Experimental and theoretical studies of linear and nonlinear optics of surface plasmon whispering gallery modes have been performed. These modes have been observed in dielectric microdroplets on the metal surfaces which support surface plasmon propagation. The plasmons have been shown to exhibit critical behavior near some surface just inside the droplet edge, where the effective refractive index seen by the surface plasmons appears to be extremely large.
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Smolyaninov et al. (2004) studied this question.