Nanotechnologies / Materials
Displays, Lidars

Crystallography of liquid crystalline cholesteric blue phases
for electro-optic applications
Division of Electrical, Electronic and Infocommunications Engineering, Graduate School of Engineering
Lecturer Hiroyuki Yoshidahttps://researchmap.jp/hiroyukiyoshida

Professor Masanori Ozaki

https://researchmap.jp/read0014092

Nanotechnologies / Materials

Abstract
Nematic liquid crystals that change their refractive index in
response to electric fields are commercialized in displays, but
research is ongoing for better materials to improve their relatively
slow response of several milliseconds. Cholesteric blue phases inwhich rod-like molecules self-organize into three-dimensional periodic structures show a fast electro-optic response in the sub-millisecond range, and are viewed as next-generation materials.
However, there are challenges for their commercialization such as
high driving voltage, hysteresis, and anisotropy in the electro-optic effect due to their crystalline structure. To better understandand control blue phases, we characterized of the structure andstability of crystal twins in blue phase devices. We also proposeda field-assisted method of achieving monodomain alignment overlarge areas.

laser processing, to next-generation devices such as LIDARs andtunable lenses; the commercialization of blue phases will thus lead
to better devices used in various scenes of everyday life.

▍ Background & Results
In nematic liquid crystals used in displays, the rod-like molecules
are aligned uniaxially over large areas, and the refractive index isswitched by an electric filed to tune the light intensity. In blue phases, the molecules form a three-dimensional periodic structure on
the order of few 100 nanometers, leading to the molecules being
switched inside tiny volumes, thereby responding quickly. However,blue phases face challenges for commercialization, such as highdriving voltage, hysteresis and alignment control. The fact that theyshow crystal structure makes the electro-optic response anisotropic, complicating their electro-optic behavior.
Our group has focused on crystallographic methods to understand and control blue phases. In recent work, we analyzed thecrystal twin that forms when the blue phase electro-optic cell isfabricated. We found that the twin plane is the (112) plane of bluephase I subphase, and that twinning can occur both in the lateraland perpendicular directions of a thin, slab-type device, with lifetimes of more than several days. Because the directions of the optic axes differ in the twinned crystals, we concluded that it should
become important to prevent the occurrence of twins for high-performance devices.
To achieve high-quality alignment of blue phases in devices, we
proposed a field-assisted method of alignment control. By exploiting the field-induced phase transition of blue phases, we found it ispossible to achieve monodomain alignment over large areas.

▍ Significance of the research and Future perspective
Our findings on twinned blue phases will provide a physical basis on which to understand the performance of blue phase-based
electro-optic devices. The field-assisted alignment control technology is easy to implement and will be useful in fabricating high-quality blue phase devices. Application of liquid crystals range fromdisplays and spatial light modulators in telecommunications andP a t e n tZhang, Yuxian; Yoshida, Hiroyuki; Ozaki, Masanori et al. Three-dimensional lattice deformation of blue phase liquid crystals under electrostriction. SoftMatter. 2020, 18, p.3328-3334, doi:10.1039/d2sm00244b
Zhang, Yuxian; Yoshida, Hiroyuki; Ozaki, Masanori et al. In Situ Optical Characterization of Twinning in Liquid Crystalline Blue Phases. ACS Appl. Mater.
Tr e a t i s eInterfaces. 2021, 13, p.36130-36137, doi: 10.1021/acsami.1c06873
Cho, SeongYong; Yoshida, Hiroyuki; Ozaki, Masanori et al. Directed self-assembly of soft 3D photonic crystals for holograms with omnidirectional circularpolarization selectivity. Commun. Mater. 2021, 2, p.39 doi: 10.1038/s43246-021-00146-x
URLK e y w o r d liquid crystals, cholesteric blue phase, crystallography, electro-optic materials