Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

We review the basic physics of surface-plasmon excitations occurring at metal/dielectric interfaces with special emphasis on the possibility of using such excitations for the localization of electromagnetic energy in one, two, and three dimensions, in a context of applications in sensing and waveguiding for functional photonic devices. Localized plasmon resonances occurring in metallic nanoparticles are discussed both for single particles and particle ensembles, focusing on the generation of confined light fields enabling enhancement of Raman-scattering and nonlinear processes. We then survey the basic properties of interface plasmons propagating along flat boundaries of thin metallic films, with applications for waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also discussed.

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Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures

Mathematica--A System for Doing Mathematics by Computer.

Carbon (C) is a crucial material for many branches of modern technology. A growing number of demanding applications in electronics and telecommunications rely on the unique properties of C allotropes. The need for microwave absorbers and radar-absorbing materials is ever growing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference among components and circuits, reduction of the back-radiation of microstrip radiators). Whatever the application for which the absorber is intended, weight reduction and optimization of the operating bandwidth are two important issues. A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (C black, C nanotube, C fiber, graphene) and the embedding matrix (rubber, thermoplastic). This paper offers a perspective on the experimental efforts toward the development of microwave absorbers composed of carbonaceous inclusions in a polymer matrix. The absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and volume fraction of the filler particles. Polymercomposites filled with carbonaceous particles provide a versatile system to probe physical properties at the nanoscale of fundamental interest and of relevance to a wide range of potential applications that span radar absorption, electromagnetic protection from natural phenomena (lightning), shielding for particle accelerators in nuclear physics, nuclear electromagnetic pulse protection, electromagnetic compatibility for electronic devices, high-intensity radiated field protection, anechoic chambers, and human exposure mitigation. Carbonaceous particles are also relevant to future applications that require environmentally benign and mechanically flexible materials.

A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles

We review the current status of Waterman's T-matrix approach which is one of the most powerful and widely used tools for accurately computing light scattering by nonspherical particles, both single and composite, based on directly solving Maxwell's equations. Specifically, we discuss the analytical method for computing orientationally-averaged light-scattering characteristics for ensembles of nonspherical particles, the methods for overcoming the numerical instability in calculating the T matrix for single nonspherical particles with large size parameters and/or extreme geometries, and the superposition approach for computing light scattering by composite/aggregated particles. Our discussion is accompanied by multiple numerical examples demonstrating the capabilities of the T-matrix approach and showing effects of nonsphericity of simple convex particles (spheroids) on light scattering.

T-Matrix Computations of Light Scattering by Nonspherical Particles: A Review

The extended boundary condition method is applied to ellipsoidal dielectric scatterers that in general have no rotational symmetry. This represents a more general study of single-object scattering in the resonant range with the goal of extending the practical applications to a wider class of targets, including irregular shapes that can be described in terms of a best-fit ellipsoid. Expressions are presented and calculated results provided for an ellipsoid in the resonant range and comparison is made with an oblate spheroid of approximately the same volume as the ellipsoid. The importance of differences in the surface and internal propagation paths provided by the two scatterers is noted. >

Differential cross section of a dielectric ellipsoid by the T-matrix extended boundary condition method

The explanation of wave behavior upon total internal reflection from a gainy medium has defied consensus for 40 years. We examine this question using both the finite-difference time-domain (FDTD) method and theoretical analyses. FDTD simulations of a localized wave impinging on a gainy half space are based directly on Maxwell's equations and make no underlying assumptions. They reveal that amplification occurs upon total internal reflection from a gainy medium; conversely, amplification does not occur for incidence below the critical angle. Excellent agreement is obtained between the FDTD results and an analytical formulation that employs a new branch cut in the complex "propagation-constant" plane.

https://www.eecs.wsu.edu/~schneidj/journal-papers/gainy.pdf

Amplified total internal reflection: theory, analysis, and demonstration of existence via FDTD

The finite-difference time-domain (FDTD) method is a numerical technique that makes no explicit physical approximations to the underlying problem. The quality of a FDTD-based solution typically is determined by the discretization of the computational domain—the smaller the spacing, the more accurate the solution. Unfortunately, for large computational domains, i.e., ones spanning many wavelengths, the small spatial step size needed to obtain a high-fidelity solution may lead to a prohibitively large number of unknowns. Here it is shown how the FDTD method can be used to model accurately scattering from pressure-release surfaces above a homogeneous water column. To keep the computational cost manageable, a number of enhancements to the standard FDTD algorithm are employed. These enhancements include correcting for numerical dispersion along the specular direction of the incident insonification, using locally conformal cells at the pressure-release boundary, and propagating the field through the homogeneous water column via an analytic method. The accuracy of the FDTD approach is demonstrated by comparison with an integral equation-based reference solution to the same rough surface scattering problem [Thorsos, Proceedings of the Reverberation and Scattering Workshop, pp. 3.2–3.20 (1994) Naval Research Laboratory Book Contribution NRL/BE/7181-96-001].

/pdf/a-finite-difference-time-domain-solution-to-scattering-from-5fcucnwowq.pdf

A finite-difference time-domain solution to scattering from a rough pressure-release surface

Two-dimensional (2-D) TEM horns are modeled using the finite-difference time-domain (FDTD) method. The boundary walls are perfect electric conductors and one wall, which does not align with the Cartesian grid, is approximated using a staircased representation. By carefully comparing the FDTD results to those of the analytic solution, one can make conclusions about the coarseness with which a boundary can be represented. It is found that staircasing errors are small when the staircase diagonal (the hypotenuse of the right triangle created by the stairstep) is smaller than half a wavelength at the highest significant frequency in the excitation. This rule-of-thumb is put forward as a necessary condition for the discretization of general problems. Results are also provided for some simple FDTD schemes that are designed to reduce staircasing errors. By using large aspect-ratio cells, a grid can be constructed that satisfies the rule-of-thumb given above. While this approach eliminates general staircasing errors, some errors persist owing to the presence of step discontinuities immediately adjacent to the horn feed. These errors can be further reduced by using a cell-splitting approach. It is shown that the contour path FDTD technique can be used to eliminate nearly all staircasing errors, while some additional improvement is shown to be provided by using a stabilized contour path FDTD approach. Finally, a recently proposed conformal technique that permits simple implementation is shown to provide results comparable with those of the stabilized contour path approach.

https://www.eecs.wsu.edu/~schneidj/journal-papers/horn.pdf

FDTD simulations of TEM horns and the implications for staircased representations

Near-to-far-field transformations require the tangential electric and magnetic fields over a surface, which we call the integration boundary. However, the staggered nature of the finite-difference time-domain grid is problematic in that the electric and magnetic fields are not collocated in either space or time. For harmonic transformations, i.e., ones which rely upon a Fourier transform of the time-domain near-fields, one can account for the temporal offset with a simple phase correction in the frequency domain. To account for spatial offsets, previously an arithmetic mean of the time-domain fields to either side of the integration boundary has been used. Here we show that superior results are obtained by instead using a geometric mean of the harmonic fields to either side of the integration boundary.

John B. Schneider

Papers

Differential cross section of a dielectric ellipsoid by the T-matrix extended boundary condition method

Amplified total internal reflection: theory, analysis, and demonstration of existence via FDTD

A finite-difference time-domain solution to scattering from a rough pressure-release surface

FDTD simulations of TEM horns and the implications for staircased representations

On the Use of the Geometric Mean in FDTD Near-to-Far-Field Transformations