Effective mode volume of nanoscale plasmon cavities

The controlled squeezing of electromagnetic energy into nanometric volumes via surface plasmon-polariton excitations in plasmonic nanoresonators is analyzed using the concept of an effective electromagnetic mode volume V-eff, while taking careful account of the plasmon-polariton dispersion and the electromagnetic energy stored in the metal. Together with the quality factor Q of the cavity resonance, this enables a comparison with dielectric optical cavities, where V-eff is limited by diffraction. For a Fabry-Perot type planar metallic cavity, a one-dimensional analytic model as well as a three-dimensional finite-difference time-domain simulation reveal that V-eff is not bounded by diffraction, and that Q/V-eff increases for decreasing cavity size. In this picture, matter-plasmon interactions can be quantified in terms of Q and V-eff, and a resonant cavity model for the enhancement of spontaneous Raman scattering is presented.