Coupled surface plasmon-phonon polariton nanocavity arrays for enhanced mid-infrared absorption.

Resonant optical cavities are essential components in mid-infrared applications. However, typical film-type cavities require multilayer stacks with a micron-thick spacer due to mid-infrared wavelengths, and their performance is limited by narrow frequency tunability and angular sensitivity. We propose and experimentally demonstrate the subwavelength-scale (≈ /150) resonant nanocavity arrays that enhance the absorption spectrum of the device in the mid-infrared (10-12 microns) via excitation of coupled surface plasmon-phonon polaritons. The proposed metal-insulator-polar dielectric (gold-silicon-silicon carbide) structure supports a guided mode of the coupled surface polaritons in the lateral direction while vertically confining the mid-infrared wave within the 80 nm thick dielectric spacer. In particular, the subwavelength-scale (≈ /10) gratings are imposed to form Fabry-Pérot cavity arrays displaying angle-insensitive and frequency-tunable absorption of up to 80% of the optical power in the mid-infrared. Our work should benefit diverse mid-infrared applications and novel designs of polariton-based photonic devices.