Identification of Dielectric, Plasmonic, and Hybrid Modes in Metal-Coated Whispering-Gallery-Mode Resonators
C. Klusmann, J. Oppermann, P. Forster, C. Rockstuhl, und H. Kalt
ACS Photonics 5, 2365–2373 (2018)
Making available and accessing in a controlled manner optical modes with largely disparate properties in a given system constitutes a prime challenge for different applications. Here, we propose, realize, and optically characterize a high-Q polymeric wedge-like whispering-gallery-mode resonator coated with a thin silver layer that supports pure surface plasmon polariton modes, pure dielectric modes, and hybrid photonic–plasmonic modes with Q-factors larger than 1000 and modal volumes as small as only a few cubic micrometers. We demonstrate both theoretically and experimentally that all three distinct kinds of cavity eigenmodes can be efficiently excited in the infrared via evanescent coupling to a tapered fiber. Performing finite-element simulations and coupled-mode theory, we develop an experimental procedure based on mode filtering to unambiguously identify the resonances observed in fiber transmission spectra. By controlling both the position of the tapered fiber with respect to the resonator and the input laser polarization, we successfully demonstrate that dielectric, plasmonic, and hybrid modes can be selectively excited, allowing for an explicit classification of the distinct cavity eigenmodes. Experimental results are in excellent agreement with the simulations.