Nano helical cholesteric liquid crystals exhibit long term bistability for energy saving smart windows

When an electric field is applied to a cholesteric liquid crystal (CLC) with negative dielectric anisotropy, a focal conic texture emerges depending on the helical pitch. We found that at high concentrations of chiral dopant, where the helical pitch becomes at the nanoscale (≈100 nm or less), the nano-helices comprising typically 5-10 turns can act as rigid entities with an effective dielectric anisotropy larger along the helix axis than perpendicular to it. These rigid nanohelices can reorient under an applied field between two stable configurations: a focal conic state, where the helix axis lies in the substrate plane, and a vertically aligned state. Based on this mechanism, a long term bistable smart window is demonstrated using homeotropically aligned, ultra-short-pitch CLCs. The device exhibits reversible switching between transparent and opaque, long termly stable states by tuning the amplitude and frequency of an AC electric field. Compared with longer-pitch (>500 nm) CLCs, the ultra-short helix (USH) device shows higher optical contrast, stronger scattering, and superior long-term stability. Electro-optical characterization, including Maltese cross observation, haze measurements, and contrast analysis, confirms the enhanced bistability at least up to 96 hours, with negligible degradation after a few months. This work demonstrates the superiority of USH CLCs as energy saving smart windows.