  Plasmonic resonators have drawn more attention due to the ability to confine
light into subwavelength scale. However, they always suffer from a low quality
(Q) factor owing to the intrinsic loss of metal. Here, we numerically propose a
plasmonic resonator with ultra-high Q factor based on plasmonic
metal-insulator-metal (MIM) waveguide structures. The resonator consists of a
disk cavity surrounded by a concentric ring cavity, possessing an ultra-small
volume. Arising from the plasmon hybridization between plasmon modes in the
disk and ring cavity, the induced bonding hybridized modes have ultra-narrow
full wave at half maximum (FWHM) as well as ultra-high Q factors. The FWHM can
be nearly 1 nm and Q factor can be more than 400. Furthermore, such device can
act as a refractive index sensor with ultra-high figure of merit (FOM). This
work provides a novel approach to design plasmonic high-Q-factor resonators,
and has potential on-chip applications such as filters, sensors and nanolasers.
