  Confining light in extreme subwavelength scales is a tantalizing task. In
this work, we report a study of individual plasmonic film-coupled nanostar
resonators where plasmonic optical modes are trapped in ultrasmall volumes.
Individual gold nanostars, separated from a flat gold film by a thin dielectric
spacer layer, exhibit a strong light confinement within the sub-10 nm volume of
the nanostar's tips and the film. Through dark field scattering measurements of
many individual nanostars, a statistical observation of the scattered spectra
is obtained and compared with extensive simulation data to reveal the origins
of the resonant peaks. We observe that an individual nanostar on a flat gold
film can result in a resonant spectrum with single, double or multiple peaks.
Further, these resonant peaks are strongly polarized under white light
illumination. Our simulation data revealed that the resonant spectrum of an
individual film-coupled nanostar resonator is related to the symmetry of the
nanostar, as well as the orientation of the nanostar relative to its placement
on the gold substrate. Our results demonstrate a simple method to create an
ultrasmall mode volume plasmonic platform which could be useful for
applications in sensing or enhanced light-matter interactions.
