Unveiling Coupled Dark Plasmon Modes in CdO Structures

Cadmium oxide (CdO) is an attractive material for plasmonic applications in the near- to mid-infrared (NIR and MIR, respectively). CdO is capable of sustaining both surface plasmon polariton (SPP) and epsilon-near-zero (ENZ) modes depending on the carrier concentration and sample thickness [1]. SPPs are dispersive with long propagation lengths, while ENZs have low dispersion but high field confinement. Previous work has shown that when CdO layers of different thicknesses and carrier concentrations are grown atop one another, hybrid polaritonic modes start to appear from the coupling of these SPP and ENZ modes [2, 3]. The coupling conditions for these hybrid modes can be further tuned by changing the sample geometry and size, which effects the modal confinement and distribution. Pillars of varying diameter were fabricated using focused ion beam (FIB) milling. Structures were made of SPP/ENZ/substrate configuration. The plasmonic response of the fabricated pillars were analyzed in a scanning transmission electron microscope (STEM) operating at 60 kV using aloof electron energy loss spectroscopy (EELS). EELS maps were acquired in the Thermo Fisher Spectra Ultra with average energy resolutions of 15-20 meV, while point spectra were acquired in the Nion HERMES with average 5 meV energy resolution. We detected a large variety of optically active/inactive plasmon modes, with a potential coupling scenario between dark modes (e.g. radial breathing modes) in the NIR range (450-500 meV – see Fig. 1). Dispersion plots reveal an evident anti-crossing behavior which is typically linked to coupling phenomenon. To further gain insight into the selection rules operating for the excitation we probed the coupled system at different tilt configurations (see Fig. 1). We observe a strong orientation dependence of the plasmonic response, with its intensity decreasing by half when tilts as low as 3° (away from the beam traversing parallel to the surface) are applied. This implies a strongly directional response. At higher mis-tilts, weak shoulders are observed on the initial doublet, indicating the presence of additional relaxation pathways through which energy can be distributed. To study the spatial distribution of those coupled modes we conducted real space mapping, which revealed an asymmetrical behavior of the mode distribution (see Fig. 2). Our work showcases heretofore unexplored areas for CdO polaritonics, where a series of both bright and dark polaritons are excited and interact with one another. The observation of these dark polaritons are promising for lossless optical propagation, especially if they are coupling with other plasmonic modes within the system. Furthermore, the presence of asymmetrical population distribution lays the foundation for further studies into how energy is dissipated at the surface of transparent conducting oxides and what channels are involved in these processes. Orientation dependence of plasmon polaritons sustained by 3.4 µm diameter CdO pillars. When the beam passes parallel to the pillar surface, the strongest resonance is sustained (labelled as 0°). Once the surface is tilted backwards (in the negative β direction), the intensity of this mode drops rapidly. Further broadening and additional shoulders are observed as the degree of mis-tilt is increased, until this mode becomes almost indistinct. Substrate surface phonon polaritons and lower order multipolar plasmon modes remain defined. Modal distribution of NIR polaritons sustained on a 3.4 µm diameter CdO pillar. EELS maps were acquired as singular scans across the entire surface. Spectra were extracted from regions indicated by the arrows, and the map was generated using the energy window noted atop the spectra. A clear asymmetry is present is how the NIR doublet is distributed across the pillar surface.

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