Far Infrared Polarization Control by GaAs Surface Phonon Polariton Nanocavities

A resonant nanocavity array utilizing coupled surface plasmon–phonon polaritons provides a versatile platform for long-wave infrared photonics. We present a passive nanocavity device that operates as a waveplate in the far-infrared spectrum, exhibiting an anisotropic polarization response to incident light. The device consists of a tri-layer structure of Au-SiO2-GaAs that acts as a surface waveguide for coupled surface plasmon-phonon polaritons. The top metallic grating structure creates a resonant nanocavity that traps the coupled mode. Because of this anisotropic structure and its resonant behavior, the reflected light undergoes polarization rotation and scattering phase shift. We analyze polarization rotation, ellipticity, and reflection-phase changes both on and off resonances. Using a Fourier-transform infrared spectrometer with a polarizer–analyzer pair, we measured polarization changes from the device. Our results show that the nanocavity array effectively converts linearly polarized light into elliptically polarized light, demonstrating its potential as a compact polarization-control element for far-infrared optical systems.