Simple Dipole Model Research Articles

Deep Subwavelength Spatial Characterization of Angular Emission from Single-Crystal Au Plasmonic Ridge Nanoantennas

We use spatially and angle-resolved cathodoluminescence imaging spectroscopy to study, with deep subwavelength resolution, the radiation mechanism of single plasmonic ridge antennas with lengths ranging from 100 to 2000 nm. We measure the antenna's standing wave resonances up to the fifth order and measure the dispersion of the strongly confined guided plasmon mode. By directly detecting the emitted antenna radiation with a 2D CCD camera, we are able to measure the angular emission patterns associated with each individual antenna resonance. We demonstrate that the shortest ridges can be modeled as a single point-dipole emitter oriented either upward (m = 0) or in-plane (m = 1). The far-field emission pattern for longer antennas (m > 2) is well described by two interfering in-plane point dipoles at the end facets, giving rise to an angular fringe pattern, where the number of fringes increases as the antenna becomes longer. Taking advantage of the deep subwavelength excitation resolution of the cathodoluminescence technique, we are able to determine the antenna radiation pattern as a function of excitation position. By including the phase of the radiating dipoles into our simple dipole model, we completely reproduce this effect. This work demonstrates how angle-resolved cathodoluminescence spectroscopy can be used to fully determine the emission properties of subwavelength ridge antennas, which ultimately can be used for the design of more complex and efficient antenna structures.

BROADBAND SPECTROSCOPY USING TWOSUZAKUOBSERVATIONS OF THE HMXB GX 301–2

We present the analysis of two Suzaku observations of GX 301-2 at two orbital phases after the periastron passage. Variations in the column density of the line-of-sight absorber are observed, consistent with accretion from a clumpy wind. In addition to a CRSF, multiple fluorescence emission lines were detected in both observations. The variations in the pulse profiles and the CRSF throughout the pulse phase have a signature of a magnetic dipole field. Using a simple dipole model we calculated the expected magnetic field values for different pulse phases and were able to extract a set of geometrical angles, loosely constraining the dipole geometry in the neutron star. From the variation of the CRSF width and energy, we found a geometrical solution for the dipole, making the inclination consistent with previously published values.

Enhanced resolution in subsurface near-field optical microscopy

We report an experimental analysis of the capabilities of scattering-type scanning near-field optical microscopy for mapping sub-surface features at varying depths. For the first time, we demonstrate experimentally that both the spatial resolution and depth contrast can be improved in subsurface microscopy by demodulating the measured near-field signal at higher harmonics of the probe's tapping frequency and by operating at smaller tapping amplitudes. Our findings are qualitatively supported by a simple dipole model.

Spatial distribution of enhanced optical fields in monolayered assemblies of metal nanoparticles: Effects of interparticle coupling

Near-field two-photon excitation images of assemblies of many gold nanospheres show characteristic feature that enhanced optical fields are confined at the rim parts of the assemblies. In the present report we analyzed the origin of this feature based on finite-difference time-domain (FDTD) approach as well as a simple point dipole model that incorporates the interparticle interaction with the dipole–dipole potential. It has been found that the simple point dipole model is useful for qualitative discussion on the optical field distribution in the metal nanoparticle assemblies. From the analysis, we have found that the interparticle interaction, which causes the propagation of the plasmon excitation in the assemblies, seems to be essential for the localization of the enhanced field at the rim. We propose that regular close-packed assemblies do not yield efficiently enhanced optical fields in visible to near-infrared region, and rather assemblies with large fluctuation are more advantageous to get highly enhanced fields.

Anisotropic properties of plasmonic nanoparticles: depolarized light scattering, dichroism, and birefringence

The properties of gold and silver nanorods and nanodisks with geometric parameters typical of wet chemical fabrication protocols were investigated. With an extended-precision T-matrix code, a representative set of simulations for extinction, light scattering, and depolarization ratio spectra of randomly oriented particles and for dichroism and birefringence spectra of particles with an arbitrary predominant orientation with respect to incident linearly polarized light were obtained. The appearance of ultra-depolarized light scattering from random ensembles was elucidated by several 3D maps illustrating the dependence of the depolarization ratio on particular orientations of rods and disks. A simple dipole model sufficed to explain the orientation dependence of dichroism and birefringence and the unusual invariant spectral localization of the depolarization ratio maxima. Reasonable agreement was obtained with experimental data on depolarized light scattering from gold nanorods.

Performance of an ST-dipole vector model for description of ST deviations in occlusive myocardial ischemia

Background: ST-deviation analysis is a cornerstone in the early triage of myocardial ischemia. The current electrocardiogram (ECG) criteria for the diagnosis of occlusive myocardial ischemia focus on ST elevations, but recent studies have proposed to include both ST depression and ST elevation in the diagnosis, hence considering all ST deviations as projections of a single underlying dipole source. In this study, we examined to which extent a dipole ST vector model accounts for the measured ST deviations in acute ischemia patients. Methods: Forty-five patients with acute myocardial infarction were selected retrospectively for the study to have an equal representation of occlusion sites among the 3 major coronary arteries left anterior descending (LAD), right coronary artery (RCA), and left circumflex (LCX). A 12-lead ECGwas recorded from each patient immediately before coronary intervention. All patients showed occlusive single-vessel disease with thrombolysis in myocardial infarction (TIMI) flow 0/1 (15 LAD, 15 RCA, 15 LCX). ST deviations were measured 60 milliseconds after the J-point in all leads using the 12SL algorithm. Three-dimensional lead vectors for each of the 12 standard leads were defined in accordancewith the spatial direction of the leads in the frontal and horizontal planes. The dipole model was established by considering ST deviations as projections of a single-dipole vector onto each of the defined lead vectors. For each recorded ECG, the best-fitting dipole vector was estimated by minimizing the sum of squared errors between measured and projected ST deviations across all leads.Overall goodness-of-fit (R) for the dipolemodelwas evaluated between measured and projected ST deviations across all recordings. Results: The goodness-of-fit across all recordings was R = 0.82-0.87 (95% confidence interval [CI]). When examining recordings grouped by culprit artery, the model fit was R = 0.75-0.85 (95% CI) for the LAD occlusions, R = 0.83-0.90 (95% CI) for the RCA recordings, and R = 0.78-0.87 (95% CI), for the LCX recordings. Analysis of variance showed no significant difference in goodness-of-fit for the different occlusion sites (P = .19). Conclusion: The single-dipole ST vector accounted for approximately 85% of the ST-deviation information in the studied ECG recordings irrespective of the site of occlusion. A simple dipole model may be a useful descriptor of ST-segment deviations, reducing ST-deviation measurements from 12 leads to a single 3-dimensional vector.

Automated performance analysis of short-vector versus long-vector electrocardiograms

Background: ST-deviation analysis is a cornerstone in the early triage of myocardial ischemia. The current electrocardiogram (ECG) criteria for the diagnosis of occlusive myocardial ischemia focus on ST elevations, but recent studies have proposed to include both ST depression and ST elevation in the diagnosis, hence considering all ST deviations as projections of a single underlying dipole source. In this study, we examined to which extent a dipole ST vector model accounts for the measured ST deviations in acute ischemia patients. Methods: Forty-five patients with acute myocardial infarction were selected retrospectively for the study to have an equal representation of occlusion sites among the 3 major coronary arteries left anterior descending (LAD), right coronary artery (RCA), and left circumflex (LCX). A 12-lead ECGwas recorded from each patient immediately before coronary intervention. All patients showed occlusive single-vessel disease with thrombolysis in myocardial infarction (TIMI) flow 0/1 (15 LAD, 15 RCA, 15 LCX). ST deviations were measured 60 milliseconds after the J-point in all leads using the 12SL algorithm. Three-dimensional lead vectors for each of the 12 standard leads were defined in accordancewith the spatial direction of the leads in the frontal and horizontal planes. The dipole model was established by considering ST deviations as projections of a single-dipole vector onto each of the defined lead vectors. For each recorded ECG, the best-fitting dipole vector was estimated by minimizing the sum of squared errors between measured and projected ST deviations across all leads.Overall goodness-of-fit (R) for the dipolemodelwas evaluated between measured and projected ST deviations across all recordings. Results: The goodness-of-fit across all recordings was R = 0.82-0.87 (95% confidence interval [CI]). When examining recordings grouped by culprit artery, the model fit was R = 0.75-0.85 (95% CI) for the LAD occlusions, R = 0.83-0.90 (95% CI) for the RCA recordings, and R = 0.78-0.87 (95% CI), for the LCX recordings. Analysis of variance showed no significant difference in goodness-of-fit for the different occlusion sites (P = .19). Conclusion: The single-dipole ST vector accounted for approximately 85% of the ST-deviation information in the studied ECG recordings irrespective of the site of occlusion. A simple dipole model may be a useful descriptor of ST-segment deviations, reducing ST-deviation measurements from 12 leads to a single 3-dimensional vector.

Performance of an ST dipole model for description of ST deviations in myocardial ischemia

Background Recent studies have proposed to include both ST depression and ST elevation in the diagnosis of myocardial ischemia, hence, considering all ST deviations relevant. In this study, we examined to which extent an ST dipole model accounts for the measured ST deviations in acute ischemia patients using 2 different sets of lead vectors. Methods 12-Lead electrocardiograms (ECGs) were recorded from 45 patients with occlusive single-vessel disease with thrombolysis in myocardial infarction (TIMI) flow 0/1 (15 anterior descending artery, 15 right coronary artery, 15 left circumflex artery), and ST deviations were measured in the J point in all leads. The dipole model was established by considering ST deviations as projections of a single dipole onto a set of lead vectors. A set of simplified unit length lead vectors and a set of lead vectors from the Dower transformation were compared. For each ECG and model, the best-fitting dipole was estimated by minimizing the sum of squared errors between measured and projected ST deviations across all leads. Results The goodness-of-fit metric ( R 2) for all recordings showed strong agreement between measured and projected ST deviations for both dipole models with R 2 of 0.77 to 0.83 (95% confidence interval [CI]) for the simplified model and R 2 of 0.91 to 0.93 (95% CI) for the Dower-based model. The Dower-based model showed superior performance for the whole population and for each of the occlusion artery subgroups ( P < .05), but only small differences were seen between the estimated ST dipoles from the 2 models. Conclusion A simple dipole model may be a useful descriptor of ST-segment deviations, reducing ST deviation measurements from 12 leads to a single 3-dimensional vector.

Pulse Phase‐Resolved Analysis of the High‐Mass X‐Ray Binary Centaurus X‐3 over Two Binary Orbits

We present a detailed analysis of observations of the high-mass X-ray binary Cen X-3, spanning two consecutive binary orbits performed with the RXTE satellite in early March 1997. During this time Cen X-3 had a luminosity of L2 − 10 keV ~ (4–5) × 1037 ergs s−1 and a pulse period of 4.814 s. The PCA and HEXTE light curves both show a clear reduction in count rate after midorbit for both binary revolutions. We therefore analyze two broadband spectra for each orbit, before and after midorbit. Consistent with earlier observations, these four joint PCA and HEXTE spectra can be well described using a phenomenological pulsar continuum model, including an iron emission line and a cyclotron resonance-scattering feature. While no strong spectral variations were detected, the second half of orbit 2 shows a tendency toward being softer and more strongly absorbed. In order to follow the orbital phase-dependent evolution of the spectrum in greater detail, we model spectra for shorter exposures, confirming that most spectral parameters show either a gradual or sudden change for the second half of the second orbit. A comparison with a simple wind model indicates the existence of an accretion wake in this system. We also present and discuss high-resolution pulse profiles for several different energy bands, as well as their hardness ratios. PCA and HEXTE spectra were created for 24 phase bins and fitted using the same model as in the phase-averaged case. Systematic pulse phase-dependent variations of several continuum and cyclotron line parameters were detected, most notably a significant increase of the cyclotron line energy during the early rise of the main peak, followed by a gradual decrease. We show that applying a simple dipole model for the magnetic field is not sufficient to describe our data.

Secular variation of the geomagnetic dipole during the past 2000 years

We have constructed a very simple model of a time‐varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe. They were also compared with the Hongre et al. (1998) time‐varying spherical harmonic model and with the CALS7K‐2 model by Korte et al. (2005). We find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. In a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to 1000 years ago, and the dipole was actually increasing from 0 until A.D. 500. The dipole moment of CALS7K is lower than the present estimate, probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data. The tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at Paris during the past 2 ka. The results show that the “archeomagnetic jerks” are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field.

Nano-Optical Investigation of Enhanced Field at Gold Nanosphere-Gold Plane Junctions

The local field distribution around gold nanosphere-gold plane junction has been studied using the finite-difference time-domain (FDTD) electrodynamics calculation procedure. We find that both the in-plane and out-of-plane polarized excitation produce enhanced field strong enough to explain the observed SERS activities of the junctions. Comparison with a simple dipole-image dipole model shows that the enhanced field primarily originates from the multipole-image multipole interaction, which indicates that the detailed fine-structures of the nanoparticles also play a significant role in the SERS activities as well.

Effect of through-space electron transfer on infrared spectrum of amorphous selenium

In this paper we present theoretical analyses on an infrared (IR) spectrum of amorphous selenium. The system is described by a 216-atom-chain model, and a set of molecular-dynamics simulations is performed to generate vitreous structures and vibrational modes. To describe an electronic structure of the system we employ a complete neglect of differential overlap model parametrized by ab initio cluster calculations. An IR intensity is evaluated with the Berry-phase formula for an electronic polarization. The effect of the through-space electron transfer on the IR spectrum is studied by artificially changing the magnitude of matrix elements associated with the electron transfer between nonbonded atoms in the chain. We find that the through-space electron transfer leads to (i) the enhancement of the bending IR peak at 135 cm(-1) and (ii) the appearance of a new low-frequency peak around 50 cm(-1), thus resulting in a good agreement with the experiment. The mechanism is discussed by a simple dipole model.

21 Years of Timing PSR B1509−58

We present an updated timing solution for the young, energetic pulsar PSR B1509-58 based on 21.3 yr of radio timing data and 7.6 yr of X-ray timing data. No glitches have occurred in this time span, in contrast to other well-studied young pulsars, which show frequent glitches. We report a measurement of the third frequency derivative of = (-1.28 ± 0.21) × 10-31 s-4. This value is 1.65 σ from (i.e., consistent with) that predicted by the simple constant magnetic dipole model of pulsar spin-down. We measured the braking index to be n = 2.839 ± 0.003 and show that it varies by 1.5% over 21.3 yr owing to contamination from timing noise. Results of a low-resolution power spectral analysis of the significant noise apparent in the data yield a spectral index of α = -4.6 ± 1.0 for the red-noise component.

Contrast and scattering efficiency of scattering-type near-field optical probes

We show that the scattering efficiency and image contrast of commercial Si tips used in “apertureless” scattering-type near-field optical microscopy can compare well with PtIr- or Au-covered tips as predicted by a simple dipole model. However, high scattering efficiency of Si tips is only achieved after etching an unexpected up to 20-nm-thick amorphous SiO2 layer generally found at the tip apex. The oxide was revealed by transmission electron microscopy. The unverified presence of SiO2 may explain contradicting reports published on the optical performance of Si probes. Bare Si tips offer advantages over metal probes due to reduced fluorescence quenching, low nonlinear optical coefficient, and less wear.

Confined Plasmons in Nanofabricated Single Silver Particle Pairs: Experimental Observations of Strong Interparticle Interactions

We report on the optical properties of single isolated silver nanodisks and pairs of disks fabricated by electron beam lithography. By systematically varying the disk size and surface separation and recording elastic scattering spectra in different polarization configurations, we found evidence for extremely strong interparticle interactions. The dipolar surface plasmon resonance for polarization parallel to the dimer axis exhibited a red shift as the interdimer separation was decreased; as expected from previous work, an extremely strong shift was observed. The scattering spectra of single particles and pairs separated by more than one particle radius can be well described by the coupled dipole approximation (CDA), where the particles are approximated as point dipoles using a modified dipole polarizability for oblate spheroids. For smaller particle separations (d < 20 nm), the simple dipole model severely underestimates the particle interaction, indicating the importance of multipolar fields and finite-size effects. The discrete dipole approximation (DDA), which is a finite-element method, describes the experimental results well even at d < 20 nm, including particles that have metallic bridges.

Generation Mechanism of Ulf Electromagnetic Emissions Before the ML = 7.1 Earthquake at Hotan

AbstractAn earthquake ML=7.1 occurred at Hotan in Xinjiang province of China on Oct.19, 1996. Three types of ULF electromagnetic precursors were recorded before the earthquake: (1) Drift of the electic potential, (2) Rotating of the major axis of horizontal polarization ellipse to perpendicular to the station epicenter line, and (3) ULF electromagnetic radiation with large amplitude. A simple dipole model for the generation of these precursors is proposed. Based on this model, we discussed the effect of static electric field inducted by a dipole on the variation of voltage and polarization axis direction.

Cylindrical induction coil to accurately imitate the ideal magnetic dipole

In this work, we optimize the geometry of a cylindrical induction coil in order to improve the imitation of the ideal dipole field. First, we analyze a single-layer coil and find that for an optimum length-to-diameter ratio ( L/ D) of 0.86:1 the imitation error, computed for a four radii distance from the coil’s center, is reduced by a factor of ∼12 (from 9.4 down to 0.73%) relative to a single-turn coil. Second, we analyze two different types of multilayer coils: one having for all the layers the same 0.86:1 optimum L/ D (a coil with a trapezoidal cross section of the winding); and another having for each layer a different L/ D (a coil with a rectangular cross section of the winding). The imitation error in this case is reduced by a factor of ∼22 and ∼33, correspondingly. We finally show that optimizing the multilayer coil makes the imitation error practically negligible (<0.28%) even at as short as four radii distances from the coil’s center. Employing such optimized coil allows one to use the simple dipole model with a high degree of accuracy. It also relieves the system of having to spend extra time for computing the exact values of the coil field.

Solar magnetic field reversal as seen at Ulysses

The rapid motion of the Ulysses spacecraft from high southern to high northern latitudes in 2000–2001 provided an excellent opportunity to make inferences regarding the solar magnetic dipole's behaviour around solar maximum. A simple dipole model is fitted to Ulysses measurements of the polarity of the heliospheric magnetic field mapped back to the solar wind source surface. Although higher order components of the field are ignored, the gradual reversal in orientation of the dipole field component can be followed during solar maximum, with the dipole axis crossing the solar equator during early 2000–early 2001. The dipole appears to exhibit a rotation at a slower rate than the Carrington frame of reference, similar to previous measurements made around solar maximum in the solar equatorial regions.

Electron affinity of plasma-hydrogenated and chemically oxidized diamond (100) surfaces

The electron affinity (EA) $\ensuremath{\chi}$ of single crystal diamond (100) is determined as a function of hydrogen and oxygen coverage by a combination of work function and photoemission experiments. For the fully hydrogenated (100)-$(2\ifmmode\times\else\texttimes\fi{}1):\mathrm{H}$ surface an EA of $\ensuremath{-}1.3 \mathrm{eV}$ and for the oxidized surface C(100)-$(1\ifmmode\times\else\texttimes\fi{}1):\mathrm{O}$ $\ensuremath{\chi}=+1.7 \mathrm{eV}$ are obtained. These are the lowest and the highest electron affinities, respectively, ever reported for any diamond surface. The variation in $\ensuremath{\chi}$ with O and H coverage is well described by a simple dipole model provided that the depolarization is properly taken into account for high adsorbate densities. This analysis favors the bridge position (etherlike) for oxygen on C(100). By mixing H and O adsorbates on a microscopic scale the EA of C(100) can be adjusted at will over 3 eV between the extreme values without jeopardizing the chemical passivation of the diamond surface afforded by H or O termination.

Tracing of proximal lumbosacral nerve conduction – a comparison of simultaneous magneto – and electroneurography

Objective: The reconstruction of nerve impulse conduction along proximal lumbosacral plexus and nerve roots is compared using simultaneous magneto- and electroneurography. Methods: In 3 healthy subjects the left tibial nerve was electrostimulated at the ankle. Evoked magnetic fields and electric surface potentials were measured simultaneously over the lumbosacral spine using a multichannel SQUID-detector with a planar measuring area and 25 surface electrodes covering a comparable area centered around L4. Based on either magnetic field or electric potential maps the depolarization front of the evoked compound action currents (CAC) was spatio-temporally reconstructed using a simple equivalent current dipole model in a half-space volume conductor. Results: The mean signal-to-noise ratio in the magnetic (electric) recordings was around 4 (8). Yet, the localization quality for the propagating CAC was lower for electric than magnetic recordings. The local nerve conduction velocity was around 47 m/s (calculated from magnetic data), but fluctuated unphysiologically for electric data. Conclusion: In comparison to electroneurography, an anatomically reasonable localization of evoked compound action currents propagating in lumbosacral roots can be obtained by magnetoneurography.