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Stephen K. Gray

Publications and source records attributed to Stephen K. Gray.

42 records · Page 3Linked to original sources
arXiv2010

Fundamental Behavior of Electric Field Enhancements in the Gaps Between Closely Spaced Nanostructures

We demonstrate that the electric field enhancement that occurs in a gap between two closely spaced nanostructures, such as metallic nanoparticles, is the result of a transverse electromagnetic waveguide mode. We derive an explicit semianalytic equation for the enhancement as a function of gap size, which we show has a universal qualitative behavior in that it applies irrespective of the material or geometry of the nanostructures and even in the presence of surface plasmons. Examples of perfect electrically conducting and Ag thin-wire antennas and a dimer of Ag spheres are presented and discussed.

cond-mat.mes-hall↗
arXiv2010

Theoretical study of the implications of causality when inferring metamaterial properties

The usual procedures to extract effective refractive indices for negative-index metamaterials are complicated by branching ambiguities in the inverse cosine function. The existing methods to eliminate ambiguities either involve calculations with varying geometries which are inherently flawed, as metamaterial properties depend strongly on geometry, or are more subjective as one must inspect and select branches to yield effec- tive parameters as a continuous function of frequency. We propose that the Kramers-Kronig relations, which hold for negative-index materials, naturally guide the selection of the proper branches, and in fact predict negative refractive index where other extraction procedures alone might not. The experimental realization of high quality negative-index materials requires a robust and reliable index retrieval procedure; the combined extraction/Kramers-Kronig retrieval can be used to design optimal metama- terials in a simple, systemmatic, and general manner.

cond-mat.mes-hall↗
arXiv2010

Calculating Nonlocal Optical Properties of Structures with Arbitrary Shape

In a recent Letter [Phys. Rev. Lett. 103, 097403 (2009)], we outlined a computational method to calculate the optical properties of structures with a spatially nonlocal dielectric function. In this Article, we detail the full method, and verify it against analytical results for cylindrical nanowires. Then, as examples of our method, we calculate the optical properties of Au nanostructures in one, two, and three dimensions. We first calculate the transmission, reflection, and absorption spectra of thin films. Because of their simplicity, these systems demonstrate clearly the longitudinal (or volume) plasmons characteristic of nonlocal effects, which result in anomalous absorption and plasmon blueshifting. We then study the optical properties of spherical nanoparticles, which also exhibit such nonlocal effects. Finally, we compare the maximum and average electric field enhancements around nanowires of various shapes to local theory predictions. We demonstrate that when nonlocal effects are included, significant decreases in such properties can occur.

physics.optics↗
arXiv2006

Finite-difference frequency-domain study of subwavelength lensing in left-handed materials

We show that the finite-difference frequency-domain method is well-suited to study subwavelength lensing effects in left-handed materials (LHM's) and related problems. The method is efficient and works in the frequency domain, eliminating the need for specifying dispersion models for the permeability and permittivity as required by the popular finite-difference time-domain method. We show that "superlensing" in a LHM slab with refractive index n = -1 can be approached by introducing an arbitrarily small loss term. We also study a thin silver slab, which can exhibit subwavelength imaging in the electrostatic limit.

physics.optics↗
arXiv2004

Controlled spatiotemporal excitation of metal nanoparticles with femtosecond pulses

We discuss finite-difference time-domain simulations of femtosecond pulses interacting with silver nanowires and nanoparticles. We show how localized hot spots near the metal surfaces can be generated and controlled in a spatiotemporal manner. The control is made possible by chirping the pulses such that the effective frequency passes through surface plasmon resonances associated with different spatial regions of the nanostructure over the course of time. The response of such nanostructures to chirped pulses could provide a novel means of encoding or decoding optical signals.

physics.optics↗