Solvent-induced self-assembly of uniform lying helix alignment of the cholesteric liquid crystal phase for the flexoelectro-optic effect

Abstract

A uniform lying helix (ULH) alignment of cholesteric liquid crystals (CLCs) is obtained using a solvent evaporation technique. The solvent evaporation method allows for the spontaneous formation of a virtually defect-free alignment, even in the absence of an external electric field. The small amount of solvent diffuses into the LC and changes its phase into the isotropic state where the individual LC molecules are more mobile. As the solvent diffuses out of the LC and consequently evaporates, additional mobility provided by the solvent allows the molecules to reach the lowest energy configuration, dictated by the boundary conditions, the solvent evaporation direction and the elastic forces among the molecules. Compared to a shear-flow-induced alignment, the solvent-induced ULH exhibits a contrast ratio between the bright and dark states that is a factor of 4 greater, due to the low number of defects in the structure. From measurements of the flexoelectro-optic effect, the difference between the splay and bend flexoelectric coefficients, e1 − e3, for the nematic LC E7 is found to be in agreement with the measured values reported in the literature (12.1±1.0 pC/m), demonstrating that the solvent self-aligning does not change the electric response of the medium, while improving its optical properties.

A Compact Full 2π Flexoelectro‐Optic Liquid Crystal Phase Modulator

“…The relatively low efficiency is due to scattering from non‐uniformities in the lying helix alignment of the LC phase modulation layer, but it is anticipated that the maximum efficiency could be significantly improved through the use of high‐quality ULH alignments such as those formed using solvent‐assisted processes. [ 30 ] Additionally, we can consider the Figure‐of‐Merit (FoM) for the modulator, as introduced by Khoo and Wu. [ 31 ] For nematic‐based devices this parameter can be defined as where K 11 is the LC splay elastic constant, Δ n is the optical anisotropy, and γ 1 is the rotational viscosity.…”

Fast and low loss flexoelectro-optic liquid crystal phase modulator with a chiral nematic reflector

“…In terms of efficiency, it was found that this varied between 16% and 44%, depending upon the alignment quality. The relatively low efficiency is due to scattering from non-uniformities in the lying helix alignment of the LC phase modulation layer and it is expected that the maximum efficiency could be significantly improved through the use of high quality ULH alignments such as those formed using solvent-assisted processes 31 . It is also worth stating that for a practical device with a silicon backplane, any unmodulated light would be reflected by the silicon backplane, pass through the LC components and would then be blocked by the polariser at the front of the device.…”

Robust measurement of flexoelectro-optic switching with different surface alignments

“…21 It has also been shown that a very high quality, near mono-domain alignment can be achieved using a solvent evaporation process, although it may be difficult to extend this technique to large areas. 22 Given the complexity of obtaining a high-quality ULH alignment, together with the limitations imposed by the simple measurement methods outlined above, it can be difficult, or at the very least inconvenient, to establish the flexoelectric coefficient combination, e 1 -e 3 . Ideally, a method is needed that works robustly with simple alignments (homeotropic and/or planar) and gives accurate results independent of the precise "quality" of the surface alignment used.…”