Brian M. Davies

Mode of incorporation of Sr2+ in calcite: Determination by X-ray absorption spectroscopy

By probing the local atomic environment of strontium coprecipitated with natural and synthetic calcites, X-ray absorption spectroscopy (XAS) reveals that the strontium is incorporated in the calcite by substitution at Ca2+ structural sites, forming a dilute solid solution. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) demonstrated that the local structural environment of Sr2+ in natural and synthetic calcites was similar to that of Ca2+ in calcite and quite different from that of Sr2+ in strontianite. The Sr2+-O2− distance derived from EXAFS, 2.58 ± 0.03A, is consistent with the sum of the radii of these two ions in six- and three-fold coordination, respectively, of 2.57 A. The XANES and EXAFS further eliminate such modes of incorporation as adsorption, occlusion, and the presence of trace amounts of strontianite or other Sr2+-rich phase. However, evidence of a possible relict Sr2+-rich aragonite was encountered in a diagenetic calcite, indicating that XAS may prove a sensitive tool for detecting incomplete mineralogical alteration in carbonates. Because Sr2+ is bound at Ca2+ lattice sites, the record of biomineralization, diagenesis and age encoded in Sr2+Ca2+ ratios, and strontium isotopes in geologic calcites is secure; likewise, 90Sr sequestered in natural calcite should not undergo significant preferential leaching.

Leed, auger, and electron energy loss studies of Ni epitaxially grown on Cu(100)

Abstract Nickel was epitaxially deposited onto a clean, flat Cu(100) surface. Low energy electron diffraction I ( E ) curves were recorded for 0.6, 1.1, and 2.7 monolayer (ML) Ni coverage. Multilayer relaxation was considered in theoretical calculations, which were compared with experiment by means of the R | ΔE | factor. The estimated relaxations of the first and second interlayer spacings are estimated to be − 2% and + 1.5% for clean Cu(100), − 2% and − 1.5% for 1 ML Ni coverage, relative to the bulk Cu interlayer spacing of 1.81 A, and −1% and 0% for 3 ML Ni coverage, relative to the bulk Ni spacing of 1.76 A. Decreasing the surface Debye temperature of the Ni layer to 268 K from the bulk value of 440 K improves the agreement between theory and experiment. The optimum inner potential values are − 9 and − 10 eV for clean Cu(100) and Ni on Cu(100), respectively. Auger electron spectroscopy was used to determine the thickness of the Ni films, and LEED indicates layer-by-layer growth until about 4 layers, when the LEED spots begin to spread, indicating island formation. Electron energy loss spectra were obtained with primary electron energies of 150 and 300 eV. The 3p core ionization transition was clearly observed after 0.5 ML Ni coverage. Peaks at 3.8 and 7.5 eV for clean Cu are ascribed to interband transitions, and shift to higher energy with Ni coverage. Peaks at 10 and 16 eV for clean Cu (ascribed to an interband transition and a surface plasmon, respectively) disappear with Ni coverage. Bulk plasmon peaks at 19 and 27 eV remain unshifted with Ni coverage. The effect of 0.9 and 1.3 ML Ni coverage of Cu(100) on the chemisorption of Co and oxygen was also studied. The behavior of the surface towards oxygen chemisorption was similar to that of the pure Ni surface. For a large exposure of oxygen (50 L and more) the EEL and Auger spectra are very similar to those observed for NiO. In the case of CO, for submonolayer Ni coverage, the surface shows a more Cu-like behavior, while for larger Ni coverage (a monolayer and more) there is a great similarity with the behavior of the pure Ni(100) surface.

Modeling and design of planar slanted volume holographic gratings for wavelength-division-multiplexing applications

Holographic gratings are modeled and designed for path-reversed substrate-guided-wave wavelength-division demultiplexing (WDDM) as a continuation of earlier research [Appl. Opt. 38, 3046 (1999)]. An efficient and practical method is developed to simulate the slanted volume holographic gratings. The trade-off between dispersion and the bandwidth of the holograms is analyzed. A 60 degrees (incident angle of the grating)/60 degrees (diffraction angle of the grating in air) grating structure is selected to demultiplex optical signals in the 1555-nm spectral region, and a 45 degrees /45 degrees grating structure is chosen for the spectral region near 800 nm. Experimental results are consistent with the simulation results for these two WDDM devices. A four-channel WDDM is also demonstrated at a center wavelength of 1555 nm and with a channel spacing of 2 nm.

Dispersion correction of surface-normal optical interconnection using two compensated holograms

Optical interconnects have advantages over electrical interconnects in applications where low transmission loss, electromagnetic interference immunity, low power budget, and high bandwidth requirements are critical. 1‐3 The need for optically-interconnected memories and processors in multichip modules is imperative due to the rapid increase of clock speed and of data throughput. Free-space interconnects must overcome packaging vulnerability in order to be practical. Guided-wave interconnects based on silica or polymeric thin films attract more attention since they can be fabricated in two-dimensional arrays using a standard very large scale integrated ~VLSI! microfabrication process. Coupling light into and out of waveguides efficiently becomes more important. Coupling methods using gratings, end-face joints, and prisms have been reported. 4,5 Surface-normal transmission holographic gratings are widely used to couple light into and out of waveguides due to their high diffraction efficiency and planarized packaging. In this letter, we investigate the light dispersion of a surface-normal input volume holographic grating. Experimental data of grating dispersion characteristics are obtained using a mode-locked femtosecond laser. A compensation method is developed to eliminate the wavelength-induced dispersion, and we demonstrate surface-normal input and output optical interconnect structures which automatically correct the dispersion resulting from the laser wavelength chirping and therefore greatly enhance the interconnection bandwidth. The basic structure of a surface-normal input and output optical interconnect using volume holographic gratings and substrate-guided waves is shown in Fig. 1. The grating structure induced by the refractive index modulation is slanted, having a tilt angle f. The grating spacing is L. Due to the high diffraction efficiency of the volume hologram and the low propagation loss due to total internal reflection, the surface-normal optical interconnect configuration is useful in photonics applications such as backplane buses, computer clock signal distribution, and waveguide-based wavelength

Azimuthal dependence of impact scattering in electron energy loss spectroscopy

Abstract The azimuthal dependence of electron energy loss spectroscopy (EELS) dipole and impact scattering intensity has been measured. Spectra for a saturation coverage of H adsorbed on W(110) exhibit loss peaks due to impact scattering from adsorbate vibrational modes. The intensity of the 160 meV loss peak has been measured as a function of the azimuthal angle between the scattering plane and a mirror plane of the s urface. The angular pattern has strong maxima oriented perpendicular to the rows of atoms on the surface, and has the C2v symmetry of the W(110) surface. This azimuthal dependence is strikingly different from the nearly isotropic angular dependence of dipole scattering from Cl adsorbed on W(110). Selection rules for impact scattering account for the general features of the angular pattern based on asymmetric stretch modes associated with bridge site H atoms. We have shown that the azimuthal dependence of the 36 meV Cl/W(110) dipole scattering loss peak is isotropic and that the 160 meV H/W(110) impact scattering loss peak exhibits a striking azimuthal pattern with C2v symmetry. The symmetry and deep minima suggest that selection rules play a central role in determining the azimuthal pattern. Application of these rules to two orthogonal directions (as in ref. 6) may be misleading, as is clear from Fig. 2, because essential features of the pattern will not be observed. Our analysis of the full pattern has suggested two bridge sites may be occupied at saturation coverage, but has still not resolved certain questions about the H/W(110) system. 1 . Impact scattering selection rules for potential adsorbate sites. The listed directions are the intersections of the planes with the (110) surface for the mirror planes and the scattering planes, and the displacement directions for the adsorbate vibrational modes. Modes are assumed to be strictly parallel to the surface. The long bridge site is between two W atoms along the direction, the short bridge site is between two W atoms along the 111 - > direction, and the distorted bridge site is displaced from the long bridge site along the 110 - > direction (ref. 6) The asterisks (*) denote that the scattering amplitude is zero for all directions in the scattering plane, otherwise it is zero only in the specular direction. The 110 - > mode of the distorted bridge is not covered by the selection rules of ref. 2. SITE LONG BRIDGE SHORT BRIDGE DISTORTED BRIDGE MIRROR PLANES [001], [1 1 - 0] NONE [1 1 - 0] 2-FOLD ABOUT Z YES YES NO PARALLEL MODES [001] [1 1 - 0] [1 1 - 1] [ 1 - 12] [001] [1 1 - 0] DIRECTIONS OF ZERO SCATTERING [001] * [1 1 - 0] [001] * [1 1 - 0] [1 1 - 1] [ 1 - 12] [1 1 - 1] [ 1 - 12] [001] * [1 1 - 0] NA

High-throughput optoelectronic interconnect for holographic memory devices

Novel optical memory systems offer ultra large storage capacity and with fast access time. The current commercial system can produce in access of 300 Mb/s aggregate data rate and near future system will yield aggregate data rates on the order of 1-10 Gb/s. However, full exploitation of this feature is possible only if memory to processor interface is fast enough to handle such a data rate. In this presentation, a unique optoelectronic interconnect architecture based on WDM and WDDM are described.

Surface-normal 4 x 4 nonblocking wavelength-selective optical crossbar interconnect using polymer-based volume holograms and substrate-guided waves

We present a 4/spl times/4 surface-normal wavelength-selective nonblocking crossbar using polymer-based volume holograms and substrate-guided waves. A prototype device is demonstrated using the center wavelengths of 750, 780, 810, and 840 nm. The employment of wavelength-division multiplexer and of address coding reduce the required 16 wavelengths to four while maintaining the 4/spl times/4 interconnects. Diffraction efficiencies of 85%, 83%, 79%, and 82% are experimentally confirmed for randomly polarized light at 750, 780, 810, and 840 mm, respectively. The measured crosstalk is less than -24 dB.

Electrocatalytic oxidation of formaldehyde on copper single crystal electrodes in alkaline solutions

The electrocatalytic oxidation of formaldehyde on copper single crystal planes has been investigated in aqueous alkaline solutions. Catalytic activity of the Cu(110) surface appeared to be significantly higher than that of Cu(100) and Cu(111) planes. Explanation of experimental results based on the geometrical arrangement of catalyst centers is proposed.

Multi-mode dense WDDM for data communications

The authors propose a novel dispersion-enhanced WDM structure using a path reversed substrate guided-wave configuration working at a center wavelength of 1555 nm. This dense WDM is expected to be cost-effective and is easy to fabricate and package for data communications.

Two-dimensional wavelength division demultiplexer with surface-normal configuration

A 2D wavelength-division demultiplexing device is demonstrated to separate and to distribute optical signals of different wavelengths by use of substrate-guided wave optical interconnects. In our experiment, two stacked input holographic gratings are fabricated to steer two optical wavelengths into two different routing directions and to zigzag within a waveguiding substrate. Input coupling efficiencies of 70 percent and 76 percent are experimentally confirmed at the input wavelengths of 780 nm and 790 nm, respectively. Two arrays of 1-to-10 cascaded output holographic grating couplers are employed to couple out the optical signals with surface-normal fanouts. The crosstalk is measured to be < -30 dB. The fanout energy fluctuation is within +/- 10 percent for each wavelength.