Near‐Field Enhancement via Plasmon–Phonon Polariton Coupling with CdO Stripes

Abstract The mid‐infrared spectral region presents significant potential for sensing and spectroscopic applications. However, traditional plasmonic materials exhibit substantial optical losses within this range, thereby constraining their effectiveness. Emerging materials such as cadmium oxide (CdO) have demonstrated promise in overcoming these limitations. In this work, a novel approach is introduced to engineer large coupling between localized surface plasmons (LSPs) in CdO and localized surface phonon polaritons (LSPhP) in sapphire. By developing a successful dry etching protocol for CdO, stripe arrays with tunable sizes are fabricated, allowing the spectral alignment between the LSP and LSPhP modes. It is demonstrated both experimentally and numerically that when these polaritons become resonant, hybrid modes emerge, resulting in coupling. Finite element simulations reveal near‐field enhancements exceeding a factor of 7000, spatially extended hundreds of nanometers around the etched structures. This approach bridges the plasmonic and phononic responses of two mid‐IR active materials, paving the way for scalable, high‐performance infrared sensing platforms.