Deterministic Fractal Plasmonic Metasurfaces for Broadband Mid‐Infrared Molecular Biosensing

Surface‐enhanced infrared absorption (SEIRA) spectroscopy faces a persistent traded‐off between sensitivity and spectral bandwidth. This arises because the strongly enhanced electromagnetic fields of plasmonic nanostructures are spectrally sparse due to narrow resonances and spatially sparse due to confinement to nanoscale hot spots, resulting in a limited effective sensing volume. Here we present deterministic fractal plasmonic metasurfaces that address this limitation by combining high sensitivity, a continuous near‐field distribution across the nanoscale aperture width, and broad spectral coverage. We employ hierarchical subwavelength gold aperture arrays with self‐similar resonator geometries supporting multiscale optical responses across the mid‐infrared. The fractal structure generates multiple distinct resonances through collective surface plasmon polariton excitations, with field enhancement distributed continuously across each aperture. In proof‐of‐concept SEIRA measurements, the platform resolves multiple vibrational fingerprints from a 2.8 nm 1‐octadecanethiol and an ultra‐thin (estimated sub‐5 nm) protein A/G layer across distinct spectral bands. A temporal coupled‐mode theory framework constrained by independently characterized single‐mode subarrays quantitatively describes plasmon–molecule interactions, including conventional and anomalous spectral modulation. These results mitigate the usual bandwidth–field‐localization trade‐off in SEIRA by combining the continuous near‐field enhancement across the aperture width with broadband response, providing a foundation for future multianalyte biosensing studies.