Browsing by Author "Wu, Po-Chang"

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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
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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).
Dielectric and Electro-Optical Properties of Polymer-Stabilized-Blue-Phase Cells
(2015-08-13)
Hsu, Hsuan-Tsung
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Wu, Po-Chang
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Lee, Wei
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光電學院
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影像與生醫光電研究所
Dielectric and phase behaviors of blue-phase liquid crystals
(2014-11-01)
Chen, Sheng-Chieh
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Wu, Po-Chang
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Lee, Wei
;
影像與生醫光電研究所
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Institute of Imaging and Biomedical Photonics
Dielectric spectra of two pristine blue-phase liquid crystals and a polymer-stabilized counterpart were acquired at various temperatures in conjunction with simultaneous observations of their optical transmission spectra and birefringent textures. The temperature-dependent dielectric data at 5.5 kHz were specifically retrieved for further examination of the feasibility of the dielectric technique as a means to identify phase transitions. With complementary identification by optical images at specific temperatures, it is suggested that investigations of the second derivative of the real-part permittivity with respect to the temperature can be considered as one of the applicable ways to depict the phase sequence of the blue-phase liquid crystals on the cooling process from the isotropic phase through the chiral nematic phase. (C) 2014 Optical Society of America
Dielectric characterization and voltage holding ratio of blue-phase cells
(2016-09)
Wu, Po-Chang
;
Hsu, Hsuan-Tsung
;
Chen, Hsin-Li
;
Lee, Wei
;
光電學院
The phase behaviors of a blue-phase (BP) liquid crystal in a planar-aligned cell were investigated by means of temperature-dependent dielectric spectroscopy. With auxiliary observations of optical transmission spectra and birefringent textures, we found that the transition temperatures of two adjacent mesophases, including the cholesteric-to-BPI, the BPI-to-BPII, and the BPII-to-isotropic, can clearly be distinguished by the first and second derivatives of the real-part dielectric permittivity function to the temperature at a specific frequency. Furthermore, an attempt to evaluate the device performance of a polymer-stabilized BP, derived from the photopolymerization of two monomers (R1V1257 and TMPTA), was achieved with a new circuital design for the voltage-holding-ratio (VHR) measurement. Preliminary results of the VHRs of samples at 180 Hz and their temperature dependence were obtained accordingly. (C) 2015 Elsevier B.V. All rights reserved.
Dielectric characterization and voltage holding ratio of blue-phase cells
(2016-09-01)
Wu, Po-Chang
;
Hsu, Hsuan-Tsung
;
Chen, Hsin-Li
;
Lee, Wei
;
光電學院
The phase behaviors of a blue-phase (BP) liquid crystal in a planar-aligned cell were investigated by means of temperature-dependent dielectric spectroscopy. With auxiliary observations of optical transmission spectra and birefringent textures, we found that the transition temperatures of two adjacent mesophases, including the cholesteric-to-BPI, the BPI-to-BPII, and the BPII-to-isotropic, can clearly be distinguished by the first and second derivatives of the real-part dielectric permittivity function to the temperature at a specific frequency. Furthermore, an attempt to evaluate the device performance of a polymer-stabilized BP, derived from the photopolymerization of two monomers (R1V1257 and TMPTA), was achieved with a new circuital design for the voltage-holding-ratio (VHR) measurement. Preliminary results of the VHRs of samples at 180 Hz and their temperature dependence were obtained accordingly. (C) 2015 Elsevier B.V. All rights reserved.
Dopant Effect and Cell-Configuration-Dependent Dielectric Properties of Nematic Liquid Crystals
(2013)
Lin, Feng-Ching
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Wu, Po-Chang
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Jian, Bo-Ru
;
Lee, Wei
;
光電學院
The dielectric polarizations induced by impurity ions in both high- and low-resistivity nematic liquid crystals (NLCs) were investigated. Upon adding carbon nanotubes (CNTs) as a dopant, the former showed no distinct change in the low-frequency (< 10(2) Hz) dielectric spectrum, whereas the latter exhibited dramatic decrease in dielectric constant in that the CNTs remarkably trapped the impurity ions in the NLC of high ionic content. Consequently, the dopant raised the voltage holding ratio by 26% and prolonged the lifetime of the cell. Also investigated were ionic behaviors in the low-resistivity NLC confined in cells with three different configurations. The diffusion constant of the ions in homeotropic cells was found to be the greatest, and the dc conductivity, determined by the diffusion constant, was also higher in the homeotropic ones.
Dye- and dye-doped liquid crystals in biosensing
(2018-01-01)
Lee, Mon-Juan
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Chiang, Yi-Lun
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Karn, Abhishek
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Wu, Po-Chang
;
Lee, Wei
Liquid crystals (LCs) have been suggested to have a place in biological sensing for detection and quantitation of biomolecules. Through many years, texture observation has long been the core technique in LC-based bioassays. Its principle stems from the texture change of LCs due to the interruption of the initially homeotropic alignment in nematic bulks or the radial-to-bipolar configuration transition in LC droplets in the presence of biomolecules. Biodetection through optical texture observation is convenient but remains a qualitative method, and alternative approaches of detection and quantitative analysis are necessary for the development of a practical LC-based biosensor. In this study, we explore the potential of two dye-enhanced LCs, a dye LC (DLC) and a dye-doped LC (DDLC), in biosensing and protein quantitation. Depending on the chromophore or dye incorporated in DLC and DDLC, the dichroic features of these LCs enable us to analyze the change in their orientation in the presence of biomolecules by transmission spectrometry, from which spectral parameters are derived to establish novel LC-based protein quantitative methods.
Dye-liquid-crystal-based biosensing for quantitative protein assay
(2018-03-01)
Wu, Po-Chang
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Karn, Abhishek
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Lee, Mon-Juan
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Lee, Wei
;
Chen, Chao-Yuan
Dye liquid crystals (DLCs) have been extensively investigated in the field of liquid crystal display, but their potential in biosensing has not been explored. To provide insights into the potential of DLCs as a means of biosensing material, a DLC-based biosensor was developed for protein detection and quantitation. Unlike dye doped LCs, the DLC used in this study contains an azobenzene LC molecule chemically modified with two azo groups as chromophores, enabling the DLC to exhibit unique optical anisotropy and dichroic absorption at 470 rim, which were exploited in the quantitation of bovine serum albumin (BSA). The change in orientation of DLC molecules from the homeotropic to a disrupted state by BSA immobilized on the aligning-layer surface was evaluated in terms of the relative loss in transmittance, expressed by the standard parameter S%, due to the absorption as well as scattering of light. The DLC-based protein assay was capable of determining the BSA concentration, with which S% varied linearly between 1 mu g/ml and 7.5 mu g/ml. When the DLC molecules were reoriented from a BSA-disrupted state to the homeotropic alignment by an externally applied electric field, a linear correlation between S% and BSA concentration could be also derived from the voltage transmittance plots. The unique electro-optical properties of the intrinsically dichroic DLC are promising toward the development of a simple and reliable sensing platform for the quantitative analysis of biomolecules.
Dynamic Tuning and Memory Switching of Defect Modes in a Hybrid Photonic Structure
(2016-10-01)
Wang, Hsiao-Tsung
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Wu, Po-Chang
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Timofeev, Ivan V.
;
Zyryanov, Victor Ya.
;
Lee, Wei
We propose a memorable and electrically tunable photonic device by infiltrating a dual-mode chiral-doped dual-frequency liquid crystal (LC) as the central defect layer in a one-dimensional photonic crystal (PC). According to the transmission properties of this structure, the wavelength tunability of defect modes is obtained by manipulating the LC layer in the dynamic mode due to the electrically controlled birefringence effect. Moreover, the switching between two memorable states, the splay and pi-twist states, creates two distinct sets of defect modes at null voltage. The spectral characteristics of this device ensure its potential application as an energy-efficient multichannel wavelength filter.
Electro-optical and dielectric properties of polymer-stabilized blue phase liquid crystal impregnated with a fluorine-containing compound
(2018-10-01)
Wu, Po-Chang
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Chen, Hsin-Li
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Rudakova, Natalya V.
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Timofeev, Ivan V.
;
Zyryanov, Victor Ya.
The effects of a fluorine-containing compound (4,4'-difluorobenzophenone; DF) on the electro-optical and dielectric properties of polymer-stabilized blue phase (PSBP) liquid crystals were investigated. When a PSBP cell was driven by an in-plane electric field, the addition of DF up to 2.7 wt% effectively reduced the operation voltage by-30%. Further inspection by dielectric spectroscopy indicated that the ionic concentration in PSBP decreased with increasing loading of OF. This finding can be ascribed to the complexation of impurity ions near the ketone group and carbon-fluorine bonds of the DF compound that restrained the ion transport after photopolymerization. As a result, the DF compound can be regarded as a superior ion-suppressor, enabling the reduction in the ionic effect and, in turn, the promotion of the electro-optical response of a PSBP. (C) 2017 The Authors. Published by Elsevier Inc.
Electro-Thermal Formation of Uniform Lying Helix Alignment in a Cholesteric Liquid Crystal Cell
(2019-04-01)
Yu, Chia-Hua
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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.
Electro-thermally tunable reflective colors in a self-organized cholesteric helical superstructure
(2018-12-01)
Wu, Po-Chang
;
Wu, Guan-Wei
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Timofeev, Ivan V.
;
Zyryanov, Victor Ya.
;
Lee, Wei
We propose to dynamically control the reflective color of a cholesteric liquid crystal (CLC) by electrically tuning the center wavelength (lambda(c)) of the bandgap. The CLC, sandwiched in a planar-aligned cell with indium-tin-oxide electrodes, possesses negative dielectric anisotropy and thermo-responsive spectral properties. The helix in the Grandjean planar state, which is subject to vertically applied voltage, should be undisturbed in that the long molecular axis is initially perpendicular to the direction of the electric field. Surprisingly, when the frequency of the applied voltage is higher than a critical value, lambda c of the CLC cell varies as a function of the voltage. The underlying mechanism is the voltage-induced temperature change through dielectric heating in the frequency regime of pseudo-dielectric relaxation, attributable to the significant equivalent resistance-capacitance circuit of the cell due to the use of electrode layers with finite conductance. The driving voltage enabling the tuning of lambda c in the entire visible spectrum is as low as 12 V-rms in a 5 pin thick cell at a frequency of 2 MHz. The proposed CLC cell, exhibiting a broad electrically tunable spectral range from the near-infrared to ultraviolet, holds great promise for developing tunable photonic devices such as multicolor reflectors, filters, and sensors. (C) 2018 Chinese Laser Press
Frequency-modulated textural formation and optical properties of a binary rod-like/bent-core cholesteric liquid crystal
(2019-11-01)
Lin, Yueh-Chern
;
Wu, Po-Chang
;
Lee, Wei
;
照明與能源光電研究所
;
影像與生醫光電研究所
We explore the mechanisms of voltage-induced textural switching and the corresponding electro-optical responses of a binary cholesteric liquid crystal (CLC) composed of the rod-like nematic LC E7 with positive dielectric anisotropy and the bent-core LC dimer CB7CB with large flexoelectric coefficients. Our results indicate that the minimal voltage (V-H) required for retaining the CLC in the homeotropic state and the optical transparency of the CLC after treating with decreasing voltage (from V-H to zero) are dependent on the voltage frequency. The observed frequency modulated electro-optical properties are characterized by the frequency regimes separated by the critical frequencies of flexoelectric polarization and dielectric relaxation in dielectric dispersion. These unusual features, specific to the CB7CB-doped CLC, are explained by the dielectric and frequency-dependent flexoelectric responses of LC molecules to an external AC voltage. Accordingly, in addition to the known Grandjean planar and focal conic states, the uniform lying helix as a third stable state can be feasibly generated in the binary CLC with 45 wt% CB7CB by two pathways. The first one is to treat the cell directly with a sufficient voltage lower than V-H ; the other is to decrease the voltage gradually from V-H to zero in the low-frequency regime, where the flexoelectric strength is significant. Manifested by its optical tristability, frequency-controllable optical transparency, and fast flexoelectro-optical response, the proposed binary rod-like/bent-core CLC system is promising for developing a variety of memory- and dynamic-mode photonic and optoelectronic devices. (C) 2019 Chinese Laser Press
Influence of methyl red as a dopant on the electrical properties and device performance of liquid crystals
(2014-12-15)
Wu, Po-Chang
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Hou, Chien-Tsung
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Hsiao, Yu-Cheng
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Lee, Wei
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影像與生醫光電研究所
The ionic effect in nematic liquid-crystal (LC) cells containing the azo dye methyl red was investigated by means of dielectric spectroscopy, measurements of voltage holding ratio (VHR) and ultraviolet/visible absorption spectroscopy. The experimental results indicated that incorporating a minute amount of the methyl red (< 0.03 wt%) in the LC host leads to the suppression of the ionic effect caused by impurity ions. Practically, the doped LC cells with a dye content of 0.02 wt% showed improved VHR and promoted lifetime by 15% and 180%, respectively, in virtually no expense of the optical transmittance. (C)2014 Optical Society of America
Label-free protein quantitation by dielectric spectroscopy of dual-frequency liquid crystal
(2019-03-01)
Lin, Ching-Min
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Wu, Po-Chang
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Lee, Mon-Juan
;
Lee, Wei
;
光電學院
A dual-frequency-liquid-crystal (DFLC)-based biosensor was developed and its frequency-dependent dielectric properties were exploited to detect and quantitate a protein standard, bovine serum albumin (BSA). By analyzing the dielectric spectra of DFLC in the presence of BSA at various concentrations, we found that the difference in dielectric permittivity between the high- and low-frequency regimes is correlated to BSA concentration, thereby permitting a DFLC-based protein quantitative method. The dielectric properties of DFLCs are fundamental in the design of liquid crystal displays and fast-switching devices. Results from this study demonstrate the extended potential of the frequency-revertible dielectric anisotropy of DFLC in biosensing and protein quantitation.
One-dimensional photonic crystals containing memory-enabling liquid-crystal defect layers
(2013)
Wu, Po-Chang
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Lee, Wei
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影像與生醫光電研究所
;
Institute of Imaging and Biomedical Photonics
Incorporating liquid crystal (LC) as a defect layer in a photonic crystal (PC) leads to the electrically tunable optical spectrum in defect modes within the photonic band gap. While the LC defect layer has bi- or multi-stable states, the profile of defect modes in each stable state can be preserved permanently without applied voltage, indicating a feature of low power consumption for photonic applications. In this paper, we report on our recent development of optical and spectral properties of multilayer PC structures containing various types of memory-enabling LC (ME-LC), including a bistable chiral-tilted homeotropic nematic (BHN), a bistable chiral-splay nematic (BCSN), a bistable dual-frequency cholesteric LC (DFCLC), a tristable polymer-stabilized cholesteric texture (PSCT), and a tristable smectic-A liquid crystal as a defect layer. The defect modes of the PC/ME-LC cell can be switched to not only the voltage-sustained states but the memory states. As a result, PC/ME-LC cells reveal several features such as the wavelength tunability, transmission tunability and optical bistability or tristability of defect modes that are of potential for realizing tunable and memorable optical devices such as low-power-consumption multichannel filters, light shutters or electrically controllable intensity modulators with green concept.
Optimized frequency regime for the electrohydrodynamic induction of a uniformly lying helix structure
(2016-12-01)
Nian, Yu-Lin
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Wu, Po-Chang
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Lee, Wei
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光電學院
;
College of Photonics
This study specifies the optimal frequency regime for the uniformly lying helix (ULH) alignment induced by the electrohydrodynamic (EHD) effect in a cholesteric liquid crystal with positive dielectric anisotropy. Based on the transport behavior of ionic charges in response to the externally applied ac electric field, four frequency regimes, divided by three critical frequencies (i.e., f (L), f (R), and f (H)), are identified by means of dielectric spectroscopy. By discussing the voltage-dependent cholesteric textural changes in each frequency regime in terms of the voltageand frequency-dependent transmission spectra and optical textures, our results suggest that the designated frequency regime f (L), < f < f (R), where electrical charges can effectively oscillate in the bulk of the cell with less ion accumulation on the electrode surfaces, is most adequate for the generation of well-structured ULH via effective induction of the EHD flow by the external voltage. As a result, this study provides a pathway to determination of the optimal frequency regime for generating the EHD-induced ULH state prior to voltage treatment. (C) 2016 Chinese Laser Press
Phase and dielectric behaviors of a polymorphic liquid crystal doped with graphene nanoplatelets
(2013-04-22)
Wu, Po-Chang
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Lee, Wei
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光電學院
;
College of Photonics
We report on the phase behavior and dielectric properties of the liquid crystal (LC) 4'-n-octyloxy-4-cyanobiphenyl dispersed with graphene nanoplatelets (GNPs). The temperature-dependent dielectric permittivity at 10(4) Hz and its derivative with respect to the temperature reveal that the incorporation of GNPs in a LC cell leads to the modification of crystalline polymorphism and shift in phase transition temperature owing to the enhanced positional and orientational order. Additionally, the dielectric data between 1 and 10(3) Hz show that the dopant reduces the ionic concentration and alters the diffusivity in the LC mesophases. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802839]
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.
Pseudo-dielectric relaxation-enabled electrothermal tuning of reflective color in a soft chiral photonic crystal
(2019-01-01)
Wu, Po-Chang
;
Wu, Guan-Wei
;
Lee, Wei
;
照明與能源光電研究所
;
影像與生醫光電研究所
Many published papers bear flaw information caused by misinterpreting the pseudo-dielectric relaxation in liquid crystal (LC) confined in thin cells. The pseudo-dielectric relaxation stems from the cell geometries or parameters, which should not be misrecognized as an inherent material property of the LC. In this work, we explored the pseudo-dielectric relaxation and demonstrated an unconventional tuning means to electrically manipulate the reflective color of a typical cholesteric LC cell based on the heating due to the pseudo-dielectric relaxation. A tuning range in the full-color spectrum can be easily achieved by a limited temperature change of merely a few degrees Celsius.