The mid-infrared spectral region presents significant potential for sensing andspectroscopic applications. However, traditional plasmonic materials exhibitsubstantial optical losses within this range, thereby constraining theireffectiveness. Emerging materials such as cadmium oxide (CdO) havedemonstrated promise in overcoming these limitations. In this work, a novelapproach is introduced to engineer large coupling between localized surfaceplasmons (LSPs) in CdO and localized surface phonon polaritons (LSPhP) insapphire. By developing a successful dry etching protocol for CdO, stripearrays with tunable sizes are fabricated, allowing the spectral alignmentbetween the LSP and LSPhP modes. It is demonstrated both experimentallyand numerically that when these polaritons become resonant, hybrid modesemerge, resulting in coupling. Finite element simulations reveal near-fieldenhancements exceeding a factor of 7000, spatially extended hundreds ofnanometers around the etched structures. This approach bridges theplasmonic and phononic responses of two mid-IR active materials, paving theway for scalable, high-performance infrared sensing platforms.
