Liquid crystal ‘blue phases’ with a wide temperature range

Abstract

Liquid crystal ‘blue phases’ are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals1. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections2 that are controllable with external fields3,4,5,6,7,8. Blue phases may be considered as examples of tuneable photonic crystals9 with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally1, is that they have limited thermal stability: they exist over a small temperature range (0.5–2 °C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 °C down to 16 °C. We prove this with optical texture analysis, selective reflection spectroscopy, Kössel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.

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Figure 1: Blue phase materials, temperature dependence of optical textures and Kössel diagrams.
Figure 2: Spectral properties of blue phases as a function of temperature and composition.
Figure 3: Voltage- and time- dependent colour switching of BP I*.

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Acknowledgements

H.J.C. thanks the EPSRC, UK, who funded the materials research under their Displays and Functional Materials initiatives. Author Contributions H.J.C. invented the flexoelectric materials, was responsible for the project planning, wrote the Letter and was responsible for the tentative explanation. M.N.P. carried out all the experimental work, including microscopic texture and data analysis.

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Correspondence to Harry J. Coles.

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Supplementary information

Supplementary Figure S1

Results of the DSC experiment.

Supplementary Figure S2

Blue Phase Textures.

Supplementary Figure S3

BPI* texture in a thick cell as a function of temperature.

Supplementary Figure Legends

Text to accompany the above Supplementary Figures.

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Coles, H., Pivnenko, M. Liquid crystal ‘blue phases’ with a wide temperature range. Nature 436, 997–1000 (2005). https://doi.org/10.1038/nature03932

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