Active resonant nanocavity based on coupled plasmon-phonon polariton surface waveguide.*

Vanadium dioxide (VO₂) emerges as a promising candidate for the spacer layer within the mid-infrared (MIR) surface waveguide, facilitating the development of active and reconfigurable nanocavities. By modeling an active resonant nanocavity array, we investigate the Fabry-Pérot resonance condition based on the MIR surface waveguide dispersion relation of coupled surface plasmon-phonon polaritons. Here, we choose a gold-VO₂-silicon carbide slab waveguide with the nanocavity structured via gold patterning atop the waveguide. The complex refractive index of VO₂ undergoes a significant change across the insulator-to-metal phase transition. Analytical solutions and full-wave simulations are compared with experimental data of reflectance measurements using a high-precision heating stage integrated with a Fourier Transform Infrared spectrometer within an infrared microscope. We studied two devices with different gold cavity widths (900 nm and 1000 nm with 100 nm gap) patterned on 100 nm thick VO₂ film on silicon carbide. The observed shift of the surface wave dispersion to longer wavelengths with increasing temperature corresponds to the resonance shift, indicating the high sensitivity of the resonance to the dielectric refractive index of VO₂.