Double-disk resonators, composed of two nearly-identical dielectric
disks separated by nanoscale air gap, can be configured to exhibit small
modal volumes with high-Q factors. Compared to the two fundamental
eigenmodes of a single-disk resonator (TE and TM), those supported
by the double-disk microcavity are split into four modes, which can be
categorized as symmetric and antisymmetric modes: TEs/TEas and
TMs/TMas, depending on the field symmetry. Theoretical descriptions
on these eigenmodes are given with regard to the cavity performance
metrics such as cavity mode dispersion, Q-factor, and mode
index. Experimental verification of these eigenmodes is provided for
a 40-nm gap double-disk/air-slot resonator. In addition to these optical
mode characterizations, the mechanical eigenmodes of double-disk
structures, which can be actuated by the optical gradient forces, are
investigated.