Thermally Tunable Mid-Infrared Polarization Rotation and Ellipticity Based on Active Surface Phonon Polariton Nanocavity Arrays*

Polarimetry is an invaluable tool for investigating material properties by observing how the material changes the polarization of light. Studies in the infrared are limited due to a lack of polarization-sensitive materials in far-infrared (IR). Here, we experimentally demonstrate an active polarization-control nanostructure, which avoids the limitations of methods that depend on bulk material effects. We measure the optical rotation and phase induced from gold subwavelength grating structures on top of vanadium dioxide (VO2) film on silicon carbide. The proposed structure has a resonance at 840 cm-1 due to the Fabry-Perot cavity array of coupled surface polaritons. Linearly polarized incident light can be decomposed into a statically reflected component and a partially absorbed temperature-dependent component. A novel polarimetric spectrometer was developed to measure the IR polarization spectra, and a simple two-polarizer method was used with laser light to quantify the polarization changes. The light near the resonance is optically rotated by 10^o, while the phase is shifted by 15^o as the temperature increases. Due to the device's small footprint, flexible operating wavelength, and potential to be used as a modulator, this work provides a solution for longwave IR polarimetry.