Mirror-coupled plasmonic bound states in the continuum for tunable perfect absorption

TL;DR

This paper introduces a novel plasmonic perfect absorber platform using mirror-coupled resonances and bound states in the continuum, enabling tunable, high-Q, multispectral absorption for advanced sensing applications.

Contribution

It combines BICs with mirror-coupled resonances to achieve tunable perfect absorption with high quality factors, expanding the capabilities of traditional gap-tuned plasmonic absorbers.

Findings

Achieved near-unity absorption with high Q-factors approaching 100 in mid-infrared.

Demonstrated broad spectral tunability over 950 to 2000 1/cm with a single spacer layer.

Enabled multispectral molecular spectroscopy using the proposed metasurface platform.

Abstract

Tailoring critical light-matter coupling is a fundamental challenge of nanophotonics, impacting diverse 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, have been instrumental for obtaining different coupling regimes by tuning the antenna-film distance. However, for on-chip uses, the ideal PA gap size can only match one wavelength, and wide range multispectral approaches remain challenging. Here, we introduce a new paradigm for plasmonic PAs by combining mirror-coupled resonances with the unique loss engineering capabilities of plasmonic bound states in the continuum (BICs). Our BIC-driven PA platform leverages the asymmetry of the constituent meta-atoms as an additional degree of freedom for reaching the critical coupling (CC) condition, delivering resonances with unity absorbance and high quality factors approaching 100 in the mid-infrared. Such a platform holds flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor to precisely control the coupling condition, resonance frequency, and selective enhancement of magnetic and electric fields while maintaining CC. We demonstrate a pixelated PA metasurface with optimal absorption over a broad range of mid-infrared frequencies (950 ~ 2000 1/cm) using only a single spacer layer thickness and apply it for multispectral surface-enhanced molecular spectroscopy in tailored coupling regimes. Our concept greatly expands the capabilities and flexibility of traditional gap-tuned PAs, opening new perspectives for miniaturized sensing platforms towards on-chip and in-situ detection.