Electrically Tunable Heliconical Smectic Superstructure in Polar Fluids

The ferroelectric nematic (N_F) phase and related polar liquid-crystalline phases form a new class of strongly polarized yet fluid soft matter. Well-recognized heliconical ferroelectric phases such as N_TBF and ${\mathrm{SmC}}_P^H$ exist, but their electro-optic functionality in smectic systems has been largely unexplored. In this study, we report a newly designed single-component achiral molecule exhibiting a hierarchical polar phase sequence: SmA_F-N_TBF-HEC- ${\mathrm{SmC}}_P^H$ . The key feature of the ${\mathrm{SmC}}_P^H$ phase is its ability to form a stable macroscopic orientation without any alignment layers and to enable continuous, reversible pitch modulation over a wide spectral range at ultralow electric fields, approximately one-third of those required for conventional heliconical nematics. The system exhibits characteristic electro-optic properties in the sub-kilohertz range, differentiating it from heliconical materials controlled by dielectric-elastic balance at higher frequencies. In addition, the ${\mathrm{SmC}}_P^H$ phase's polar heliconical smectic structure facilitates enhanced second-harmonic generation via favorable phase matching, thereby establishing a simple and robust platform for low-voltage photonic applications.