Mid‐Infrared Extreme Grazing Brewster Effect in Natural Crystals for Surface‐Enhanced Molecular Spectroscopy

The realization of the Brewster effect at extreme grazing incidence is fundamentally constrained by the requirement for ultra‐high permittivity materials, rendering it impractical for wide‐angle optical applications and confining wide‐angle metamaterial implementations largely to the microwave spectrum. Here, we circumvent these challenges by utilizing natural crystals with Reststrahlen bands to realize a mid‐infrared extreme grazing Brewster effect (>80°, surpassing the traditional limit of ≈70°). By leveraging the strong electric field enhancement associated with the grazing incidence configuration, we demonstrate enhanced detection of molecular vibrational signatures in polystyrene nanolayers. Additionally, we introduce active tunability of the grazing Brewster angle (>85°) through crystal rotation and dynamic graphene modulation. Supported by rigorous theoretical analysis and experimental validation, our approach establishes a robust platform for grazing‐incidence optics, enabling advancements in surface‐enhanced molecular spectroscopy and ultra‐wide‐angle planar optics.