Eric L. Shirley

Nonlocal pseudopotentials and diffusion Monte Carlo

We have applied the technique of evaluating a nonlocal pseudopotential with a trial function to give an approximate, local many‐body pseudopotential which was used in a valence‐only diffusion Monte Carlo (DMC) calculation. The pair and triple correlation terms in the trial function have been carefully optimized to minimize the effect of the locality approximation. We discuss the accuracy and computational demands of the nonlocal pseudopotential evaluation for the DMC method. Calculations of Si, Sc, and Cu ionic and atomic states and the Si2 dimer are reported. In most cases ∼90% of the correlation energy was recovered at the variational level and excellent estimations of the ground state energies were obtained by the DMC simulations. The small statistical error allowed us to determine the quality of the assumed pseudopotentials by comparison of the DMC results with experimental values.

Bethe-Salpeter equation calculations of core excitation spectra

We present a hybrid approach for Bethe-Salpeter equation (BSE) calculations of core excitation spectra, including x-ray absorption (XAS), electron energy loss spectra (EELS), and nonresonant inelastic x-ray scattering (NRIXS). The method is based on ab initio wave functions from the plane-wave pseudopotential code ABINIT; atomic core-level states and projector augmented wave (PAW) transition matrix elements; the NIST core-level BSE solver; and a many-pole self-energy model to account for final-state broadening and self-energy shifts. Multiplet effects are also approximately accounted for. The approach is implemented using an interface dubbed OCEAN (Obtaining Core Excitations using ABINIT and NBSE). To demonstrate the utility of the code we present results for the K edges in LiF as probed by XAS and NRIXS, the K edges of KCl as probed by XAS, the Ti L2,3 edge in SrTiO3 as probed by XAS, and the Mg L2,3 edge in MgO as probed by XAS. These results are compared with experiment and with other theoretical approaches.

Symmetry of spatial-dispersion-induced birefringence and its implications for CaF2 ultraviolet optics

The discovery of a significant spatial-dispersion-induced birefringence (intrinsic birefringence) in CaF2 at ultraviolet wavelengths has had a major impact on the design of 157 nm lithography systems, requiring complete redesign of the optics to take account of the imaging aberrations resulting from the birefringence and the accompanying index anisotropy. This intrinsic birefringence phenomena results from a symmetry-breaking effect of the finite wave vector of the photon on the symmetry of the light-matter interaction in fluorite-structure cubic crystals. As a follow-up to our original concise report of measurements and theory of the effect in CaF2 and BaF2, we present here a more detailed analysis of the theory, focusing on the symmetry and its consequences. We also provide the full directional dependence of the effect in useful closed forms. We analyze the implications for precision optical design with CaF2 optical elements, and discuss qualitatively the approaches being considered to compensate for it.

Theoretical optical and x-ray spectra of liquid and solid H2O

Theoretical optical and x-ray spectra of model structures of water and ice are calculated using a many-body perturbation theory, Bethe-Salpeter equation (BSE) approach implemented in the valence- and core-excitation codes ai2nbse and ocean. These codes use ab initio density functional theory wave functions from a plane-wave, pseudopotential code, quasiparticle self-energy corrections, and a BSE treatment of particle-hole interactions. This approach improves upon independent-particle methods through the inclusion of a complex, energy-dependent self-energy and screened particle-hole interactions to account for inelastic losses and excitonic effects. These many-body effects are found to be crucial for quantitative calculations of ice and water spectra.

High-index materials for 193 nm immersion lithography

193 nm immersion lithography optical projection systems using conventional UV optical materials and water as the immersion fluid, with planar lens/fluid interfaces, have a practical numerical aperture (NA) limit near 1.3. The bottleneck for pushing the NA further is the refractive index of the final lens element. Higher-index immersion fluids cannot alone give much improvement, because the NA is limited by the lowest material index. In this paper we consider the possibility of using novel high-index materials in the last lens element to get around this bottleneck and to push the NA limit to at least 1.5, while containing the lens system size and complexity. We discuss three classes of high-index (n>1.8), wide-band-gap, oxide-based materials that have the potential for being fabricated with optical properties appropriate for lithography optics: group-II oxides, magnesium-aluminum-spinel-related materials, and ceramic forms of spinel. We present theoretical calculations and experimental measurements of the optical properties of these materials, including intrinsic birefringence, and we assess their prospects.

Sources of Differences in On-Orbital Total Solar Irradiance Measurements and Description of a Proposed Laboratory Intercomparison.

There is a 5 W/m2 (about 0.35 %) difference between current on-orbit Total Solar Irradiance (TSI) measurements. On 18–20 July 2005, a workshop was held at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland that focused on understanding possible reasons for this difference, through an examination of the instrument designs, calibration approaches, and appropriate measurement equations. The instruments studied in that workshop included the Active Cavity Radiometer Irradiance Monitor III (ACRIM III) on the Active Cavity Radiometer Irradiance Monitor SATellite (ACRIMSAT), the Total Irradiance Monitor (TIM) on the Solar Radiation and Climate Experiment (SORCE), the Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliospheric Observatory (SOHO), and the Earth Radiation Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS). Presentations for each instrument included descriptions of its design, its measurement equation and uncertainty budget, and the methods used to assess on-orbit degradation. The workshop also included a session on satellite- and ground-based instrument comparisons and a session on laboratory-based comparisons and the application of new laboratory comparison techniques. The workshop has led to investigations of the effects of diffraction and of aperture area measurements on the differences between instruments. In addition, a laboratory-based instrument comparison is proposed that uses optical power measurements (with lasers that underfill the apertures of the TSI instruments), irradiance measurements (with lasers that overfill the apertures of the TSI instrument), and a cryogenic electrical substitution radiometer as a standard for comparing the instruments. A summary of the workshop and an overview of the proposed research efforts are presented here.

Revised formulas for diffraction effects with point and extended sources

Revised formulas to estimate diffraction effects in radiometry for point and extended sources are derived. They are found to work as well as or better than previous formulas. In some instances the formulas can be written in closed form; otherwise their evaluation entails performing simple integrations as indicated. Formulas have been found for nonlimiting apertures, large defining apertures, and pinhole apertures. Examples of all three types of application are presented.

Electronic structure of lithium battery interphase compounds: Comparison between inelastic x-ray scattering measurements and theory

In lithium ion batteries, decomposition of the electrolyte and its associated passivation of the electrode surface occurs at low potentials, resulting in an electronically insulating, but Li-ion conducting, solid electrolyte interphase (SEI). The products of the SEI and their chemical constituents/properties play an important role in the long-term stability and performance of the battery. Reactivity and the sub-keV core binding energies of lithium, carbon, oxygen, and fluorine species in the SEI present technical challenges in the spectroscopy of these compounds. Using an alternative approach, nonresonant inelastic x-ray scattering, we examine the near-edge spectra of bulk specimens of common SEI compounds, including LiF, Li(2)CO(3), LiOH, LiOH·H(2)O, and Li(2)O. By working at hard x-ray energies, we also experimentally differentiate the s- and p-symmetry components of lithiums unoccupied states using the evolution of its K edge with momentum transfer. We find good agreement with theoretical spectra calculated using a Bethe-Salpeter approach in all cases. These results provide an analytical and diagnostic foundation for better understanding of the makeup of SEIs and the mechanism of their formation.

Optical to UV spectra and birefringence of SiO2 and TiO2: First-principles calculations with excitonic effects

A first-principles approach is presented for calculations of optical, ultraviolet spectra including excitonic effects. The approach is based on Bethe-Salpeter equation calculations using the NBSE code combined with ground-state density-functional theory calculations from the electronic structure code ABINIT. Test calculations for bulk Si are presented, and the approach is illustrated with calculations of the optical spectra and birefringence of

Theory of probing a photonic crystal with transmission near-field optical microscopy

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