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Optical isotropy and iridescence in a smectic ‘blue phase’


When liquid crystal molecules are chiral, the twisted structure competes with spatially uniform liquid crystalline orders, resulting in a variety of modulated liquid crystal phases, such as the cholesteric blue phase1, twist grain boundary2,3,4 and smectic blue phases5. Here we report a liquid crystal smectic blue phase (SmBPiso), formed from a two-component mixture containing a chiral monomer and a ‘twin’ containing two repeat units of the first molecule connected by a linear hydrocarbon spacer. The phase exhibits the simultaneous presence of finite local-order parameters of helices and smectic layers, without any discontinuity on a mesoscopic length scale. The anomalous softening of elasticity due to a strong reduction in entropy caused by mixing the monomer and the twin permits the seamless coexistence of these two competing liquid crystal orders. The new phase spontaneously exhibits an optically isotropic but uniformly iridescent colour and automatically acquires spherical symmetry, so that the associated photonic band gap6,7,8,9 maintains the same symmetry despite the local liquid crystalline order. We expect a range of unusual optical transmission properties based on this three-dimensional isotropic structure, and complete tunability due to the intrinsic softness and responsiveness of the liquid crystalline order against external fields.

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Figure 1: Phase diagram for the twin/monomer mixture.
Figure 2: Polarizing microscope photographs in SmBPs.
Figure 3: Macroscopic and microscopic structural analysis in SmBPs.
Figure 4: Mechanical properties in SmBPs.

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We thank L. Hough and N. A. Clark for suggesting methods of preparing the frozen replica of the TEM observation.

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Correspondence to Jun Yamamoto.

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Supplementary information

Supplementary Figure S1

A freeze-fracture replica of the TEM photograph of a fractured SmBPIso. The scale bar corresponds to 1 µm. We would like to emphasize that the fractured surface profile represents the spatial distribution of the layers since they can be fractured easily along the plane of the layers. (JPG 130 kb)

Supplementary Figure S2

A magnified photograph of Supplementary Fig. S1. The scale bar corresponds to 200 nm. The thin string-like pattern indicates that the interconnected structure was formed by the multi-layer bundle. (JPG 122 kb)

Supplementary Figure S3

A frozen replica of the TEM photograph of the free air interface of SmBPIso. The scale bar corresponds to 1 µm. The surface step of the smectic layer curved automatically and formed a wavy structure due to the bulk of the multi-lamellar interconnected structure. (JPG 128 kb)

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Yamamoto, J., Nishiyama, I., Inoue, M. et al. Optical isotropy and iridescence in a smectic ‘blue phase’. Nature 437, 525–528 (2005).

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