Nano scale modal confinement in 3D gap plasmon polariton cavities

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