Au nanoantennas embedded into silicon for surface enhanced infrared absorption spectroscopy

Surface-enhanced infrared absorption (SEIRA) spectroscopy based on plasmonic nanoantennas is a powerful tool for ultrasensitive molecular detection; however, further enhancement of its performance remains a key challenge. In this work, we demonstrate arrays of gold nanoantennas embedded into a silicon substrate using metal-assisted chemical etching (MACE). This approach enables precise control of the nanoantenna embedment depth and formation of nanoscale capillary gaps. We show that both the embedment depth and the capillary gap critically influence the localized surface plasmon resonance (LSPR) energy and near-field distribution. Full-wave electromagnetic simulations reveal that the capillary gap acts as a key parameter governing field confinement. By comparing experimental and simulated IR transmission spectra, the gap size is determined to be 6±1 nm. The embedded geometry enhances the effective interaction volume between the analyte and the plasmonic field, resulting in improved SEIRA sensitivity by factor 3. This approach provides a robust platform for designing highly sensitive mid-IR plasmonic sensors.