arXiv subjects

Gino Wegner

Publications and source records attributed to Gino Wegner.

7 recordsLinked to original sources

arXiv2026

Nonconcentric Multi-shell Nanowires: Geometry-Induced Plasmon Hybridization and Near-Field Localization

Localized surface plasmons (LSPs) in multi-shell nanostructures provide a versatile route for controlling optical fields at the nanoscale, yet the influence of deviations from concentric geometries remains insufficiently understood. Here, we investigate the impact of shell nonconcentricity on the quasistatic optical response of core-single-shell and core-multi-shell nanowires. Exploiting the conformal properties of bipolar coordinates, we derive analytical solutions for nonconcentric cylindrical interfaces and systematically analyze the evolution of LSP resonances, absorption spectra, and near-field distributions. Starting from single-shell structures, we show that nonconcentricity enables finite coupling of incident radiation to higher-order plasmon modes that are optically inactive in the quasistatic concentric limit. Extending the analysis to multi-shell bull's eye wires, we identify how shell thickness, number of fixed-thickness shell units, each defining a set of a dielectric and metal shell, as well as interface nonconcentricness shape the hybridized plasmon spectrum. Increasing the number of metal-dielectric interfaces broadens the spectral response, while nonconcentric geometries additionally increase the density of accessible resonances and localize electromagnetic fields preferentially within and around the thinner shell sections. Eventually, comparison of concentric with bipolar and Doppler-grating-inspired nonconcentric bull's eye wires based on Mie theory and full-wave Discontinuous Galerkin Time-Domain simulations, respectively, allows to assess the impact of nonconcentricness for typical nanowire dimensions. These results provide insight into geometry-induced plasmon hybridization and suggest routes toward nanoscale control of optical energy localization for applications in active nanophotonics and plasmon-assisted photochemistry.

physics.optics↗

arXiv2026

Optical extinction and near-field properties of plasmonic dimers: Role of particle shape and separation

Localized surface plasmons (LSPs) supported by metallic nanostructures exhibit a great tunability of their resonance frequencies and associated spatial field distributions. In particular, nanoscale gaps and sharp corners exhibit promising near-field enhancements for applications such as biochemical sensing and nanoantennas. In this work, we investigate the optical properties of bowtie nanowire dimers excited by linearly polarized light, including polarization orthogonal to the dimer axis. To distinguish dimer-separation-induced and monomer-shape-induced effects, we compare bowtie, circular-cylindrical dimer and triangular wires. We emphasize the advantages of a Bézier-type corner parametrization with tunable curvature and employ the discontinuous-Galerkin time-domain finite-element method for numerical calculations. This enables the identification of hybrid resonances and the systematic analysis of the role of geometrical parameters. For all investigated geometries, we compare the spatially local Drude model with the spatially nonlocal Halevi model. For the specific case of silver, we demonstrate quantitative curvature-dependent LSP line shifts arising from the longitudinal nonlocality captured by the Halevi model. For higher-order LSPs, this nonlocality gives rise to a sequence of resonances that may be exploited for light-harvesting applications. Furthermore, we investigate the field enhancement associated with LSP resonances and, motivated by the occurrence of both hot and cold spots, introduce geometry-dependent measures for assessing the ability of nanostructures to enhance nonlinear optical effects, such as through the optimal positioning of SERS-active molecules. To facilitate the simultaneous study these measures, we propose the use of suitable radar charts.

physics.optics↗

arXiv2025

Roadmap on Nonlocality in Photonic Materials and Metamaterials

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally made in optical material modeling. On one end, the growing interest in plasmonic, polaritonic and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials--metamaterials and metasurfaces--can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In the case of metasurfaces, in particular, nonlocality engineering has become a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward--toward new scientific discoveries and technological advancements.

cond-mat.mes-hall↗

arXiv2022

Halevi's extension of the Euler-Drude model for plasmonic systems

The nonlocal response of plasmonic materials and nanostructures is usually described within a hydrodynamic approach which is based on the Euler-Drude equation. In this work, we reconsider this approach within linear response theory and employ Halevi's extension to this standard hydrodynamic model. After discussing the impact of this improved model, which we term the Halevi model, on the propagation of longitudinal volume modes, we accordingly extend the Mie-Ruppin theory. Specifically, we derive the dispersion relation of cylindrical surface plasmons. This reveals a nonlocal, collisional damping term which is related to earlier phenomenological considerations of limited-mean-free-path effects and influences both, peak width and amplitude of corresponding resonances in the extinction spectrum. In addition, we transfer the Halevi model into the time-domain thereby revealing a novel, diffusive contribution to the current which shares certain similarities with Cattaneo-type currents and analyze the resulting hybrid, diffusive-wave-like motion. Further, we discuss the relation of the Halevi model to other approaches commonly used in the literature. Finally, we demonstrate how to implement the Halevi model into the Discontinuous-Galerkin Time-Domain finite-element Maxwell solver and are able to identify an oscillatory contribution to the diffusive current. The Halevi model thus captures a number of relevant features beyond the standard hydrodynamic model. Contrary to other extensions of the standard hydrodynamic model, its use in time-domain Maxwell solvers is straightforward -- especially due its affinity to a class of descriptions that allow for a clear distinction between bulk and surface response. This is of particular importance for applications in nano-plasmonics where nano-gap structures and other nano-scale features have to be modeled efficiently and accurately.

physics.optics↗

arXiv2022

Time-domain modeling of interband transitions in plasmonic systems

Efficient modeling of dispersive materials via time-domain simulations of the Maxwell equations relies on the technique of auxiliary differential equations. In this approach, a material's frequency-dependent permittivity is represented via a sum of rational functions, e.g. Lorentz-poles, and the associated free parameters are determined by fitting to experimental data. In the present work, we present a modified approach for plasmonic materials that requires considerably fewer fit parameters than traditional approaches. Specifically, we consider the underlying microscopic theory and, in the frequency domain, separate the hydrodynamic contributions of the quasi-free electrons in partially filled bands from the interband transitions. As an illustration, we apply our approach to gold and demonstrate how to treat the interband transitions within the effective model via connecting to the underlying electronic bandstructure, thereby assigning physical meaning to the remaining fit parameters. Finally, we show how to utilize this approach within the technique of auxiliary differential equations. Our approach can be extended to other plasmonic materials and leads to efficient time-domain simulations of plasmonic structures for frequency ranges where interband transitions have to be considered.

physics.optics↗

arXiv2021

No-slip boundary conditions for electron hydrodynamics and the thermal Casimir pressure

We derive modified reflection coefficients for electromagnetic waves in the THz and far infrared range. The idea is based on hydrodynamic boundary conditions for metallic conduction electrons. The temperature-dependent part of the Casimir pressure between metal plates is evaluated. The results should shed light on the "thermal anomaly" where measurements deviate from the standard fluctuation electrodynamics for conducting metals.

quant-ph↗

arXiv2020

Remarks about surface plasmons and their stability

We comment on the macroscopic model for surface plasmons of H.-Y. Deng [New J. Phys. 21 (2019) 043055; arXiv:1712.06101] and a claim, based on energy conversion from charges to the electric field, that surface plasmons on metallic surfaces may become unstable [J. Phys.: Cond. Matt. 29 (2017) 455002; arXiv:1606.06239, 1701.01060]. The discussion revolves around the formulation of charge conservation in the bulk and the surface of a metal. We elaborate in particular on the role of a finite electric current normal to the surface. Using a scheme of Cercignani and Lampis and of Zaremba, we point out that the model chosen by Deng for the non-specular scattering of electrons needs to be amended to prevent the disappearance of charges at the surface. Different models and approaches in the literature on surface plasmons are reviewed: the interfacial excess field approach of Bedeaux and Vlieger which contains Deng's macroscopic model, the assumption of specular reflection of Ritchie and Marusak, a hydrodynamic model with a composite charge density (partially localized at the surface), the local dielectric model, and a macroscopic method with (anti)symmetric fictitious stimuli (used, e.g., by Garc\'ıa-Moliner and Flores). This puts Deng's results into perspective and illustrates problems with his approach.

physics.optics↗