Quasinormal mode analysis of extremely localized optical field in body-of-revolution plasmonic structures

Surface plasmons in metallic nanostructures can confine the optical field within the region of subwavelength, even nanometer scale, and thus enhance the light-matter interaction and other physical processes, which will lead the plasmon optics to possess attractive applications in many areas. However, the " mode volume” often used to characterize field confinement in plasmonic structures is only defined phe-nomenologically and suffers ambiguity when applied to complex structures. In this work, we develop a theoretical method to characterize the field confinement based on quasi-normal mode analysis. We recognize the fact that a plasmonic resonance may result from many eigen-modes, which together contribute to the observed field confinement. An effective mode volume is introduced for quasi-normal modes and used to characterize the field confinement when the plasmonic resonance is dominated by a single quasi-normal mode. Two typical kinds of plasmonic structures are systematically examined, and the field confinement on the order of 10 nm3–100 nm3 is confirmed. In pursuit of the ultimate field confinement, we revisit the so-called " pico-cavity” formed by an atomistic protrusion in the nano gap of the particle-on-mirror configuration. The apparent hot spot is shown to have contributions from several quasi-normal modes. The dominant one exhibits a further squeezed mode volume compared with the scenario without the protrusion, but is still well above 10 nm3.