Quasi-bound states in the continuum with stable dual-resonance wavelengths in mid-infrared metasurface for ultrasensitive refractive index sensing and molecular vibrations of strong coupling.

In this paper, a dual-resonances mid-infrared all-dielectric metasurface sensor based on asymmetric cross dimer, which is driven by quasi-bound states in the continuum (QBIC), is proposed and investigated. The metasurface sensor maintains the total permittivity constant when the asymmetric parameter is adjusted, thereby ensuring the stability of the QBIC resonance wavelengths, which exhibit Q-factors of 6351 and 13561, respectively. The multiple decompositions and electromagnetic field distributions reveal that the toroidal dipole is the dominant component of the dual-resonance modes. The sensitivities to the refractive index are 3559 nm/RIU and 1146 nm/RIU, with corresponding figures of merit of 4449 RIU^-1 and 2453 RIU^-1, respectively. Further numerical simulations have demonstrated a strong coupling phenomenon between the QBIC and the molecular vibrations of polymethyl methacrylate (PMMA), resulting in a significant enhancement of the infrared absorption signal. With the 50-nm-thick PMMA layer, the enhancement of molecular signal is 90.614%.