Abstract

We study the electromagnetic field scattered by a metallic nanoparticle with dispersive material parameters in a resonant regime. We consider the particle placed in a homogeneous medium in a low-frequency regime. We define modes for the non-Hermitian problem as perturbations of electro-static modes, and obtain a modal approximation of the scattered field in the frequency domain. The poles of the expansion correspond to the eigenvalues of a singular boundary integral operator and are shown to lie in a bounded region near the origin of the lower-half complex plane. Finally, we show that this modal representation gives a very good approximation of the field in the time domain. We present numerical simulations in two dimensions to corroborate our results.

Highlights

  • When describing the interaction of light with a resonating particle, summing the natural resonant modes of the system is an intuitive and attractive approach

  • The goal of this paper is to obtain an approximation of the low-frequency part of the scattered field by a dispersive obstacle in the time domain as a finite sum of modes oscillating at complex frequencies

  • We show that even though they are irrelevant for frequency domain representation, quasi-normal modes can be used to approximate the field in the time domain

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Summary

Context

When describing the interaction of light with a resonating particle, summing the natural resonant modes of the system is an intuitive and attractive approach. The modes are computed as they are eigenmode solutions to a source-free problem. They are intrinsic quantities of the system and give insights to understand the underlying physics. Once they are calculated, the response of the system to any given excitation can be computed at a low com-. This article is part of the section “Applications of PDEs” edited by Hyeonbae Kang.

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Scope of the paper
Previous work on plasmonic resonances and layer potentials
Contributions and organisation of the paper
Problem setting
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Helmholtz equation for a subwavelength resonator
Layer potential formulation
Scaling and small-volume approximation
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Modal decomposition of the field
Modal expansion truncation
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The two-dimensional case
Modal decomposition
Size dependent resonant frequencies
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Plasmonic quasi-normal modes
Time domain approximation of the scattered field
The modal approximation
Alternative formulation with non-diverging causal quasi-normal modes
Proof of Theorem 3
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Numerical simulations
Domains and physical parameters
Modes contribution decay
Plasmonic resonances
Validation of theorem 5
Reference solution
Asymptotic solution
Comparison in the far-field for the diamond
Extension to a nearer-field for the ellipse and flower
About the high frequencies
About the computational cost
Concluding remarks
A Properties of the layer potentials
Definitions and notations
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Invertibility of the boundary operators
B Scaling properties for a finite volume particle
Asymptotic expansions of the boundary operators
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Findings
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