Soft Matter Macro and Micro-Photonics workshop

Self-organization and polar order in ferroelectric nematics for nonlinear optics

Nerea Sebastian

Jožef Stefan Institute, Ljubljana, Slovenia

The discovery of the ferroelectric nematic (NF) phase established the first example of a liquid with long-range ferroelectric order, opening new opportunities for functional soft materials. The absence of inversion symmetry in this phase enables second-order nonlinear optical processes such as second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC), making ferroelectric liquid crystals an appealing platform for nonlinear optics. However, the realization of strong nonlinear responses is closely tied to molecular architecture, posing a challenge for the development of materials since the stability of the ferroelectric nematic phase itself depends sensitively on molecular structure. Recent developments in molecular design have expanded the family of polar liquid crystalline phases, revealing a broader “ferroelectric nematic realm” that includes phases such as SmAF, SmCF, and the polar heliconical nematic (NTBF). These discoveries demonstrate how variations in molecular structure can produce new polar mesophases and enable the formation of self-assembled periodic architectures.

In this presentation, I will present measurements of second-order nonlinear susceptibilities in selected materials. Then, I will focus on the emergence of modulated structures in ferroelectric chiral nematic systems near the transition to the N*TBF. We investigate how these structures form under confinement and how their morphology evolves with cell thickness. Using second-harmonic generation microscopy, we probe the associated nonlinear optical response. Results indicate that the observed instabilities arise from a softening of the bend elastic constant upon approaching the twist-bend ferroelectric nematic phase, providing a mechanism for the spontaneous formation of periodic polar structures.