Extended duration bistability in ultra-short-pitch cholesteric liquid crystals for smart window and rewritable display applications

When cholesteric liquid crystals (CLCs) are doped with sufficiently high concentrations of a chiral dopant, the helical pitch is reduced to ultra-short values (≈100 nm or less). Under these conditions, the CLC tends to form nano-helical domains comprising typically 5–10 turns, which behave as rigid entities with an effective dielectric anisotropy that is larger along the helix axis than perpendicular to it if the molecular dielectric anisotropy is negative. These rigid helices can reorient under an applied electric field between two stable configurations: a focal-conic state, in which the helix axis lies predominantly in the substrate plane, and a vertically aligned state. Exploiting this mechanism, we demonstrate a novel highly bistable smart window based on homeotropically aligned ultra-short-pitch CLCs. The device exhibits reversible switching between transparent and opaque stable states through appropriate control of the amplitude and frequency of an applied AC electric field. Compared with longer-pitch (>500 nm) CLCs, the ultra-short-pitch device delivers higher optical contrast, stronger light scattering, and significantly enhanced long-term stability. Electro-optical characterization, including haze and contrast measurements, confirms robust bistability for up to 96 hours, with only minimal degradation observed over several months. These results highlight the superiority of ultra-short-pitch CLCs for energy-saving smart window and rewritable display technologies.