Nanocast sodium metasurfaces for enhanced light-matter coupling

Sodium has been proven to be a plasmonic material superior to noble metals. However, its high chemical reactivity poses a major obstacle to direct nanopatterning via conventional techniques, hindering the pursuit of sodium metasurfaces that simultaneously deliver high coupling efficiency and high quality factor. In this work, we developed a nanocast sodium-argon metasurface by rapidly cooling liquid sodium on a polymethyl methacrylate nanomold. This metasurface supports high-Q plasmonic resonances with record-high performance, surpassing previously reported alkali metal plasmonic systems. The liquid sodium consistently maintains an ultra-low filling depth regardless of the mold depth, which not only creates a nonlocal photonic configuration but also affords precise control over the radiative-to-dissipative loss ratio. Such casting behavior enables the metasurface to transition from under-coupling to critical-coupling regimes, thereby achieving high-efficiency, tunable second harmonic generation. We further provide direct experimental evidence that sodium outperforms silver in nonlinear optical performance. By leveraging the robustness inherent to the nonlocal nanocasting technique for sodium-argon metasurfaces, we demonstrate a large-area second harmonic generation metasurface with polarization-switchable imaging. The proposed sodium nanocasting technique opens an alternative avenue for sustainable high-performance plasmonics.