Mid-infrared Polarization Rotation and Ellipticity Tuning via Surface Phonon Polaritonic Nanocavity Arrays

Active polarization rotation and ellipticity control are experimentally demonstrated using both broadband and monochromatic mid-infrared light. The Fabry-Pérot nanocavity array consists of gold subwavelength gratings atop a phase-changing vanadium dioxide film deposited on a polar dielectric 6H-SiC substrate. Thermally tunable anisotropic reflection near the insulator-to-metal transition enables rotation of the polarization axis and a phase shift linked to polarization ellipticity. Within the 3 ^oC narrow temperature window, a 10^o polarization rotation and a notable scattering phase shift were observed at the 12 μm resonance. The measured polarization rotation and ellipticity spectra agree with full-wave simulations and Jones-matrix analysis. The scattering phase of the reflected light is attributed to surface plasmon polaritons in the gold cavities, where the cavity mode interacts with surface phonon polaritons in the SiC substrate. This scattering phase enables gradient-metasurface design for polarization-tailored wavefronts, offering a compact, tunable platform for long-wave infrared polarimetry and modulation applications.