Spatial Distribution and Sensitivity of Localized Plasmon Modes in Individual Infrared SPP/ENZ Resonators Probed via EELS

Controlling the spatial distribution and field localization of plasmonic elements is crucial to designing systems which rely on modal overlap. Such overlaps can lead to strong coupling, as has previously been demonstrated for mid-infrared surface plasmon polariton (SPP) and epsilon-near-zero (ENZ) modes in cadmium oxide (CdO) thin films. Insight into how local cavity confinement alters modal frequency, spatial field distribution, and coupling behavior is desirable for further control of CdO-supported polaritons. Here, we utilize monochromated electron energy loss spectroscopy in the scanning transmission electron microscope to locally probe and map out the plasmonic response of CdO pillars comprised of SPP- and ENZ-sustaining layers with simultaneously high spatial and energy resolution. We demonstrate that multiple localized surface plasmons (LSP) appear strongly asymmetric due to convoluting effects of uneven carbon coating and crosstalk between microresonators. Several of these modes are nonradiative in nature, and the diameter-independent modes convolve with diameter-dependent modes. Effects of Ga+ irradiation and hydration are additionally investigated as factors impacting the plasmonic response. These results provide fundamental insights into how radiant and subradiant modes manifest within propagating infrared plasmonic materials that have been shaped into individual resonators, offering relevant insights into the photonic density of states of hybrid resonators.