Plasmon-Molecular Coupling-Driven Off-Resonant SEIRA for Self-Referenced Microplastic Detection

Abstract Conventional surface-enhanced infrared absorption (SEIRA) relies on precise spectral overlap between molecular vibrational resonances and plasmonic modes, requiring strict nanofabrication and resulting in inherently narrowband operation. Here, we experimentally demonstrate an angle-tunable off-resonant SEIRA platform on a simple flexible plasmonic 1D metagrating (FPM). The FPM is fabricated in a single step and has an extended spectrally steerable Fano mode. The deterministic angle tuning supports the on-demand selection of enhanced molecular vibrational modes within the off-resonance window, allowing unprecedented spectral selectivity for broadband sensing. The FPM has been used for sensing with three coexisting optical channels: (i) plasmon-independent molecular absorption also visible in TE polarization; (ii) self-referenced Fano mode for simultaneous refractive index sensing; and (iii) angle-tunable hybrid plasmon–molecule off-resonant channel. Temporal coupled mode theory quantitatively predicts the vibrational absorption via an extracted plasmon–molecule coupling coefficient. The FPM exhibits enhancement factors of 4.3 × 104 for purely molecular mode and 473 for off-resonance mode at grazing incidence, comparable to the existing off-resonant benchmarks while eliminating the need for high-cost, sophisticated nanofabrication. Polyethylene terephthalate (PET) microplastics leached from commercial plastic water bottles are detected with a sensitivity of 834.234 cm–1/g and a figure of merit up to 1.82 × 106/g. The limit of detection and the quantification are 48 and 145 μg, respectively. The results highlight the potential of extended plasmon-mediated plasmon–molecule hybridization toward broadband and fabrication-tolerant SEIRA spectroscopy.