Nematic twist-bend phase of a bent liquid crystal dimer: field-induced deformations of the helical structure, macroscopic polarization and fast switching speeds

TL;DR

This study explores the polar properties, dielectric behavior, and fast electro-optic switching of the nematic twist-bend phase in a bent liquid crystal dimer, revealing potential for advanced device applications.

Contribution

It provides the first experimental evidence of net polar order and fast switching in the nematic twist-bend phase of a bent liquid crystal dimer, linking structural deformations to electro-optic properties.

Findings

High polarization values across the mesophase range.

Sub-millisecond switching times observed.

Electric field-induced deformation of helical structures confirmed.

Abstract

The twist-bend nematic (Ntb) phase is a recent addition to the nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntb phase under an external electric field was predicted in several recent theoretical studies but yet to be experimentally realized. We investigated the polar nature, dielectric properties, electro-optical switching and optical transmission properties of a bent LC dimer CB7CB. The LC showed a relatively high-temperature nematic (N) phase and a lower-temperature nematic (NX) phase (also called Ntb in literature). A threshold-dependent polarization current response with large polarization values was obtained in the entire mesophase range. The associated switching times were found in sub-millisecond region. This ferroelectric-like polarization resulted from collective reorientation of polar cybotactic clusters. In NX phase, electric field-induced deformation of twisted helical structures also contributed to net polarization. Dielectric measurements confirmed the presence of cybotactic clusters via relaxation processes with large activation energies. Deformation of the NX helical structure under external electric field was corroborated by polarized optical microscopy and optical transmission studies. The field-induced deformations, net polar order and fast switching will contribute towards greater understanding of the NX (or Ntb) phase dynamics. It may also find applications in next-generation electro-optic devices.