Ultrafast Plasmon Dynamics of Low-Loss Sodium Metasurfaces.

Alkali metals are considered as a promising alternative to conventional noble metals for plasmonic applications, offering lower optical loss and significantly reduced material costs. The recent development of a thermo-assisted spin-coating process paired with phase-shift photolithography has enabled the creation of stable nanostructured sodium, which exhibits narrow resonances in the near-infrared (NIR) region and demonstrates free electron relaxation times comparable to noble metals. Through the control of nanostructure pitch and light incident angle, the surface plasmon polariton (SPP) resonance wavelength can be tuned throughout the visible and NIR regions, making nanostructured sodium particularly attractive for nanophotonics, surface-enhanced sensing, and photocatalytic applications. In this work, we investigate hot electron dynamics in nanostructured sodium thin films on polyurethane supports by leveraging the high sensitivity of SPPs to their metal's bulk properties. Through optical transient reflectance measurements, we probe the distinct signatures of electron-electron and electron-phonon interactions in sodium at ultrafast time scales. Our results show the unique early time response of sodium that differs from those observed in noble metals, providing key insight into sodium-based plasmonics. This comprehensive understanding of hot electron dynamics will enable more efficient design and implementation of sodium in next-generation plasmonic devices and applications where hot electron processes are critical considerations.