Plasmonic Graded-Chains as Deep-Subwavelength Light Concentrators

TL;DR

This paper investigates plasmonic graded-chains of nanoparticles that concentrate electromagnetic fields at deep subwavelength scales, providing a universal theory and analytical solutions with potential applications in sensing and energy harvesting.

Contribution

It introduces a novel class of plasmonic graded-chains that act as deep-subwavelength light concentrators and develops a universal scaling theory applicable to various configurations.

Findings

Arrays concentrate electromagnetic fields even at long wavelengths.

Universal curves describe the dependence on system parameters.

Analytical solutions for homogeneous chains are provided.

Abstract

We have studied the plasmonic properties of aperiodic arrays of identical nanoparticles (NPs) formed by two opposite and equal graded-chains (a chain where interactions change gradually). We found that these arrays concentrate the external electromagnetic fields even in the long wavelength limit. The phenomenon was understood by identifying the system with an effective cavity where plasmonics excitations are trapped between effective band edges, resulting from the change of passband with NP's position. Dependence of excitation concentration on several system's parameter was also assessed. This includes, different gradings as well as NP's couplings, damping, and resonant frequencies. In the spirit of the scaling laws in condensed matter physics, we developed a theory that allows us to rationalize all these system's parameters into universal curves. The theory is quite general and can also be used on many other situations (different arrays for example). Additionally, we also provided an analytical solution, in the tight-binding limit, for the plasmonic response of homogeneous linear chains of NPs illuminated by a plane wave. Our results can find applications on sensing, near field imaging, plasmon-enhanced photodetectors, as well as to increase solar cell efficiency.