  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.
