Tungsten–Carbon Nanohelix Arrays by Helium-Ion Direct Writing: Geometry-Dependent Circular-Polarization Response in the Visible

Abstract 3D nanohelices provide a direct route to control light–matter interactions with circular polarization at the nanoscale. Here, we report periodic arrays of W–C nanohelices fabricated by He+ focused-ion-beam-induced deposition (FIBID), combining direct-write 3D growth with high spatial resolution and W-rich deposits (∼65 at. % W). Two arrays with the same lateral geometry were fabricated while varying only the vertical pitch, corresponding to helical ratios HR ≈ 1 and HR ≈ 2. Transmittance measurements under circularly polarized light reveal a clear geometry-dependent response. The compact nanohelices (HR ≈ 1) show a larger circular-polarization-dependent transmission contrast (g ≈ 0.17, θ ≈ 2.35°), moderate transmittance (∼30–50%), and a sign inversion near 600 nm. The larger-pitch helices (HR ≈ 2) show higher transmittance (∼35–100%) with a weaker but stable response (g ≈ 0.13, θ ≈ 1.60°). Qualitative simulations reproduce the main spectral trends and support distinct optical responses of the two pitch geometries.