Mirror-Coupled Plasmonic Bound States in the Continuum for Tunable Perfect Absorption

Abstract

Tailoring critical light-matter coupling is a fundamental challenge of nanophotonics, impacting fields from higher harmonic generation and energy conversion to surface-enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, excel at obtaining different coupling regimes by tuning the antenna-film gap size. However, practical PA applications require constant gap size, making it impossible to maintain critical coupling beyond singular wavelengths. Here, a new approach for plasmonic PAs is introduced by combining mirror-coupled resonances with the unique loss engineering capabilities of plasmonic quasi-bound states in the continuum. This novel combination allows to tailor the light–matter interaction within the under-coupling, over-coupling, and critical coupling regimes using flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor. The study demonstrates a pixelated PA metasurface with optimal absorption over a broad range of mid-infrared wavenumbers (950–2000 cm−1) using only a single gap size and applies it for multispectral surface-enhanced molecular spectroscopy. Moreover, the asymmetry parameter enables convenient adjustment of the quality factor and resonance amplitude. This concept expands the capabilities and flexibility of traditional gap-tuned PAs, opening new perspectives for miniaturized sensing platforms towards on-chip and in situ detection.