  Modal volumes at the nano-scale, much smaller than the "diffraction-limit",
with appreciable quality factors, are calculated for a dielectric cavity
embedded in a space between metal plates. The modal field is bounded between
the metal interfaces in one dimension and can be reduced in size almost
indefinitely in this dimension. But more surprisingly, due to the "plasmonic"
slow wave effect, this reduction is accompanied by a similar in-plane modal
size reduction. Another interesting result is that higher order cavity modes
exhibit lower radiation loss. The scheme is studied with effective index
analysis, and validated by FDTD simulations.
