A ferroelectric liquid crystal confined in cylindrical nanopores: Reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations

TL;DR

This study investigates ferroelectric liquid crystals confined in nanopores, revealing reversible smectic layer buckling, enhanced optical rotation, and ultra-fast electro-optical excitations due to confinement effects and surface anchoring conditions.

Contribution

It demonstrates how nanopore confinement and surface treatment induce reversible layer buckling, enhance optical properties, and significantly increase the speed of Goldstone excitations in ferroelectric liquid crystals.

Findings

Reversible smectic layer buckling observed in confined liquid crystals.

Optical rotatory power increased up to tenfold compared to bulk.

Goldstone phason excitations are two orders of magnitude faster in confinement.

Abstract

The orientational and translational order of a thermotropic ferroelectric liquid crystal (2MBOCBC) imbibed in self-organized, parallel, cylindrical pores with radii of 10, 15, or 20 nm in anodic aluminium oxide monoliths (AAO) are explored by high-resolution linear and circular optical birefringence as well as neutron diffraction texture analysis. The results are compared to experiments on the bulk system. The native oxidic pore walls do not provide a stable smectogen wall anchoring. By contrast, a polymeric wall grafting enforcing planar molecular anchoring results in a thermal-history independent formation of smectic C* helices and a reversible chevron-like layer buckling. An enhancement of the optical rotatory power by up to one order of magnitude of the confined compared to the bulk liquid crystal is traced to the pretransitional formation of helical structures at the smectic-A*-to-smectic-C* transformation. A linear electro-optical birefringence effect evidences collective fluctuations in the molecular tilt vector direction along the confined helical superstructures, i.e. the Goldstone phason excitations typical of the para-to-ferroelectric transition. Their relaxation frequencies increase with the square of the inverse pore radii as characteristic of plane-wave excitations and are two orders of magnitude larger than in the bulk, evidencing an exceptionally fast electro-optical functionality of the liquid-crystalline-AAO nanohybrids.