Wide spectral control of long-wave infrared multilayer nanocavity resonances within Reststrahlen band.

Long-wave infrared (LWIR) is of great technological significance for sensing and imaging applications. One of the primary challenges to advancing LWIR photonics lies in the limited efficiency of plasmonic resonators due to high intrinsic optical losses. We recently developed a resonant nanocavity that utilizes surface phonon polaritons (SPhP) at LWIR and far infrared. This approach has emerged as a promising solution for controlling LWIR and even longer wavelength far infrared because phonon polaritons – the collective oscillations of infrared photons and polarized ions – are being explored as a substitute for plasmon polaritons due to minimal optical power loss. Our current design creates a surface waveguide cavity that supports coupled surface plasmon-phonon polaritons. Our design is limited to tuning resonances within the Reststrahlen band due to the dielectric spacer layer's constraint on the dispersion relation at higher frequencies close to the longitudinal optical phonon resonance. We propose analytical and numerical approaches for a tunable, multilayer surface waveguide to overcome these limitations using various polar dielectric and active materials. Our goal is to develop low-loss nanocavities for advanced, broad-spectrum LWIR metasurfaces.