A spherical dielectric particle can sustain the so-called
whispering-gallery
modes (WGMs), which can be regarded as circulating electromagnetic
waves, resulting in the spatial confinement of light inside the particle.
Despite the wide adoption of optical WGMs as a major light confinement
mechanism in salient practical applications, direct imaging of the
mode fields is still lacking and only partially addressed by simple
photography and simulation work. The present study comprehensively
covers this research gap by demonstrating the nanoscale optical-field
visualization of self-interference of light extracted from excited
modes through experimentally obtained photon maps that directly portray
the field distributions of the excited eigenmodes. To selectively
choose the specific modes at a given light emission detection angle
and resonance wavelength, we use cathodoluminescence-based scanning
transmission electron microscopy supplemented with angle-, polarization-,
and wavelength-resolved capabilities. Equipped with semi-analytical
simulation tools, the internal field distributions of the whispering-gallery
modes reveal that radiation emitted by a spherical resonator at a
given resonance frequency is composed of the interference between
multiple modes, with one or more of them being comparatively dominant,
leading to a resulting distribution featuring complex patterns that
explicitly depend on the detection angle and polarization. Direct
visualization of the internal fields inside resonators enables a comprehensive
understanding of WGMs that can shed light on the design of nanophotonic
applications.