Ultrafast Plasmon Dynamics of Low-Loss Sodium Metasurfaces

Alkali metals have emerged as a promising alternative to conventional noble metals for plasmonic applications, offering lower optical loss and significantly reduced materials costs. While sodium, in particular, has demonstrated exceptional potential in the near-infrared (NIR), its reactive nature has limited the study of its plasmonic properties. The recent development of a thermo-assisted spin-coating process paired with phase-shift photolithography has enabled the creation of stable nanostructured sodium thin films, allowing for their extensive optical investigation.

These sodium thin films exhibit notably narrow resonances in the NIR region and demonstrate electron relaxation times competitive with their noble metal counterparts. Through the control of nanostructure pitch and incident angle of probing light, the surface plasmon polariton (SPP) resonance wavelength can be precisely tuned throughout the visible and NIR regions, making these sodium films particularly attractive for nanophotonics, surface-enhanced sensing, and super-resolution imaging applications. However, a gap remains in the fundamental understanding of the ultrafast dynamics of hot carriers in sodium metal, which is essential for realizing its full potential in plasmonic applications.

In this work, we leverage the high sensitivity of SPPs to their metal's bulk properties to investigate hot electron dynamics in nanostructured sodium thin films on polyurethane supports. Through transient reflection measurements, we probe the distinct signatures of electron-electron and electron-phonon interactions in sodium at ultrafast time scales. Our results reveal a unique early-time response 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.