Polarization-independent dielectric gradient near-perfect absorbers for aqueous mid-infrared molecular sensing

-symmetric unit cells ensure polarization-independent resonances, enabling efficient utilization of incident light under arbitrary polarization states. The gradient architecture independently controls radiative loss and resonance wavelength, allowing distinct coupling regimes within one metasurface while achieving high absorbance (>0.8). We demonstrate poly(methyl methacrylate) sensing in air with ~20% absorbance envelope modulation under arbitrary polarization. Furthermore, we introduce a sensing configuration utilizing a 700 nm residual thin-water film that preserves qBIC (absorbance ~0.5) near the prominent water absorption peak. This enables the first demonstration of dielectric metasurface-based mid-infrared molecular sensing under a water background, achieving >30% absorbance envelope modulation. This platform extends the utility of dielectric metasurfaces to aqueous environments and supports versatile, spectrometer-less sensing schemes.