Ultra-narrowband dielectric metasurfaces for surface-enhanced infrared absorption

Quasi-bound states in the continuum (QBICs) in the mid-infrared region have been widely explored to enhance light–matter interactions for biochemical sensing. However, their steep dispersion bands often limit Q-factor stability under focused light. Here, we design and experimentally demonstrate a super QBIC within a flatband by introducing lateral geometric perturbations in a square lattice of etched holes in high-index Ge films, enabling strong coupling between energy bands. The super QBIC achieves a theoretical Q-factor two orders of magnitude higher than conventional QBICs, with nearly zero group velocity, and supports ultra-narrowband resonances with slow light effect. Experimentally, we report a robust Q-factor of ∼270 at a tilted angle, a full width at half maximum of ∼22 nm, and a field enhancement of ∼23 in accessible optical cavities. Vibrational strong coupling is achieved between the super QBIC mode and the C=O vibrational mode of polymethylmethacrylate molecules, with Rabi mode splitting and a Rabi energy of 24.2 cm−1 at zero detuning. Differential transmission spectra reveal a fivefold enhancement in molecular absorption due to strong field localization. These results demonstrate the potential of flatband metasurfaces for advancing surface-enhanced infrared absorption at the micrometer scale.