  Whispering-gallery mode resonators host multiple trapped narrowband
circulating optical resonances that find applications in quantum
electrodynamics, optomechanics, and sensing. However, the spherical symmetry
and low field leakage of dielectric microspheres make it difficult to probe
their high-quality optical modes using far-field radiation. Even so, local
field enhancement from metallic nanoparticles (MNPs) coupled to the resonators
can interface the optical far-field and the bounded cavity modes. In this work,
we study the interaction between whispering-gallery modes and MNP surface
plasmons with nanometric spatial resolution by using electron-beam
spectroscopies in a scanning transmission electron microscope. We show that
gallery modes are induced over a selective spectral range of the nanoparticle
plasmons, and additionally, their polarization can be controlled by the induced
dipole moment of the MNP. Our study demonstrates a viable mechanism to
effectively excite high-quality-factor whispering-gallery modes and holds
potential for applications in optical sensing and light manipulation.
