Electrostatic and optical resonances of cylinder pairs

We study the polarizability α(σ) of a pair of identical cylinders, each of dielectric constant σ and immersed in a medium of unit dielectric constant. In the case of non-touching cylinders, the spectrum of resonant solutions is shown to be discrete, whereas when the cylinders touch it is continuous, and corresponds to a branch-cut of α(σ). This behaviour is compared with that of the spectrum of ε(σ), the dielectric constant of an infinite array of cylinders. We derive a simple expression for α(σ), and use it to obtain ε(σ) for a dilute array of touching cylinder pairs. The resulting formula is shown to have properties differing from those of the widely-used Maxwell-Garnett and effective medium theories, and consequently it may prove useful in studies of solar-selective columnar cermets.     

Transport properties of arrays of intersecting cylinders

Formalisms are presented which enable the accurate calculation of the polarizability of an isolated square prism, and of the transport properties of square arrays of both circular cylinders and square prisms, over the entire range of volume fractions. For volume fractions close to zero and to unity, the results are shown to agree with dipolar formulae based on the appropriate polarizability. For volume fractions close to touching, the transport property varies rapidly but smoothly through the multipolar region. The results are shown to be in good qualitative agreement with experimental curves.     

Electrostatic and electromagnetic surface shape resonances

The homogeneous integral equations that give the electromagnetic field in the vicinity of a protuberance or a depression on the otherwise planar interface with vacuum of a semi-infinite dielectric medium or a thin dielectric film on a semi-infinite substrate have been obtained. The Rayleigh hypothesis, the vectorial equivalent of the Kirchhoff integral, and the extinction theorem have been used for this purpose. The assumption that the perturbation of the vacuum dielectric interface has cylindrical symmetry about the normal to the nominal surface allows a significant simplification of these integral equations to be carried out. We have used Gaussian quadrature schemes to convert the resulting integral equations into matrix equations, and have obtained the frequencies of the shape resonances by equating to zero the determinants of the matrices obtained. Calculations have been carried out for Gaussian (x ₃=Aexp(–x ² /R ²)) and exponential (x ₃=Aexp(–x /R)) surface profiles, and convergent results obtained for values ofA/R of the order of unity.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Electrostatic spray deposition of spinel Li4Ti5O12 thin films for rechargeable lithium batteries

Spinel Li₄Ti₅O₁₂ thin films are important for the fabrication of rechargeable lithium microbatteries. Porous thin films of Li₄Ti₅O₁₂ were prepared by electrostatic spray deposition (ESD) technique with lithium acetate and titanium butoxide as the precursors. The structures of these films were analyzed by scanning electron microscopy and X-ray diffraction. Coin-type cells with a liquid electrolyte were made with the Li₄Ti₅O₁₂ films against metallic lithium. Their electrochemical performance was investigated by means of galvanostatic cell cycling, cyclic voltammetry and Ac impedance spectroscopy. It was found that pure spinel phase of Li₄Ti₅O₁₂ was obtained. After annealing at the optimal temperature of 700 °C, the films can deliver a reversible specific capacity of about 150 mAh/g with excellent capacity retention after 70 cycles. Their electrochemical characteristics were quite comparable with those of the Li₄Ti₅O₁₂ laminate electrodes containing carbon black additive.     

Dielectric resonances of ordered passive arrays

The electrical and optical properties of ordered passive arrays, constituted of inductive and capacitive components, are usually deduced from Kirchhoffs rules. Under the assumption of periodic boundary conditions, comparable results may be obtained via an approach employing transfer matrices. In particular, resonances in the dielectric spectrum are demonstrated to occur if all eigenvalues of the transfer matrix of the entire array are unity. The latter condition, which is shown to be equivalent to the habitual definition of a resonance in impedance for an array between electrodes, allows for a convenient and accurate determination of the resonance frequencies, and may thus be used as a tool for the design of materials with a specific dielectric response. For the opposite case of linear arrays in a large network, where periodic boundary condition do not apply, several asymptotic properties are derived. Throughout the article, the derived analytic results are compared to numerical models, based on either Exact Numerical Renormalisation or the spectral method.     

Electronic and hole minibands in quantum wire arrays of different crystallographic structure

We consider semiconductor heterostructures consisting of GaAs rods embedded in Al_xGa_1 − xAs and disposed in sites of a square or triangular lattice. The electronic and hole spectra around the conduction band bottom and the valence band top are examined for electrons and holes propagating in plane of periodicity, versus geometry of the lattice formed by the rods, concentration of Al in the matrix material, and structural parameters including the filling fraction and the lattice constant. Our calculations use the envelope function and are based on the effective-mass approximation. We show that the electronic and hole spectra resulting from the periodicity of the heterostructure, depend on the factors considered and that the effect of lattice geometry varies substantially with lattice constant. For low lattice constant values the minigaps are significantly wider in the case of triangular lattice, while for high lattice constant values only slightly thiner minigaps occur in the square lattice-based arrays. We discuss the consequences of our findings for the efficiency of solar cells.     

Singularity expansions for cylinders of finite conductivity

A new rigorous differential formulation to compute the scattering matrix of an obliquely oriented cylinder of finite conductivity and arbitrary shape is given. The analytic continuation of the scattering operator is examined in the frequency domain as well as in the propagation constant domain. The corresponding computer program is able to compute the singularity expansion for such a cylinder, as well as the propagating mode along it. Numerical examples are given.     

Symmetry group and natural resonances for a cylinder of finite conductivity

The connection of the natural resonances and the symmetry group of the cross-section of the diffracting-cylinders is established. As is done in spectroscopy, selection rules for the propagation directions of the diffracted field are deduced. Some remarks about the case of propagating modes along a dielectric rod are also made.     

Cavity resonances and mutual coupling in concave waveguide arrays

A method is presented for the mutual coupling analysis of concave, circular cylindrical or spherical waveguide arrays. The array is considered as a set of waveguides that are coupled through a prefectly conducting, lossless cavity, which leads to an equivalent multiport network. The corresponding scattering matrix is obtained by matching the cavity and waveguide fields over the guide apertures and solving the resulting equation by Galerkin's method. In particular the complications are discussed, which arise from the possibly large number of degenerate, resonant cavity modes. A result obtained is the general condition that at a resonance the total aperture field (for all waveguide apertures) must be orthogonal to the set of degenerate, resonant cavity modes. This leads, with certain exceptions, to zero coupling between the guides whenever the total number of aperture modes (for all waveguides) is less than or equal to the number of resonant cavity modes.     

Acoustical scattering by arrays of cylinders in waveguides

Multiple scattering of acoustic waves in a planar horizontal waveguide by finite-length cylinders is considered. Cylinder height equals the waveguide depth, and both are vertically constrained by the pressure-release boundaries. An analytically exact solution is obtained via normal mode expansion method in conjunction with the concept of the T matrix. The problem is decomposed into an infinite number of two-dimensional multiple scattering problems, modulated by waveguide mode shapes. Examples are presented for an isovelocity waveguide in which the medium is uniform and the waveguide depth is constant. It is found that, in numerical computations, including one or two evanescent modes captures the essence of the evanescent modes. Multiple scattering in the waveguide is compared with the corresponding two-dimensional case. It is concluded that, in low frequencies, the wave patterns in the two cases are very similar, with a shift in the frequency. The similarity diminishes when there are multiple propagating modes. Despite the mode mixing, some key features in the scattering as observed in the two-dimensional problem remain observable in the waveguide case.     

Coupled-channels optical calculation of positron-hydrogen resonances

An application of the coupled-channels optical method is given for the energy-dependent phenomena of positron-hydrogen resonances below the n=2 excitation threshold.The equivalent local optical potential is used to account for the target polarization and positronium formation.The calculation includes 9 explicitly physical coupled channels.The lowest S-wave resonance energy position and new resonances are found.Angular dependence of the cross section in the resonance region are investigated.     

Electrostatic surface shape resonances of a finite number of ridges

The resonance properties of localized electrostatic surface modes associated with a finite number of ridges on an otherwise planar surface are investigated. Numerical solutions of the homogeneous integral equations that describe the electromagnetic fields in the vicinity of the ridges are used to obtain the dispersion relation of surface plasmons. The frequencies of the electrostatic surface shape resonances are calculated for ridges with Gaussian, Lorentzian, sinusoidal, exponential, and triangular profiles. We show the existence of splittings of the plasmon frequencies, which depends on the surface profile function and on the distance between the ridges. Considering the ridge with a sinusoidal profile, we obtain the limit on the number of ridges which generates a frequency splitting of the electrostatic surface shape resonances, whose frequency values converge to those of the dispersion relation of surface plasmons on one-dimensional sinusoidal grating. Received: 24 June 1997 / Received in final form: 5 September 1997 / Accepted: 15 September 1997     

Boundary-value problems in near-field optical microscopy and optical size resonances

We have solved a boundary-value problem for a ball probe interacting with a flat dielectric surface in an external optical radiation field. This interaction gives rise to the optical size resonance at frequencies significantly different from the natural frequencies of two-level atoms both in the medium and in the probe with allowance for the local field corrections. These resonances depend significantly on the distance from the probe center to the surface, on the ball probe size, on the concentration of two-level atoms in the probe and in the medium, on the spectral line width, and on the atomic inversion. The field strengths inside and outside the ball probe and a semiinfinite dielectric medium have been calculated in the near-field and wave zones. It is shown that the proposed electrodynamic theory of optical near-field microscopy agrees with the results of experimental measurements.     

Dynamics of arrays of cylinders with internal and external axial flow

Various aspects of the dynamics and stability of clusters of tubular cylinders containing internally flowing fluid and surrounded by a bounded external axial flow are examined. The general character of free motions is established by obtaining the eigenfrequencies of the system and studying their evolution with increasing flow, internal or external. Stability diagrams have been obtained for the critical flow velocities, beyond which the system would lose stability by buckling (divergence), under the combined effect of internal and external flow. Free vibration, following an initial disturbance of one of the cylinders, is studied, in order further to examine the effect of hydrodynamic coupling. It is found that beating phenomena may arise, implying energy transfer between cylinders and the possibility of transient amplitudes much larger than the initial disturbances. Also, the vibration of the system (in still fluid) when one cylinder is constrained to oscillate in a prescribed manner is examined, establishing that transmission of vibration from cylinder to cylinder can be very rapid; indeed, such constrained motion of one cylinder at certain frequencies may induce large amplified motions of others.     

Coupled-channels optical calculation of positronben hydrogen resonances

An application of the coupled-channels optical method is given for the energy-dependent phenomena of positron-hydrogen resonances below the n=2 excitation threshold. The equivalent local optical potential is used to account for the target polarization and positronium formation. The calculation includes 9 explicitly physical coupled channels. The lowest S-wave resonance energy position and new resonances are found. Angular dependence of the cross section in the resonance region are investigated.     

Tunneling resonances and coherence in an optical lattice

The center-of-mass quantization of atoms trapped in a gray optical lattice is observed to manifest itself in the steady-state properties of the atoms. Modulations in the lifetime and macroscopic magnetization as a function of an applied B field are attributed to quantum mechanical tunneling resonances and are shown to exist only under conditions which afford spatial coherence of the trapped atoms over several lattice wells and coherence times that exceed the tunneling period.     

阵列光镊的发展与应用

在生命科学研究中和在微量液体环境下分离液体中的细胞、生物大分子或胶体颗粒一直是一项具有挑战性的工作。"光镊"技术自20世纪80年代被提出到现在,在生命科学研究领域已经得到了日益广泛的运用。激光对细胞捕获的作用已得到进一步扩展,二维"光镊阵列"技术是近年来"光镊"技术中最重要的发展之一。讨论了阵列光镊的发展现状及基本原理,分析了它在生命科学中的应用,并对其发展趋势进行了展望。     

Hydrogenated microvoids and light-induced degradation of amorphous-silicon solar cells

Amorphous-silicon solar cells generally exhibit a degradation of conversion efficiency in the first few weeks of exposure to sun light. This degradation is associated with metastable defect centers that influence many of the properties of hydrogenated amorphous-silicon. This paper discusses a model for these metastable centers that is based on the assumption that holes can be trapped near microvoids and can induce the motion of hydrogen on the internal surfaces of the microvoids.     

Metastructural systems of activated nanospheres and optical near-field resonances

The near-field interaction of two spherical nanoparticles containing dense ensembles of two-or multilevel atoms in an external field of optical low-intensity radiation is shown to result in the formation of resonances whose frequencies differ considerably from the transition frequencies in the spectrum of the interacting atoms. Optical near-field resonances are shown to play an important role in metastructural systems composed of activated nanospheres. The reflectance of a metastructural system of activated nanospheres oriented along a certain direction depends strongly on the polarization and the frequency of external radiation, as well as on the concentration of impurity atoms inside the nanospheres and on their sizes.     

Electrostatic resonance of clusters of dielectric cylinders: A finite element simulation

Based on the finite-element method, it is demonstrated that the electrostatic resonance features of a range of heterostructures made of clusters of dielectric cylinders depend sensitively on the shape and arrangement of the inclusions in the background matrix, and on the polarization of the applied electric field.