Optical 3D cavity modes below the diffractionlimit using slow-wave   surface-plasmonpolaritons
  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. 
Eyal Feigenbaum and Meir Orenstein (Technion - Isreal institue of   technology)