Large Near-Infrared Refractive Index Modulation in Ion-Gel-Gated BaSnO₃ for Active Metasurfaces

Electrostatically gated materials can modulate the optical responses of metamaterials through large, high-speed (>MHz) property changes induced in nanometer-scale charge accumulation layers. Transparent conducting oxides are popular materials for electrostatically tunable metasurfaces due to their voltage-tunable plasma frequencies along with epsilon-near-zero dispersion points that fall in the infrared. Ion-gel-gated films of the transparent conductor perovskite BaSnO3 show large, low power changes in carrier density and electron mobility under electron accumulation. However, the corresponding optical changes in ion-gel-gated BaSnO3 are unknown. Here, we study near-infrared refractive index modulation in ion-gel-gated La-doped BSO for active metasurfaces through optical modeling rooted in realistic material data. Near-infrared spectroscopic ellipsometry of as-grown n-type BaSnO3 films establishes Drude optical parameters vs carrier density. We then ion-gel-gate BaSnO3 into electron accumulation, where in situ Hall effect measurements and subsequent electrostatic modeling reveal a ∼4-fold carrier density enhancement near the film surface despite high initial doping of >1020 cm–3. We map doping-dependent Drude parameters onto the measured carrier density modulation, enabling us to model the voltage-dependent near-infrared refractive index changes in BaSnO3, which exceed unity at the 1550 nm telecom band. Finally, our voltage-dependent refractive index data enable simulations of plasmonic metasurfaces with BaSnO3, which we design to achieve reconfigurable beam steering at near-telecom wavelengths. These findings frame ion-gel gated BaSnO3 as a promising material for reconfigurable infrared metasurfaces and motivate the design of similar tunable photonic devices based on its large refractive index changes.