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Electrically induced bandwidth broadening in polymer stabilized cholesteric liquid crystals
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) CLCs with no polymer stabilization. (a) Positive Δɛ material response to low DC fields; the blue shift indicates the tilting of the helix while the broaden and reduced transmission (scatter) results from the unwinding of the helical structure. (b) Negative Δɛ response to DC fields shows little effect even with large fields. Similar responses are seen in both materials if AC fields are applied rather than DC fields.

Image of FIG. 2.
FIG. 2.

(Color online) CLCs (CB15 and NLC ZLI-2079) samples with low crosslink polymerization. (a) Transmission spectra with applied AC field show no symmetrical broadening behavior and require high fields to switch. (b) Reflection and transmission spectra showing symmetrical broadening at low DC fields gives a 3.7 fold increase in bandwidth.

Image of FIG. 3.
FIG. 3.

(Color online) Bandwidth with dc field for CLCs containing CB15 with different Δn NLCs. The data shows near parallel curves suggest little dependence on the Δn of the material. The fitted lines show a comparison of Δλ as a function of simulated maximum pitch distortion (Δp/po bottom x-axis) at various birefringence values and those experimentally obtained as a function of applied DC electric field.

Image of FIG. 4.
FIG. 4.

(Color online) (a) Increase of bandwidth with cell thickness. CLC mixture contained CB15 and NLC ZLI-2079. (b) Band broadening with dc field for two CLC mixtures. The S811 mixture has a −Δɛ, while the CB15 mixture has a more positive Δɛ value. The nematic LC was ZLI-2079.

Image of FIG. 5.
FIG. 5.

(Color online) (a) The approximate structure of a cholesteric slab using the Berreman 4 × 4 method. (b) Various models for symmetrical broadening such as no distortion (blue bold dashed), sinusoidal distortions (red bold dash-dot-dot and red thin dash), and linear (black bold solid). (c) The resulting transmission spectra from the pitch variations shown in b) with po = 0.35 μm, d = 10 μm,  = 1.56, Δn = 0.17, and Δp/po = 0.2.

Image of FIG. 6.
FIG. 6.

(Color online) Comparison between the experimental and theoretical dependences of the transmission spectra for a CLC mixture of (a) 2079:CB15 in an 8.8 μm cell and (b) 2806:CB15 in a 10 μm cell. The simulations show similar features to that of the experimental spectra that indicate pitch distortions of approximately 13%–15% occur for electric fields between 2.8 and 3 V/μm.

Image of FIG. 7.
FIG. 7.

(Color online) (a) Reflection spectra from the exposed side of the sample with zero, forward, and reverse field applied. (b) Pitch variation with applied dc field.

Image of FIG. 8.
FIG. 8.

(Color online) Band broadening dependence on LC cross-linking monomer (RM257) used for polymer stabilization. CLC samples were made from CB15 and NLC ZLI-2079.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Electrically induced bandwidth broadening in polymer stabilized cholesteric liquid crystals