Critical coupling for efficient energy trapping in gold plasmonic chain

Integrating plasmonic chains onto a dielectric waveguide enables efficient individual excitation of plasmonic nanostructures. Nevertheless, the broad operation bandwidth related to plasmonic resonance remains a drawback for some applications. We propose a novel design based on planarized encapsulated waveguides enabling flexible 3D positioning of the plasmonic chain with respect to the waveguide. Through numerical and experimental demonstrations, we show that a critical coupling regime between the waveguide and the resonant chain can be achieved by properly engineering the geometric parameters. This configuration allows for trapping all the waveguide mode energy inside the chain, resulting in a sharper spectral response with a strongly reduced group velocity. We apply temporal coupled mode theory to analyze this system and discuss the characteristics of this specific excited state of the plasmonic chain.