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收集所有交通大學研究人員與學生個人或團體所發表之各項學術著作，涵蓋期刊論文、教師專書、學位論文等項。;;Collections of Publications of NCTU Researchers and Students Like Journal Papers and Thesis.

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Author: search.filters.author.Lee, WeiSubject: search.filters.subject.cholesteric liquid crystals

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Now showing 1 - 5 of 5
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    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.
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    Electrothermo-optical effect in liquid crystals and its applications
    (2017-01-01)
    Hsiao, Yu-Cheng
    ;
    Lee, Wei
    ;
    影像與生醫光電研究所
    ;
    Institute of Imaging and Biomedical Photonics
    Electro-optical effects in liquid crystals (LCs) have been widely utilized in many optical components and photonic devices, thanks to the anisotropic media that can be easily manipulated by an electric field to modulate the light. In general, dielectric heating in LC applications is negligible because their orientational dielectric relaxations occur at high frequencies. Here we focus on a dual-frequency LC characterized by its much lower relaxation frequency. The field-induced heat strongly affects the LC ordering and optical properties. The electrothermo-optical effect reveals an unusual behavior compared with the well-known electro-optical effect in regular LCs. Based on the electrothermo-optical effect, some applications such as optical modulators or tunable optical shutters are demonstrated.
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    Thermodielectric effect in dual-frequency cholesteric liquid crystals
    (2015-01-01)
    Hsiao, Yu-Cheng
    ;
    Lee, Wei
    ;
    影像與生醫光電研究所
    ;
    Institute of Imaging and Biomedical Photonics
    Thermodielectric effect in dual-frequency cholesteric liquid crystals (DFCLCs) is an important issue and has rarely been studied in the past. DFCLC materials have many applications such as fast-switching CLCs, light modulators, and tunable photonic devices. However, DFCLCs characteristically need high operation voltage, which hinders their further development in thin-film-transistor operation. Here we present a lower-voltage switching method based on thermodielectric effect. Dielectric heating effect entails applying an electromagnetic wave to occasion dielectric oscillation heating so to induce the increase in crossover frequency. The subsequent change in dielectric anisotropy of the DFCLC permits the switching, with a lower voltage, from the planar state to the focal conic or homeotropic state. Furthermore, we also demonstrate the local deformation of the CLC helical structure achieved by means of the thermodielectric effect. The wavelength of the deformation-induced defect mode can be tuned upon varying the dielectric heating power. The physics and the calculation of dielectric heating in DFCLCs are described.
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    Photonic Bandgap-Cholesteric Device with Electrical Tunability and Optical Tristability in Its Defect Modes
    (2017-07-01)
    Wu, Po-Chang
    ;
    Hsiao, Chih-Yuan
    ;
    Lee, Wei
    ;
    光電系統研究所
    ;
    影像與生醫光電研究所
    This study proposes a hybrid structure for a one-dimensional (1D) photonic crystal (PC) comprising a tristable cholesteric liquid crystal (CLC) as the defect layer. The CLC exhibits three optically stable states: the Grandjean planar (P), focal conic (FC), and uniform lying helix (ULH) configurations. Specifically, the reflection band of the CLC is set within the photonic bandgap (PBG) of the 1D PC. While the ULH and the FC states can be regarded as the light-on and light-off states for defect-mode peaks in the visible spectrum, respectively, switching the device from the ULH to the P state enables suppression of the transmission of partial defect modes within the PBG. This device possesses many alluring features, such as optical tristability at null applied voltage and transmission tunability of the defect modes, providing a new pathway for the design of multifunctional and energy-efficient optical switches, light shutters, multichannels, and wavelength selectors.
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    Red, Green, and Blue Reflections Enabled in an Electrically Tunable Helical Superstructure
    (2018-03-05)
    Hsiao, Yu-Cheng
    ;
    Yang, Zong-Han
    ;
    Shen, Dong
    ;
    Lee, Wei
    ;
    光電學院
    Materials with tunable optical properties in the visible spectrum are intriguingly important in science and technology. Such desired tunability can be conveniently accomplished by controlling optical materials with electric field. Now an organic film of self-organized helical superstructure producing electrically tunable reflection of visible light is introduced in this work. The composite is a dual-frequency thermosensitive cholesteric (DFTC) material consisting of a liquid crystal host doped with a binary chiral-dopant mixture including a thermosensitive chiral switch. The red, green, and blue reflections in the DFTC film can be obtained by varying the frequencies of a 45 V-rms applied voltage to induce dielectric heating. The resulting variation in temperature and, in turn, in helical pitch length, enables various wavelengths of reflection to render full-color display in a single film without the need of film stacking. As far as it is known, this is the first work to demonstrate broad electrical tuning of cholesteric reflection in red, green, and blue.