High- Q plasmonic and dielectric modes in a metal-coated whispering-gallery microcavity

This paper investigates the confinement and quality (Q) factors of the dielectric mode, exterior surface plasmonic mode, and interior surface plasmonic mode in a metal-nanolayer-coated silica microtoroid. It is found that the confinement of these modes relies mainly on the real part of the coating-metal permittivity, and a smaller negative real part produces a lower effective potential barrier which plays distinct roles in these three types of modes. For the exterior plasmonic mode, a lower potential leads to a better confinement and thus lower radiation losses, while for the interior plasmonic mode and the dielectric mode, a higher potential can play a very positive role in suppressing the radiation losses. The metal absorption loss, as expected, is directly related to the imaginary part of the metal permittivity and the energy fraction of the mode in the metal layer, and the latter depends on the confinement or, namely, the effective potential barrier induced by the metal nanolayer. We also compare the plasmonic modes in the coated microcavity and the dielectric modes in the uncoated microcavity at different cavity sizes. An interesting finding is that when the cavity is small enough, which is highly desirable for compact photonic devices, the Q factors of the plasmonic modes even exceed significantly that of the uncoated dielectric modes. DOI: 10.1103/PhysRevA.87.023824