Title: Mid-infrared waveplate based on the resonant nanocavity of silicon carbide coupled surface polaritons

A resonant nanocavity utilizing surface phonon polaritons offers a promising platform for long-wave infrared and far infrared photonics. Recently, we have developed a passive nanocavity device with an anisotropic response to mid-infrared light. When illuminated with linearly polarized light, the device exhibits high reflection for the polarization parallel to the metallic grating and absorption for the polarization perpendicular to the grating. In this study, we explore its potential as a mid-infrared waveplate capable of generating circularly polarized light. We investigate the polarization rotation and possible ellipticity or reflection phase changes occurring both on and off resonances. Using a Fourier transform infrared spectrometer and a polarizer pair, we systematically measured the device's response to various polarization angles. The first polarizer generates a linear polarization and changes the initial polarization direction relative to the device's optical axis perpendicular to the grating. The second polarizer functions as an analyzer to probe polarization changes. Our results demonstrate the device's ability to effectively convert linearly polarized light into circularly polarized light, providing a valuable component for mid-infrared optical systems.