  We analyze a whispering gallery mode biosensor with a metallic nanorod bound
to its surface. It is found that a localized surface plasmon resonance in the
nanorod can reduce the optical mode volume of the resonator by as much as four
orders of magnitude via a local enhancement of the electric field, thus
improving the detection sensitivity. Optical frequency shifts as large as 15
MHz are predicted for typical proteins and, for typical experimental
parameters, the biosensor is predicted to be limited by laser frequency noise,
leading to a minimum detectable polarizability on the order of 10 cubic
angstroms.
