Browsing by Author "Lee, Wei"

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A Comparative Study of Carrier Transport for Overlapped and Nonoverlapped Multiple-Gate SOI MOSFETs
(2009-07-01)
Lee, Wei
;
Su, Pin
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
This paper provides a comparative study of carrier transport characteristics for multiple-gate silicon-on-insulator MOSFETs with and without the nonoverlapped source/drain structure. For the overlapped devices, we observed Boltzmann law in subthreshold characteristics and phonon-limited behavior in the inversion regime. For the nonoverlapped devices, however, we found insensitive temperature dependence for drain current in both subthreshold and inversion regimes. Our low-temperature measurements indicate that the intersubband scattering is the dominant carrier transport mechanism for narrow overlapped multigate field-effect transistors (MuGFETs). For the nonoverlapped MuGFETs, the voltage-controlled potential barriers in the nonoverlapped regions may give rise to the weak localization effect (conductance reduction) and the quantum interference fluctuations.
A Comprehensive Study of Single-Electron Effects in Multiple-Gate MOSFETs
(2008)
Lee, Wei
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Su, Pin
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電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
A thermally tunable narrowband selector based on a chiral nematic containing a binary thermosensitive chiral dopant mixture
(2017-01-01)
Yang, Zong-Han
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Hsiao, Yu-Cheng
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Shen, Dong
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Lee, Wei
;
光電系統研究所
We employed a thermosensitive chiral dopant (CD), characterized by its thermoinduced change in helical twisting power (HTP) in terms of the handedness and strength, to shift the Bragg reflection band of a CLC. A thermally tunable photonic bandgap (PBG) device was demonstrated using a one-dimensional asymmetric photonic crystal (PC) infiltrated with the CLC as a defect layer. With a temperature controller to shift the PBG of the CLC, two narrow transmission bands can be tuned in the spectrum dominated by the asymmetric PC structure. This optical device can find applications as a tunable narrowband wavelength selector or an intensity controller.
Advanced color-reflective dual-frequency cholesteric liquid crystal displays and the driving matrix
(2017-01-01)
Hsiao, Yu-Cheng
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Yeh, En-Rong
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Lee, Wei
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影像與生醫光電研究所
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Institute of Imaging and Biomedical Photonics
We propose a novel color-reflective, dual-frequency cholesteric liquid crystal display (DFCLCD) by employing hybrid alignment treatment. The switching voltage is significantly lowered to favor the thin-film transistors used in the current display market and the response time is drastically reduced to the millisecond level while the bistability remains. Test cells with three distinct helical pitches were fabricated to demonstrate the concept of color pixels. The DFCLCD can be directly switched from the bright planar state to the light-scattering state by a low-frequency voltage pulse, and reversibly by a high-frequency pulse. A drive scheme is presented for the advanced color DFCLCD.
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
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Lee, Wei
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光電學院
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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).
Bovine serum albumin detection and quantitation based on capacitance measurements of liquid crystals
(2016-08-29)
Lin, Chi-Hao
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Lee, Mon-Juan
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Lee, Wei
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影像與生醫光電研究所
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Institute of Imaging and Biomedical Photonics
Liquid crystal (LC)-based biosensing is generally limited by the lack of accurate quantitative strategies. This study exploits the unique electric capacitance properties of LCs to establish quantitative assay methods for bovine serum albumin (BSA) biomolecules. By measuring the voltage-dependent electric capacitance of LCs under an alternating-current field with increasing amplitude, positive correlations were derived between the BSA concentration and the electric capacitance parameters of LCs. This study demonstrates that quantitative analysis can be achieved in LC-based biosensing through electric capacitance measurements extensively employed in LCD research and development. Published by AIP Publishing.
Chiral liquid crystals as biosensing platforms
(2016-01-01)
Lee, Mon-Juan
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Sung, Yu-Chien
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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 texture observation has long been the core technique in liquid crystal (LC)-based bioassays. Its working principle stems from the dark-to-bright texture change induced by the interruption of the initially homeotropic alignment in nematic bulks or from the radial-to-bipolar configuration transition in LC droplets in the presence of biomolecules. One of the drawbacks of this observational scheme, which requires a polarizing optical microscope, is the difficulty in quantitative analysis. In this invited paper, we report on our recent development of alternative optical biosensing techniques based on cholesteric LCs (CLCs) without the use of a polarizer. The increase in structural order in a vertically anchored CLC cell in the quasi-planar state provides a means to allow detection and quantification of the concentration of biomolecules immobilized on the interface between the mesophase and the surfactant DMOAP for LC vertical alignment.
Chiral Optical Tamm States at the Interface between an All-Dielectric Polarization-Preserving Anisotropic Mirror and a Cholesteric Liquid Crystal
(2019-10-01)
Rudakova, Natalya, V
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Timofeev, Ivan, V
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Bikbaev, Rashid
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Pyatnov, Maxim, V
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Vetrov, Stepan Ya
As a new localized state of light, the chiral optical Tamm state exists at the interface between a polarization-retaining anisotropic mirror and a substance with optical activity. Considering a hybrid structure comprising a metal-free polarization-preserving mirror and a cholesteric liquid crystal, we highlight the high Q factor arising from the all-dielectric framework. The intensity of localized light decreases exponentially with increasing distance from the interface. The penetration of the field into the cholesteric liquid crystal is essentially prohibited for wavelengths lying in the photonic bandgap and close to the cholesteric pitch length. The dielectric mirror has its own photonic bandgap. The energy transfer along the interface can be effectively switched off by setting the tangential wave vector to zero. The spectral behavior of the chiral optical Tamm state is observed both as reflection and transmission resonance. This Fano resonance is analogous to the Kopp-Genack effect. Our analytics are well in line with precise calculations, which may pave a new route for the future development of intelligent design for laser and sensing applications.
Chiral Optical Tamm States: Temporal Coupled-Mode Theory
(2017-04-01)
Timofeev, Ivan V.
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Pankin, Pavel S.
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Vetrov, Stepan Ya.
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Arkhipkin, Vasily G.
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Lee, Wei
The chiral optical Tamm state (COTS) is a special localized state at the interface of a handedness-preserving mirror and a structurally chiral medium such as a cholesteric liquid crystal or a chiral sculptured thin film. The spectral behavior of COTS, observed as reflection resonances, is described by the temporal coupled-mode theory. Mode coupling is different for two circular light polarizations because COTS has a helical structure replicating that of the cholesteric. The mode coupling for co-handed circularly polarized light exponentially attenuates with the cholesteric layer thickness since the COTS frequency falls into the stop band. Cross-handed circularly polarized light freely goes through the cholesteric layer and can excite COTS when reflected from the handedness-preserving mirror. The coupling in this case is proportional to anisotropy of the cholesteric and theoretically only anisotropy in magnetic permittivity can ultimately cancel this coupling. These two couplings being equal result in a polarization crossover (the Kopp-Genack effect) for which a linear polarization is optimal to excite COTS. The corresponding cholesteric thickness and scattering matrix for COTS are generally described by simple expressions.
Color tuning in thermo-sensitive chiral photonic liquid crystals based on the pseudo-dielectric heating effect
(2019-12-15)
Yeh, Min-Chih
;
Yang, Sheng-Hsiung
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Lee, Wei
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照明與能源光電研究所
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影像與生醫光電研究所
The ability of electro-thermal tuning in center wavelength (X,) of the Bragg reflection band is promoted in cholesteric liquid crystal (CLC) containing a positive-dielectric-anisotropy liquid crystal host (E7) and a binary chiral dopant (R-811 and R-5011). In the CLC mixtures with a fixed R-811 content of 30 wt%, the transition temperature of the smectic-A phase to CLC phase decreases when the R-5011 loading increases. The CLC systems are temperature-sensitive, each exhibiting a temperature range of about 9-12 degrees C in the CLC phase. Optimization of the content of R-5011 at 1.5 wt% allows for the remarkably effective thermal tuning in lambda(c )from 780 nm to 380 nm in response to a temperature increase of merely 3 degrees C from the room temperature. Under the mechanism of pseudo-dielectric heating, the tunable lambda(c) can shift in the entire visible spectrum by an applied voltage as low as 7 V-rms at a frequency of 2 MHz across the cell gap of a 4.8-mu m-thick cell. Furthermore, a multifunctional and operation-reversible device is demonstrated by the control of the applied AC voltage, yielding an optical device in which the reflection band can be tuned and the mode can be switched among the reflection, scattering, and transparent states. (C) 2019 Elsevier B.V. All rights reserved.
Conditions for the formation of a uniformly lying helix structure induced by the electrohydrodynamic effect
(2017-01-01)
Nian, Yu-Lin
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Wu, Po-Cheng
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Lee, Wei
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影像與生醫光電研究所
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Institute of Imaging and Biomedical Photonics
The uniform lying helix (ULH) structure in a cholesteric liquid crystal (CLC) cell has been widely discussed in the literature. Based on the electrohydrodynamic (EHD) effect, we have suggested that the voltage-induction of a well-aligned ULH state can be fine-tuned in a CLC cell by optimizing both the amplitude and the frequency of the applied voltage. However, since the EHD effect is correlated with the migration of mobile ions within the bulk of the cell, the ionic conditions for such an EHD-induced ULH state have not been clarified. In this research, we focus on studying the influence of ion density and the diffusivity on the formation of the EHD-induced ULH structure in a CLC cell, followed by finding out some key conditions for the generation of well-aligned ULH states.
Derivation of extended Maxwell Garnett formula for carbon-nanotube-doped nematic liquid crystal
(2013-06-01)
Fuh, Andy Ying-Guey
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Lee, Wei
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Huang, Kevin Yu-Chia
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光電學院
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College of Photonics
A numerical effort for modelling the dielectric behaviour of the novel binary mixture of nematic liquid crystal and carbon nanotube (NLC:CNT) has been carried out using the Maxwell Garnett theory. The CNTs are treated as conductive anisotropic inclusions while the NLCs as dielectric isotropic or anisotropic matrix depending on temperature. Thus, the numerical expressions for the dielectric anisotropy for the scenarios of aligned CNTs in the NLC matrix of either nematic or isotropic phases have been derived, and the results show that the Debye parameter of CNT at optic limit seems to be the decisive factor in the enhancement of the dielectric anisotropy of the nematic system.
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
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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
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Hsu, Hsuan-Tsung
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Chen, Hsin-Li
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Lee, Wei
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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
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Lee, Wei
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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
;
Chiang, Yi-Lun
;
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
;
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.
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Zyryanov, Victor Ya.
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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.