  Vertical gold-nanogaps are created on microtubular cavities to explore the
coupling between resonant light supported by the microcavities and surface
plasmons localized at the nanogaps. Selective coupling of optical axial modes
and localized surface plasmons critically depends on the exact location of the
gold-nanogap on the microcavities which is conveniently achieved by rolling-up
specially designed thin dielectric films into three dimensional microtube ring
resonators. The coupling phenomenon is explained by a modified quasi-potential
model based on perturbation theory. Our work reveals the coupling of surface
plasmon resonances localized at the nanoscale to optical resonances confined in
microtubular cavities at the microscale, implying a promising strategy for the
investigation of light-matter interactions.
