  Nanoparticle-induced modifications of the spectrum of
whispering-gallery-modes (WGM) of optical spheroidal resonators are studied
theoretically. Combining an ab initio solution of a single resonator problem
with a dipole approximation for the particle, we derive simple analytical
expressions for frequencies and widths of the particle-modified resonances,
which are valid for resonators with moderate deviations from the spherical
shape. The derived expressions are used to analyze spectral properties of the
resonator-particle system as functions of the particle's position, the size of
the resonators and the characteristics of WGMs. The obtained results are shown
to agree well with available experimental data. It is also demonstrated that
the particle-induced spectral effects can be significantly enhanced by careful
selection of resonator's size, refractive index and other experimental
parameters. The results presented in the paper can be useful for applications
of WGM resonators in biosensing, cavity QED, optomechanics and others.
