Electric-field controllable optical activity in the nano-segregated system composed of rod- and bent-core liquid crystals [Invited]
Spotlight summary: One of the most exciting aspects of soft-matter research is the constantly expanding number of complex fluid phases that nature provides. And when a newly discovered phase has the potential for unique applications, the level of interest is even higher.
Such is the case when a nematic liquid crystal and a bent-core liquid crystal with a B4 phase are mixed. The B4 phase is one in which there is spontaneous chiral segregation. Even though the bent-core compound is not chiral, regions with chiral order of opposite handedness spontaneously form.
Prior work suggests that such a bent-core material forms helical nanofilaments when mixed with a nematic liquid crystal. When the nematic liquid-crystal material is isotropic, the helical nanofilaments induce little order in the isotropic regions. But when the liquid-crystal material is in its nematic phase, the helical organization of the nanofilaments induces chiral order within the orientationally ordered nematic liquid crystal. This is similar to what happens when a chiral dopant is added to a nematic liquid crystal.
In a nematic–B4 mixture, the chiral order in the nematic liquid crystal produces local regions that possess circular dichroism and optical activity. These optical properties can be measured by focusing light on small regions of the sample while observing the changes in the polarization state of the light.
Araoka et al. report that the application of an electric field to the material serves to reorient the nematic liquid crystal parallel to the electric field, thereby decreasing the chiral order and hence the local circular dichroism and optical activity. The optical activity depends on wavelength, but at one wavelength, varying the applied voltage from 0 to 15 V changes the optical activity from 0.7 to 0 deg/µm. This magnitude of optical activity is similar to what is observed in oriented chirally doped nematic liquid crystals and is surpassed only in such materials close to the condition in which the wavelength of the light in the material and the pitch of the helix of the material are matched.
Interestingly, the application of an electric field to an oriented chiral nematic liquid crystal also reduces its optical activity, but often the original texture and thus the original optical activity does not return when the electric field is removed. The helical structure of the B4 nanofilaments must not be affected—or at least not permanently affected—by the application of the electric field, because Araoka et al. demonstrate that the original optical activity always returns after multiple switching of the electric field on and off. Clearly, nematic–B4 mixtures have some unique properties that may find their way into innovative optical devices.
--Peter J. Collings
ToC Category: Chiral Optical Materials
|OCIS Codes:||(160.3710) Materials : Liquid crystals|
|(160.1585) Materials : Chiral media|
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