Near-Field Mapping of Photonic Eigenmodes in Patterned Silicon Nanocavities by Electron Energy-Loss Spectroscopy

Recently, there has
been significant interest in using dielectric
nanocavities for the controlled scattering of light, owing to the
diverse electromagnetic modes that they support. For plasmonic systems,
electron energy-loss spectroscopy (EELS) is now an established method
enabling structure–optical property analysis at the scale of
the nanostructure. Here, we instead test its potential for the near-field
mapping of photonic eigenmodes supported in planar dielectric nanocavities,
which are lithographically patterned from amorphous silicon according
to standard photonic principles. By correlating results with finite
element simulations, we demonstrate how many of the EELS excitations
can be directly corresponded to various optical eigenmodes of interest
for photonic engineering. The EELS maps present a high spatial definition,
displaying intensity features that correlate precisely to the impact
parameters giving the highest probability of modal excitation. Further,
eigenmode characteristics translate into their EELS signatures, such
as the spatially and energetically extended signal of the low Q-factor electric dipole and nodal intensity patterns emerging
from excitation of toroidal and second-order magnetic modes within
the nanocavity volumes. Overall, the spatial–spectral nature
of the data, combined with our experimental–simulation toolbox,
enables interpretation of subtle changes in the EELS response across
a range of nanocavity dimensions and forms, with certain simulated
resonances matching the excitation energies within ±0.01 eV.
By connecting results to far-field simulations, perspectives are offered
for tailoring the nanophotonic resonances via manipulating
nanocavity size and shape.