Rainbows at the End of Subwavelength Discontinuities: Plasmonic Light Trapping for Sensing Applications

Abstract

Surface plasmon polaritons (SPPs) which exist at the metal-dielectric interface are guided by the coupling of electromagnetic waves to oscillations in the electron gas plasma created at metal surfaces. Plasmonics, a subset of the field of nanophotonics pertaining to all things beyond the diffraction limit, has flourished and evolved significantly over the past decade offering practical utility to a variety of applications. [1] Notwithstanding the ohmic loss in metals and commensurate limited propagation lengths of SPPs, the ability to effectively concentrate light in nanoscale volumes and generate extremely high electric field intensities at discontinuities or subwavelength patterned surfaces offers unique opportunities to enhance lightmatter interactions for a diverse range of functionalities. [2] Trapping broadband electromagnetic radiation over a range of deep subwavelength guided modes in a given structure provides opportunities for light-matter interactions at the nanoscale. Use of materials-commonly metals-that exhibit negative dielectric permittivity (ε < 0) at optical frequencies provide an optimum solution for light localization. Nanometallic light concentrators, in contrast to their dielectric counterparts, can squeeze and localize light into subwavelength volumes with greater control and higher efficiency. [3] Such plasmonic light concentration can be achieved by either resonant or non-resonant structures. In resonant structures, SPPs are created by time-varying electric fields that exert a force on the negatively charged electron gas inside a metal. These oscillations are resonantly driven leading to a strong charge displacement at specific optical frequencies and concentration of the light field within the structures. [4] For non-resonant light concentration effects, Schuller et al. demonstrated subwavelength light localization by introducing a feed gap in the metal structure for retardation-based resonators. The gap builds up opposite charges across it whereby SPPs encounter a longitudinal electric field component causing strong sub-wavelength light localization. [2] The focus of this article is plasmonic devices that enable rainbow light trapping, a specific modality of nanoscale field enhancement in which trapped light is spatially separated This article presents recent advances in plasmonic multiwavelength rainbow light trapping, a field that has evolved over the last decade and today is an active area of research interest encompassing a manifold of potential applications which include optical biosensing, photodetection, spectroscopy, and medicine. Conventional plasmonic devices are designed and optimized to enhance optical performance at single wavelengths, and as such are not suitable for applications that require electromagnetic field localization at multiple frequencies or broad frequency ranges of interest. To overcome these limitations, the ability to slow and trap light at multiple wavelengths and at different spatial locations has attracted significant scientific attention and opened up n...

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