Deep-ultraviolet chiral interband plasmonics in silicon nanohelices.

Ultraviolet (UV) plasmonic materials are promising for biosensing and nanophotonics, but practical deployment has been limited by oxidation and corrosion in UV-active metals. Here we introduce chiral silicon nanohelices as a robust and scalable platform for UV plasmonics enabled by interband-driven negative permittivity of silicon below ~300 nm. Wafer-scale arrays of three-dimensional chiral Si nanohelices are fabricated by glancing angle deposition and exhibit pronounced UV chiroptical responses, including strong circular dichroism near 270 nm. The nanohelices show exceptional stability, retaining their UV chiral plasmonic signatures after >4 years of ambient storage and under strongly acidic conditions (pH 1.9). As a proof of concept, we translate the UV chiroptical resonance shifts into a refractive index sensing scheme, demonstrating competitive detection performance in the UV. These results establish silicon nanohelices as a chemically robust, application-ready platform for UV chiral plasmonics.