  Photonic modes in dielectric nanostructures, e.g., wide gap semiconductor
like CeO2 (ceria), has potential for various applications such as light
harvesting and information transmission. To fully understand the properties of
such phenomenon in nanoscale, we applied electron energy-loss spectroscopy
(EELS) in scanning transmission electron microscope (STEM) to detect such modes
in a well-defined ceria nanocube. Through spectra and mapping, we demonstrated
a geometrical difference of mode excitation. By comparing various spectra taken
at different location relative to the cube, we also showed the transmission
properties of the mode. To confirm our observation, we performed EELS
simulation with finite-element dielectric calculations in COMSOL Multiphysics.
We also revealed the origin of the modes through the calculation. We purposed a
simple analytical model to estimate the energy of photonic modes as well. In
all, this work gave a fine description of the photonic modes' properties in
nanostructures, while demonstrating the advantage of EELS in characterizing
optical phenomena in nanoscale.
