Elasticity-Driven Periodic Polarization Patterns in a Chiral Ferroelectric Nematic Fluid.

Ferroelectric nematic phases are a new class of polar fluids in which spontaneous polarization is directly coupled to the orientational order, providing unique opportunities for creating self-organized materials with spatially modulated electric polarization and nonlinear optical response. Here we report the spontaneous emergence of polarization-modulated textures in a chiral ferroelectric nematic material close to the transition to the chiral twist-bend ferroelectric nematic phase. By systematically varying cell thickness and surface anchoring conditions, we map the formation of these modulated states, revealing stripe, square, and hexagonal morphologies determined via confinement conditions. These structures are directly translated into periodic modulation of the nonlinear optical response, as evidenced by second harmonic generation imaging. Comparison with an elasticity-based theoretical framework and numerical free energy minimization shows that the instability originates from the softening of the bend elastic constant in the chiral nematic phase as the system approaches the lower-temperature heliconical polar phase. The resulting elastic frustration, combined with confinement, drives the formation of spatially periodic director distortions, highlighting ferroelectric nematic fluids as a promising platform for self-assembled nonlinear optical materials.