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Spotlight on Optics


  • November 2010

Optics InfoBase > Spotlight on Optics > Cholesteric liquid crystal devices with nanoparticle aggregation

Cholesteric liquid crystal devices with nanoparticle aggregation

Published in Optics Express, Vol. 18 Issue 21, pp.22572-22577 (2010)
by Shie-Chang Jeng, Shug-June Hwang, Yu-Hsiang Hung, and Sheng-Chieh Chen

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Spotlight summary: Cholesteric liquid crystals (CLCs) have been intensively studied for their unique selective reflection characteristics, since they reflect only circularly polarized light coinciding with the LC helical rotation handedness. Because of their inherent Bragg reflection properties, their center wavelength is given by λ0 = <n>*P, and their bandwidth is δλ = δn*P = δ*λ0&slash;<n>, where <n> δn, and P are the average LC refractive index, LC birefringence, and the helix pitch, respectively. The birefringence of LC materials with proper physical/chemical properties (such as good alignment capability) is typically less than 0.3. Thus, the reflection bandwidth is usually limited to about 100 nm, resulting in monocolored reflective displays. To obtain more user-friendly white-black color displays, broadband reflection is necessary. There are generally two effective strategies for achieving this: (1) the generation of multiple helical pitches, e.g., through a pitch gradient along the cell thickness direction, and (2) the formation of multidomains of similar pitches by inclining different helices at different angles. In the later strategy, differently inclined planar structures lead to an averaging effect of the various angles between incident light and the helix axis of each domain, covering more wavelengths to achieve a white color. One successful approach is the polymer stabilized CLC cells, which create multidomains by the polymer network.

In this paper, the authors have demonstrated an alternative new approach that achieves broadband CLC displays by using the aggregation of polyhedral oligomeric silsesquioxane (POSS) for multidomain formation. POSS aggregates are known for their capability of providing a stable homeotropic alignment of LC materials. In addition, owing to the hydrogen bond formed between hydroxyl compounds in the POSS side chain, clusters could be formed to create multidomain in CLC cells. A mixture of POSS nanoparticles and CLC material is first heated into isotropic phase and injected into cells without the conventional polyimide alignment material. By controlling the POSS concentration and cooling rate, one can adjust the POSS clusters and therefore the CLC multidomain properties. From the reported results, a low POSS concentration of less than 10% and a fast cooling rate of around 80¬oC/min are preferred to provide a small feature size for the clusters. By breaking the uniform planar helix structure into multidomains, the wavelength range of strong reflection could be expanded to range from 400 to greater than 600 nm. In addition, the light network between POSS clusters leads to relatively weak scattering, which results in a good dark state when the CLC cell is switched into the focal conic state. Further suppression of light leakage in the POSS-dispersed CLC cells in the black state and improvement of the device stability will be important for this new technique in the near future.

The POSS-dispersed CLC technology presented by the authors is really exciting owing to the following two major aspects: (1) it can lead to LC alignment by avoiding the conventional PI layer, which requires high temperature treatment, and (2) it can also lead to broadband/white reflection, providing users with a comfortable and natural reading experience. These properties could make it quite suitable for flexible bistable reflective CLC displays with good outdoor readability and low power consumption. It can be foreseen that extensive research interest and efforts will lead to further enhancement of the device’s performance.

--Zhibing Ge

Technical Division: Optoelectronics
ToC Category: Optical Devices
OCIS Codes: (160.3710) Materials : Liquid crystals
(160.4236) Materials : Nanomaterials

Posted on November 12, 2010

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