Self‐Stratification of Polymer Films by Strong Coupling Between Molecular Absorbers and Infrared Grating Modes

Strong coupling between deep metal grating modes and molecular vibrations provides a powerful route to sculpt the optical response of chemically uniform polymer films. Here, we integrate poly(methyl methacrylate) (PMMA) with a deep silver (Ag) grating to realize vibrational strong coupling (VSC) to the C=O stretching mode, and use full‐wave simulations to disentangle polaritonic, dark, and uncoupled molecular manifolds. We show that the evanescent field of the grating defines an effective mode volume within which PMMA molecules are hybridized into upper and lower vibrational polaritons, while molecules outside this region retain bulk‐like absorption at the bare vibrational frequency. Exploiting this binary optical behavior, we demonstrate a VSC‐based open‐cavity solvent‐diffusion sensor. Ethanol‐induced swelling of PMMA modulates the polaritonic dispersion and absorption at the bare molecular frequency in a depth‐dependent fashion, enabling us to detect the arrival of the solvent front at the strongly coupled interface. Our results establish deep metal gratings coupled to molecular films as a versatile platform for engineering spatially selective strong coupling, with prospects for advanced photonic sensing and spatial control of light–matter coupling in soft materials.