Metallic Spheroids Research Articles

Extending the applicability of the T-matrix method to light scattering by flat particles on a substrate via truncation of sommerfeld integrals

The simulation of light scattering by particles on a substrate with the T-matrix method relies on the expansion of the scattered field in spherical waves, followed by a plane wave expansion to allow the evaluation of the reflection from the substrate. In practice, the plane wave expansion (i.e., the Sommerfeld integrals) needs to be truncated at a maximal in-plane wavenumber κmax. An appropriate selection of κmax is essential: counter-intuitively, the overall accuracy can degrade significantly if the integrals are truncated with a too large value. In this paper, we propose an empirical formula for the selection of κmax and discuss its application using a number of example simulations with dielectric and metallic oblate spheroids on dielectric and metallic substrates. The computed differential scattering cross sections are compared to results obtained from the discrete-sources method.

Investigation of particle shape and size effects in SERS using T-matrix calculations

The influence of particle size and shape effects on average and punctual surface-enhanced Raman scattering (SERS) enhancement factors (EFs) is investigated using exact T-matrix electrodynamic calculations of silver and gold spheroids over a large parameter space. This study extends the conventional treatment of these effects within the frameworks of the electrostatics approximation, its generalizations, or Mie theory for spheres. It confirms the qualitative features of these approaches, but provides in addition quantitative predictions of SERS EFs in the case of large non-spherical particles, where the lightning-rod effect (shape effect) and radiation damping (size effect) operate simultaneously. Finally, the localization effect at large SERS EF (hot-spots) is shown to be dictated only by shape, not size, in the case of metallic spheroids at the dipolar localized surface plasmon resonance.

From slow to superluminal propagation: Dispersive properties of surface plasmon polaritons in linear chains of metallic nanospheroids

We consider propagation of surface plasmon polaritons (SPPs) in linear periodic chains (LPCs) of prolate and oblate metallic spheroids. We show that the SPP group velocity can be efficiently controlled by varying the aspect ratio of the spheroids. For sufficiently small aspect ratios, a gap appears in the first Brillouin zone of the chain lattice in which propagating modes do not exist. Depending on the SPP polarization, the gap extends to certain intervals of the Bloch wave number $q$. Thus, for transverse polarization, no propagating SPPs exist with wave numbers $q$ such that ${q}_{c}^{\ensuremath{\perp}}<|q|<\ensuremath{\pi}/h$, $h$ being the chain period. For longitudinally polarized SPPs, the gap spans the interval $|q|<{q}_{c}^{\ensuremath{\parallel}}$. Here ${q}_{c}^{\ensuremath{\perp}}$ and ${q}_{c}^{\ensuremath{\parallel}}$ are different constants, which depend on the chain parameters, spheroid aspect ratio, and its orientation with respect to the chain axis. The dependence of the dispersion curves on the spheroid aspect ratio leads to a number of interesting effects. In particular, bandwidth of SPPs that can propagate in an LPC can be substantially increased by utilizing prolate or oblate spheroids. When $q$ is close to a critical value, so that $|q\ensuremath{-}{q}_{c}^{\ensuremath{\perp}}|⪡\ensuremath{\pi}/h$ or $|q\ensuremath{-}{q}_{c}^{\ensuremath{\parallel}}|⪡\ensuremath{\pi}/h$, the decay length of the SPPs is dramatically increased. In addition, the dispersion curves acquire a very large positive or negative slope. This can be used to achieve superluminal group velocity for realistic chain parameters. We demonstrate superluminal propagation of Gaussian wave packets in numerical simulations. Both theory and simulations are based on Maxwell equations with account of retardation and, therefore, are fully relativistic.

Subwavelength lasers

Composite media comprised of a host with amplification and high aspect ratio metallic spheroids exhibit a dielectric resonance which is accompanied by both a gigantic index of refraction and large collective gain. Expressions for the complex index of refraction near this dielectric anomaly are derived for the case of oriented spheroids in an amplifying host. The large index of refraction and gain can be used to satisfy the laser cavity resonance condition in nanoscale subwavelength structures with EH11 transverse modes allowing for the future miniaturization of laser devices.

Surface-plasmon resonances in indium nitride with metal-enriched nano-particles

Plasmonic resonances in In-enriched nano-particles, spontaneously formed during growth, can dramatically modify optical properties of InN. Experimental support for this is provided from detailed studies of absorption and infrared emission in InN. In particular, thermally detected optical absorption and photoluminescence excitation spectroscopy reveal a peak below the region of strong absorption in InN. A higher-energy part of the infrared emission having a noticeable p-polarization is markedly enhanced with excitation along the surface. These peculiarities are discussed in terms of the Mie resonances, arising in metallic spheroids with different aspect ratio, and their coupling with recombining states, whose strength depends on energy separation between the states and the resonances.

Layer-multiple-scattering method for photonic crystals of nonspherical particles

We present an extension of the layer-multiple-scattering method to photonic crystals of nonspherical particles in a homogeneous host medium. The efficiency of the method is demonstrated on a specific example of a crystal of metallic spheroids. We report a thorough analysis of the optical properties of this crystal and discuss aspects of the underlying physics that relate to the nonspherical shape of the particles.

Exact radiation for dipoles on metallic spheroids at the interface between isorefractive half-spaces

Oblate and prolate metallic spheroids located at the interface between isorefractive half-spaces are considered. The electromagnetic fields produced by electric and magnetic dipoles located on the symmetry axis of the structure are determined exactly. Particular cases are discussed, and numerical results are presented for far fields and surface currents.

Biomonitoring of air quality in the Cologne conurbation using pine needles as a passive sampler—Part I: magnetic properties

High resolution temporal and spatial control of atmospheric pollutants is of crucial importance for environmental health monitoring. Passive sampling using natural vegetation biomonitoring allows acquisition of well-defined samples at affordable costs. We here present results from a study conducted in the conurbation of Cologne, Germany, based on airborne pollutants accumulated on pine needles. This integrated study includes (i) the microscopic analysis of pine needles and analysis of their magnetic properties, (ii) PAH, and (iii) selected trace elements (Fe, Cd, Pb, Ni, Cr, Cu). A major proportion of atmospheric pollutants is bound to particles, for which in part I of the study we present data on magnetic susceptibility, remanence measurements (IRM, ARM) and total Fe content. SEM-analysis indicates that particles accumulated on needles are mostly <2.5 μm in diameter and comprise pollen or spores, mineral dust and silica-glassy or metallic spheroids. The latter were identified as magnetite with minor pyrrhotite. These particles derive from combustion of coal in power plants or fuels in vehicular engines. A close correlation of magnetic properties (susceptibility, SIRM, ARM) and Fe content shows that non-destructive, time-efficient enviromagnetics of needles serves as an excellent proxy for biomonitoring of combustion pollutants. Their spatial distribution within the conurbation of Cologne was determined for 43 locations integrated in a GIS-database. The dominant sources of fine metallic particulates (PM 2.5 ) are emissions from road traffic, including fuel combustion, corrosion and brake-wear and from railroad and tram traffic preferentially due to material wear. Parks, forests and agricultural areas show the lowest levels of pollution by magnetic particles, followed by residential areas. This implies that traffic emissions with short transportation distances (<1.0 km) are dominant in the Cologne conurbation, whereas the contribution from power plants is negligible.

Stable nickel isotopes and cosmogenic beryllium‐10 and aluminum‐26 in metallic spheroids from Meteor Crater, Arizona

Abstract— The isotopic abundances of Ni in 17 metallic spheroids from Meteor Crater, Arizona, were determined by inductively coupled plasma mass spectrometry (ICP‐MS). Sixteen spheroids have normal isotopic abundances. A 17th shows a marginally detectable mass fractionation of 0.40 ± 0.14 %/AMU in favor of the heavier isotopes. The general absence of mass fractionation indicates that open system evaporation caused little loss of Ni.Variable activities of the cosmogenic radionuclides 10Be and 26Al were measured by accelerator mass spectrometry in separate suites of spheroids. Activities of 26Al in most samples and of 10Be in metal cores separated from spheroids indicate that they either (1) come from greater depths in the parent meteoroid than do hand specimens, or (2) lost Al and Be during the process of spheroid formation. One individual spheroid has 10Be and 26Al activities comparable to those of bulk specimens. This result suggests that spheroid formation may occasionally include material from the outermost meter or so of the impactor. Relatively high activities of 10Be, ∼3 dpm/kg, in the siliceous shells of Canyon Diablo spheroids very likely have a meteoric origin.

Observations of impact-induced molten metal-silicate partitioning

Observations of molten mid-ocean ridge basalt (MORB)-molybdenum (Mo) interactions produced by shock experiments provide insight into impact and differentiation processes involving metal-silicate partitioning. Analysis of fragments recovered from experiments (achieving MORB liquid shock pressures from 0.8 to 6 GPa) revealed significant changes in the composition of the MORB and Mo due to reaction of the silicate and metal liquids on a short time scale ( < 13 s). The FeO concentration of the shocked liquid de creases systematically with increasing pressure. In fact, the most highly shocked liquid (6 GPa) contains only 0.1 wt% FeO compared to an initial concentration of 9 wt% in the MORB. We infer from the presence of micrometer-sized Fe-, Si- and Mo-rich metallic spheres in the shocked glass that the Fe and Si oxides in the MORB were reduced in an estimated oxygen fugacity of 10 −17 bar and subsequently alloyed with the Mo. The in-situ reduction of FeO in the shocked molten basalt implies that shock-induced reduction of impact melt should be considered a viable mechanism for the formation of metallic phases. Similar metallic phases may form during impact accretion of planets and in impacted material found on the lunar surface and near terrestrial impact craters. In particular, the minute, isolated Fe particles found in lunar soils may have formed by such a process. Furthermore, the metallic spheres within the shocked glass have a globular texture similar to the textures of metallic spheroids from lunar samples and the estimated, slow cooling rate of ⩽ 140°C/s for our spheres is consistent with the interpretation that the lunar spheroids formed by slow cooling within a melted target.

Simple treatment of the enhancement of Raman scattering due to a two-dimensional array of metallic spheroids.

A simple naive treatment of the enhancement of Raman scattering due to the presence of a two-dimensional array of metallic spheroids has been worked out as an extension of the existing particle-plasmon model. Calculated results, which are based on the ${r}^{\mathrm{\ensuremath{-}}3}$ term of dipolar fields, conform to the experiment. Comparisons and discussions show that this treatment, rather complete and accurate within the ${r}^{\mathrm{\ensuremath{-}}3}$ regime, provides the basic understanding of the physics of the spheroidal array experiment and also can serve to justify the existing single-spheroid approximations.

Identification of Metallic Spheroids by Classification of Their Electromagnetic Induction Responses

An investigation into the feasibility of applying pattern recognition concepts to the classification of metallic objects by their electromagnetic induction responses was performed. The effect on the response of a limited set of steel spheroids due to various factors such as object shape, size, and orientation was examined and a pattern recognition scheme based on these results was proposed. Implementation of the scheme involved the development of a novel extension to the nearest mean vector type of classifier in which the concept of the class mean as a point in feature space was generalized to be a curve. The resultant pattern recognition scheme was tested on a representative test set which included 815 responses, corresponding to 104 variations in object and orientation. A success rate of greater than 96 percent was achieved. It is noted that the classifier extension developed provides a viable approach to classification of responses that very continuously with respect to a single parameter.

SULPHUR ISOTOPE RATIOS IN THE CANYON DIABLO METALLIC SPHEROIDS*

Sulphur contents and δ34S values have been determined for metallic spheroids collected from the vicinity of Meteor Crater, Arizona. Large spheroids with diameters ∼ 1.3 mm contain about 20% more sulphur than do small spheroids with diameters ∼ 0.5 mm. The mean δ34S value for large spheroids is + 0.23°/∞, while that for small spheroids is + 0.4°/∞, both measured relative to Canyon Diablo troilite. Abrasion experiments on large spheroids show that δ34S varies radially within them, with values of ∼ 1.9δ/∞ at their surfaces, falling to zero at their centres. The favoured explanation for the observed sulphur content and δ34S patterns is low temperature oxidation during post‐formation weathering.

The chemical composition of metallic spheroids and metallic particles within impactite from Barringer Meteorite Crater, Arizona

Atomic absorption analyses of 25 metallic spheroids from Barringer Meteorite Crater have been carried out for Fe, Ni, Co and Cu. In addition, electron microprobe analyses of 58 impactite metallic particles have been carried out for Fe, Ni and Co from four different impactite samples. The normalized Ni, Co and Cu contents of the spheroids were from 13–22 per cent, 0.8–1.3 per cent and 260–430 μg/g, respectively. These figures represent enrichment factors of 2–3 in the spheroids compared to analyses of the bulk meteorite. The Co/Ni and Cu/Ni ratios in the spheroids are close to the respective ratios in the bulk meteorite. This suggests that the spheroids were formed by preferential removal of iron by oxidation from a chemically homogeneous liquid. The impactite metallic particles had Ni contents from 10 to 95 per cent and Co contents from 0·3 to 4 per cent. The Co contents of these particles showed a positive correlation with Ni up to 60 per cent Ni and a negative correlation beyond 60 per cent Ni. Reaction of the impactite metallic particles with SiO 2 of the target can explain these variations. Our findings show that extensive chemical reaction between projectile and target occurs at impact.

Investigation of the Canyon Diablo metallic spheroids and their relationship to the breakup of the Canyon Diablo meteorite

Metallic spheroids approximately 1 mm in diameter and having a dendritic structure appropriate to a rapidly solidified liquid alloy are encountered in conjunction with Canyon Diablo meteorites in the vicinity of the Barringer crater, Arizona. Compositionally and structurally similar particles have been encountered in lunar soils returned from Apollo 11, 12, 14, and 15 expeditions. The present study is devoted entirely to Canyon Diablo spheroids, which are shown by means of electron microprobe techniques to be enriched in Ni and considerably enriched in S and P relative to the metallic phases of Canyon Diablo meteorites. In contrast to earlier theories that considered only selective oxidation, a process of formation is now put forward involving the earlier stage of the shock-induced melting of sulphide-rich areas in the meteoritic projectile when it hits the earth. This process is also applicable to metallic spheroids formed on the moon, where oxidation processes will be minimal. Measuring the interarm spacing of the dendrites leads to estimates of 500°–30,000°C/sec for the rates at which these spheroids cooled through the solidification range. The breakup of the Canyon Diablo iron meteorite projectile is described by a series of events involving increasing shock pressures.

The Preparation of Metallic Prolate Spheroids

A technique is described for the electrical discharge machining of prolate spheroid specimens from metallic rods using conventionally milled electrodes. Specimens of Nb, Ta, Re, and Nb–Ta alloys have been satisfactorily prepared. For single crystals of Nb, x-ray studies indicate the absence of any recrystallization after heat treatment of the prolate spheriods to remove strains.

The analysis of metallic spheroids from Meteor Crater, Arizona

The analysis of metallic spheroids from Meteor Crater, Arizona