Self-referenced SEIRA with an all-dielectric tilted grating metasurface using mid-infrared GMR and QBIC

The mid-infrared (MIR) spectral region provides molecular fingerprint signatures for chemical identification, where surface-enhanced infrared absorption (SEIRA) and refractive index sensing offer complementary detection capabilities. However, refractive index sensing requires radiative coupling to the environment, whereas SEIRA relies on strong near-field confinement, making their integration in simple low-loss dielectric metasurfaces challenging. Here, we report an all-dielectric tilted grating metasurface that enables dual-band resonances for multifunctional MIR sensing. By introducing an out-of-plane tilt, a symmetry-protected bound state in the continuum (BIC) is continuously transformed into a radiatively accessible quasi-BIC (QBIC), while preserving a guided-mode resonance (GMR) at shorter wavelengths. The GMR mode exhibits strong interaction with the surrounding medium, yielding a high bulk refractive index sensitivity of 862 nm/RIU. In contrast, the QBIC mode supports an ultrahigh-Q resonance (Q ≈ 1436) with pronounced near-field localization and a refractive index sensitivity of 466 nm/RIU. Multipole analysis reveals that the GMR mode is dominated by electric and magnetic dipoles, whereas QBIC arises from magnetic dipole and quadrupole contributions. The QBIC resonance aligns with molecular vibrational fingerprints and enables SEIRA. This design achieves a maximum absorption enhancement factor β of 24.5 dB for analyte layers as thin as 8 nm, enabling trace detection of 2,4-dinitrotoluene. This dual-mode coexistence establishes a self-referenced sensing paradigm: the GMR mode continuously monitors environmental refractive index fluctuations (e.g., temperature or buffer drifts), while the QBIC mode simultaneously captures molecular vibrational fingerprints. Their spectral and physical orthogonality enables the retrieval of clean, reliable molecular signatures in complex real-world scenarios, which is a key advantage over single-mode or functionally degenerate dual-mode sensors. This work offers a scalable, low-loss platform for integrated MIR sensing and provides a powerful route for tailoring light-matter interactions in dielectric metasurfaces.