Engineering the Dispersion of Surface Plasmon Polariton/Epsilon-Near-Zero Modes through Modal Separation and Optical Confinement

Doped cadmium oxide (CdO) films support tunable, low-loss surface plasmon polaritons (SPPs) and epsilon-near-zero (ENZ) modes throughout the midinfrared. Prior work demonstrated that polaritonic strong coupling could be realized in bilayer films of CdO supporting adjacent SPP and ENZ modes. The hybridized SPP-ENZ dispersion displays a prominent anti-crossing with an amplitude dependent on the spectral and spatial overlap of the modes. Here, we leverage the low carrier diffusivity, in concert with the broad tunability of the plasma frequency in CdO to control the SPP-ENZ dispersion within monolithic, multilayer CdO structures featuring dielectric, unintentionally doped CdO coatings or spacer layers. In the case of the former, the coating induces further compression of the polaritonic fields, altering the dielectric environment. In contrast, when incorporated as a spacer layer, the spatial overlap of the constituent polaritonic near-fields is reduced. In all cases, the anti-crossing behavior of the hybrid SPP-ENZ modes is maintained, thus inheriting the extreme light confinement of the ENZ mode and extended propagation length of the SPPs. Through simultaneous control over the SPP-ENZ coupling strength and group velocities, significant tunability over the SPP-ENZ dispersion is demonstrated, enabling applications such as in-plane waveguiding, tailored thermal emission, and tunable near-field heat transfer.