation-of-motion) formulations have been developed. The self-energy operators that occur in these formulations can provide the fundamental basis for a self-consistent determination of the non-equilibrium and coherent electronic-state kinetics together with the homogeneous spectral-line shapes. This density-matrix description can be adapted for the computer simulation of electromagnetic processes. From first-principles electronic-structure calculations or from semi-empirical approaches, the parameters describing the elementary collisional and radiative interactions can be evaluated and organized into the basic data set for the application of the density-matrix description. The final product is a theoretical prediction for the linear or non-linear optical absorption or emission spectrum corresponding to a given set of values for the appropriate physical variables, such as temperatures, densities, and electric or magnetic field strengths.

Electronic states in self-assembled quantum dots with a lens geometry are studied. A conformal analytical map is used to transform the quantum dot boundary into a dot with semi-spherical shape. The Hamiltonian for a carrier confined in the quantum lens is also mapped into an equivalent operator and its eigenvalues and eigenfunctions for the corresponding Dirichlet problem, of confinement by infinite walls, are analyzed. A modified Rayleigh-Schrddinger perturbation theory is developed to obtain analytical expressions for the energy levels and wavefunctions depending on the height b and radius a of the circular cross section of the spherical cap lens. Numerical calculations are shown for typical cases. The effects of decreasing rotational symmetry on the energy states and eigenfunctions of the quantum dot with lens-shape are presented: The degeneracy due to the z-component of the angular momentum m is broken for b ∦ a. Energy states and wavefunctions with m = 0 present the most pronounced influence on the b ∦ a case. Analytical expressions presented here can be used to estimate the sizes of actual self-assembled quantum dots.

We present our implementation of the length-gauge formalism of Sipe and coworkers (Phys. Rev. B 48, 11705 (1993); ibid 52, 14636 (1995)) using the linearized muffin-tin orbital (LMTO) method and discuss its application to the calculation of second order response functions. The importance of gap corrections beyond LDA is discussed. As primary application, we discuss the second harmonic generation (SHG) coefficients of the SiC polytypes and of the chalcopyrites of both the II-IV-V2 and I-III-VI2 families. These examples illustrate the relation of the second order response function to the modification of the crystal structure and chemical substitutions.

We report new theoretical studies of the electronic and structural response of materials to fast intense laser pulses, with durations ∼ 10–100 feintoseconds and intensities up to ∼10 terawatts/cm2. The results provide still stronger evidence that GaAs undergoes a true nonthermal phase transition as the intensity is varied at constant pulse duration. For C60, there are also different regimes of behavior as a function of pulse intensity. At low intensity, various optically-active modes are observed. At high intensity, the breathing moode is by far the most dominant. At still higher intensities, there is photofragmentation, with the evolution of dinmers and other products. These results were all obtained in simulation-. using tight-binding electron-ion dyanamics (TED).

Over the past several years there has been continuing interest in the design of highly conjugated monomers, oligomers and polymers for a variety of photonics applications dependent on a nonlinear response to intense laser radiation. Materials design criteria for these various applications depend on accurate structure-property relationships, and while these relationships can best be determined experimentally, computational approaches that can accurately predict trends in series are extremely useful in establishing target molecules having the desired photonic properties. We have recently reviewed the relationship between incorporation of polaronic or bipolaronic charge states in oligomeric or polymeric conjugation sequences' and enhanced third-order optical nonlinearity, particularly for finite-length polyenes. In this presentation we will focus on how these charge states can be stabilized by electron-donating substituents in dithienylpolyenes, and in model dendrimers based on bis(diphenylamino)stilbene repeat units, and illustrate why these criteria are important for the design of new optical limiting chromophores.

We report herein the results of coupled perturbed Hartree-Fock (CPHF) ab initio extended basis set calculations on the geometric structures, dipole moments, static first-order (α), second-order (β), and third-order polarizabilities (λ) of a series of fused heterocyclic aromatic compounds based on quinoline. The effects of the presence/absence of the heteroatom as well as the introduction of other substituents at various positions in the ring system on these molecular properties are described. The effect of the presence of N-oxide is also examined. Suggestions for the design of heterocyclic systems with enhanced polarizabilities are made.