Browsing by Author "Yu, Chia-Hua"

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Alternative generation of well-aligned uniform lying helix texture in a cholesteric liquid crystal cell
(2017-10-01)
Yu, Chia-Hua
;
Wu, Po-Chang
;
Lee, Wei
;
光電學院
;
College of Photonics
This work demonstrates a simple approach for obtaining a well-aligned uniform lying helix (ULH) texture and a tri-bistable feature at ambient temperature in a typical 90 degrees-twisted cell filled with a short-pitch cholesteric liquid crystal. This ULH texture is obtained at room temperature from initially field-induced helix-free homeotropic state by gradually decreasing the applied voltage. Depending on the way and rate of reducing the voltage, three stable states (i.e., Grandjean planar, focal conic, and ULH) are generated and switching between any two of them is realized. Moreover, the electrical operation of the cell in the ULH state enables the tunability in phase retardation via the deformation of the ULH. The observations made in this work may be useful for applications such as tunable phase modulators and energy-efficient photonic devices. (C) 2017 Author(s).
Electro-Thermal Formation of Uniform Lying Helix Alignment in a Cholesteric Liquid Crystal Cell
(2019-04-01)
Yu, Chia-Hua
;
Wu, Po-Chang
;
Lee, Wei
;
影像與生醫光電研究所
;
Institute of Imaging and Biomedical Photonics
We demonstrated previously that the temperature of a sandwich-type liquid crystal cell with unignorable electrode resistivity could be electrically increased as a result of dielectric heating. In this study, we take advantage of such an electro-thermal effect and report on a unique electric-field approach to the formation of uniform lying helix (ULH) texture in a cholesteric liquid crystal (CLC) cell. The technique entails a hybrid voltage pulse at frequencies f(1) and, subsequently, f(2), which are higher and lower than the onset frequency for the induction of dielectric heating, respectively. When the cell is electrically sustained in the isotropic phase by the voltage pulse of V = 35 V-rms at f(1) = 55 kHz or in the homeotropic state with the enhanced ionic effect at V = 30 V-rms and f(1) = 55 kHz, our results indicate that switching of the voltage frequency from f(1) to f(2) enables the succeeding formation of well-aligned ULH during either the isotropic-to-CLC phase transition at f(2) = 1 kHz or by the electrohydrodynamic effect at f(2) = 30 Hz. For practical use, the aligning technique proposed for the first time in this study is more applicable than existing alternatives in that the obtained ULH is adoptable to CLCs with positive dielectric anisotropy in a simple cell geometry where complicated surface pretreatment is not required. Moreover, it is electrically switchable to other CLC textures such as Grandjean planar and focal conic states without the need of a temperature controller for the phase transition, the use of ion-rich LC materials, or mechanical shearing for textural transition.
Voltage-induced pseudo-dielectric heating and its application for color tuning in a thermally sensitive cholesteric liquid crystal
(1970-01-01)
Wu, Po-Chang
;
Wu, Guan-Wei
;
Yu, Chia-Hua
;
Lee, Wei
;
照明與能源光電研究所
We previously proposed an electrical approach enabling the tuning of the center wavelength lambda(c) of the cholesteric liquid crystal (CLC) bandgap in the full-color visible spectrum based on the electro-thermal effect. The idea involved the design of a negative CLC with a thermally sensitive bandgap feature and the use of a sandwich-type cell with finite electrode conductivity, allowing the control of cell temperature by applied voltage via pseudo-dielectric heating. It has been demonstrated experimentally that the induced pseudo-dielectric heating is predominated by the pseudo-dielectric relaxation originating from the designated cell geometry. On the basis of this technique, key factors determining the tuning efficacy of the temperature and thus lambda(c) are primarily investigated in this study. Our results suggest that lowering the electrode resistivity and the specific heat conductivity of the cell can promote the maximum tunable temperature range. Expectedly, optimizing the electrode area, cell gap and dielectric permittivity of the CLC favors a decreased relaxation frequency and, in turn, reducing the voltage as well as the frequency required for lambda(c) tuning.