Broadband Purcell enhancement in plasmonic ring cavities

We present a theoretical study of a surface plasmon ring resonator geometry that shows strong spontaneous emission control due to an extremely small optical cavity mode volume. V grooves made in the surface of a metal confine whispering gallery surface plasmon polariton modes at the bottom of the groove, with the confinement determined by groove depth and width. Resonances in ringlike cavities defined by grooves patterned into circular shape, determined using the boundary-element method, are in agreement with calculations based on dispersion for linear V grooves. Cavity quality factors are relatively insensitive to cavity geometry (Q=10-50 ) while mode volume is very sensitive to small differences in the cavity shape. The smallest mode volume V=0.000 73 lambda(3)(0) is found for a ring with a 10-nm wide, 100-nm deep groove with straight sidewalls and a diameter of 180 nm. Purcell factors well above 2000 are found in the energy range from E =1.0-1.8 eV depending on cavity geometry. For a given cavity geometry the Purcell enhancement is observed over a broad spectral range (50-100 meV), enabling application of these cavities beyond the typical low-temperature cavity quantum electrodynamics experiments.