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1941 KiB

Open AccessArticle

Electro-optical Effect of Gold Nanoparticle Dispersed in Nematic Liquid Crystals

by Che-Ju Hsu, Li-Jyuan Lin, Mao-Kun Huang and Chi-Yen Huang

Crystals 2017, 7(10), 287; https://doi.org/10.3390/cryst7100287 - 23 Sep 2017

Cited by 56 | Viewed by 7370

The electro-optical performance of nematic liquid crystals (NLCs) dispersed with gold nanoparticles (GNPs) was investigated in this study. The addition of a small amount of GNP dopant decreased the threshold voltage for LC reorientation due to the decreased elastic constant and increased dielectric [...] Read more.

The electro-optical performance of nematic liquid crystals (NLCs) dispersed with gold nanoparticles (GNPs) was investigated in this study. The addition of a small amount of GNP dopant decreased the threshold voltage for LC reorientation due to the decreased elastic constant and increased dielectric anisotropy of the LC mixture. The response time of the LC cell was decreased with the addition of tiny amounts of GNPs because of decrease in rotational viscosity of LCs. The doped GNPs also shorten LC reorientation angle during voltage switching, further decreasing the response time of the LC cell. The addition of high amounts of GNPs slowed down the response time of the LC cell, because excess GNPs aggregated and formed networks in the cell, thus disturbing LC alignment and hindering LC reorientation. The measured dielectric spectra of the GNP-LC mixture revealed that the addition of GNPs decreased the relaxation time constant of the LCs. This result confirmed that the GNP dopant decreased the rotational viscosity and elastic constant of the LCs. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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12404 KiB

Open AccessCommunication

Electrohydrodynamics-Induced Abnormal Electro-Optic Characteristics in a Polymer-Dispersed Liquid Crystal Film

by Sheng-Kuang Wu, Ting-Shan Mo, Jia-De Lin, Shuan-Yu Huang, Chia-Yi Huang, Hui-Chen Yeh, Lin-Jer Chen and Chia-Rong Lee

Crystals 2017, 7(7), 227; https://doi.org/10.3390/cryst7070227 - 21 Jul 2017

Viewed by 4589

Abstract

This study demonstrates for the first time abnormal electro-optic (EO) characteristics induced by electrohydrodynamics (EHD) in a polymer-dispersed liquid crystal (PDLC) film in the presence of a low-frequency (1 kHz) AC voltage. Large LC droplets (20−40 µm) buried in the film can be [...] Read more.

This study demonstrates for the first time abnormal electro-optic (EO) characteristics induced by electrohydrodynamics (EHD) in a polymer-dispersed liquid crystal (PDLC) film in the presence of a low-frequency (1 kHz) AC voltage. Large LC droplets (20−40 µm) buried in the film can be obtained after the illumination of one UV light with a weak intensity (~0.96 mW/cm²) for 12 h. This film exhibits abnormal EO features, including the transmittance’s decay at a high voltage regime at normal incidence and the conversion between polarization independence and polarization dependence for the transmittance-voltage curve at normal and oblique incidences, respectively, of which properties are different from those shown in traditional PDLC films with small droplets. The abnormal EO characteristics of the large-droplet PDLC at the high voltage regime are attributed to a strong scattering effect associated with the formation of the foggy LC droplets in the cell. This effect is induced by a vortex-like LC director field with a rotational axis normal to the cell substrates in each dome-like droplet of the cell at the high voltage regime. The vortex-like director field is induced by a vortex-like turbulence of charged impurity generated by the EHD effect under the action of the AC electric field along the cell normal and the confinement of the dome-like boundary of the droplet on the charged impurities in each droplet. The scattering is decided by the degrees of mismatch between the refractive indices of the LC droplet and polymer, and the local fluctuation of the vortex-like director field in the droplet, resulting in the abnormal EO behaviors of the large-droplet PDLC. This investigation provides novel insight into the EHD effect in three dimensional (3D) microdroplets with anisotropic fluid. Such a large-droplet PDLC has potential in photonic applications, such as electrically controlled polarization-based optical components or optical converters between polarization independence and polarization dependence. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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17842 KiB

Open AccessArticle

An Electrically Tunable Liquid Crystal Lens with Coaxial Bi-Focus and Single Focus Switching Modes

by Chun-Yu Chien, Cheng-Hau Li and Chia-Rong Sheu

Crystals 2017, 7(7), 209; https://doi.org/10.3390/cryst7070209 - 07 Jul 2017

Cited by 13 | Viewed by 7407

Abstract

A hole-patterned electrode liquid crystal (LC) lens with electrically switching coaxial bi-focus and single focus modes of tuning is demonstrated. The proposed LC lens mainly consists of a two LC layer (TLCL) structure with different thicknesses to achieve higher focusing power than the [...] Read more.

A hole-patterned electrode liquid crystal (LC) lens with electrically switching coaxial bi-focus and single focus modes of tuning is demonstrated. The proposed LC lens mainly consists of a two LC layer (TLCL) structure with different thicknesses to achieve higher focusing power than the conventional hole-patterned electrode LC lens with the same aperture size. In the TLCL structure, one LC layer, doped with 3 wt % RM257, was photopolymerized to achieve a fixed focusing power of 18.5 Diopter. Due to polarization dependence in TLCL lenses, an additional 90° twisted nematic (TN) cell was used to change the incident polarization in order to switch lens functions on or off. As a result, a fixed focusing power of 18.5 Diopter was achieved when voltages of 10 V_rms were applied to the 90° TN cell. In addition, the switching capabilities of the bi-focus and single focus modes were achieved when operating individually with applied voltages from 20 V_rms to 90 V_rms, and higher voltages of over 90 V_rms, respectively. The maximum focusing power in the fabricated TLCL lens is 30.9 Diopter. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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6289 KiB

Open AccessArticle

Doping Liquid Crystal Cells with Photocurable Monomer via Holographic Exposure to Realize Optical-Scattering-Free Infrared Phase Modulators with Fast Response Time

by Chun-Yu Chien and Chia-Rong Sheu

Crystals 2017, 7(7), 208; https://doi.org/10.3390/cryst7070208 - 07 Jul 2017

Cited by 6 | Viewed by 4845

Abstract

Photocurable monomer-doped liquid crystal (LC) cells were processed via holographic exposure using a low-power He–Ne laser to generate holographic polymer networks. The polymer network LC (PNLC) cells are used to fabricate infrared phase modulators at 1550 nm wavelength possessing favorable electro-optical performance. Compared [...] Read more.

Photocurable monomer-doped liquid crystal (LC) cells were processed via holographic exposure using a low-power He–Ne laser to generate holographic polymer networks. The polymer network LC (PNLC) cells are used to fabricate infrared phase modulators at 1550 nm wavelength possessing favorable electro-optical performance. Compared with our previous work, the percentages of ingredients in the LC mixture filled in PNLC cells underwent a slight change. The 2 wt% concentration of anisotropic monomer RM257 were in place of isotropic monomer N–vinyl–2–pyrrolidinone (NVP). As a result, the fabricated phase modulators also maintained well homogeneous LC alignments and optical-scattering-free characteristics. Furthermore, NVP dopant successfully reduced the operating voltages from 95 V_rms to 79 V_rms to prevent polymer network deformation when electrically operating with higher voltages. The fabricated infrared phase modulators had a good average response time (i.e., rising time of 0.88 ms and falling time of 0.40 ms). Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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3050 KiB

Open AccessCommunication

External-Voltage-Free Dielectrophoresis of Liquid Crystal Droplets

by Sheng-Kuang Wu, Ting-Shan Mo, Jia-De Lin, Shuan-Yu Huang, Hui-Chen Yeh, Lin-Jer Chen and Chia-Rong Lee

Crystals 2017, 7(7), 202; https://doi.org/10.3390/cryst7070202 - 03 Jul 2017

Viewed by 3719

Abstract

This work reports, for the first time, a dielectrophoresis (DEP) effect-induced motion of liquid crystal (LC) droplets in an LC/monomer mixture sample with a poly-(N-vinyl carbazole) PVK-coated substrate without an external voltage. With the UV pre-irradiation of the PVK layer through [...] Read more.

This work reports, for the first time, a dielectrophoresis (DEP) effect-induced motion of liquid crystal (LC) droplets in an LC/monomer mixture sample with a poly-(N-vinyl carbazole) PVK-coated substrate without an external voltage. With the UV pre-irradiation of the PVK layer through a binary mask, a laterally non-uniform electric field can be induced between the pre-illuminated regions and the neighboring non-pre-illuminated PVK regions near the borders of the two regions. The phase separation occurs once the temperature is lower than 50 °C and the LC droplets can form in the sample. The pre-formed non-uniform field provides a DEP-like force to manipulate the small LC microdroplets in the pre-illuminated regions to effectively migrate to the adjacent non-pre-illuminated regions. The continuous supply of the LC from the pre-illuminated regions to the adjacent non-pre-illuminated regions significantly increases the diffraction efficiency of the grating sample. This study provides an insight into developing new external-voltage-free DEP-based devices that can be applied on various fields, such as photonics, displays, and biomedicines. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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2801 KiB

Open AccessArticle

Fast Turn-Off Switching of Vertically-Aligned Negative Liquid Crystals by Fine Patterning of Pixel Electrodes

by Yeongyu Choi, Tae-Hoon Choi, Jae-Hyeon Woo, Byoung-Gyu Jeon and Tae-Hoon Yoon

Crystals 2017, 7(7), 201; https://doi.org/10.3390/cryst7070201 - 03 Jul 2017

Cited by 13 | Viewed by 4519

Abstract

We investigated the two-dimensional (2D) confinement effect on the switching of vertically-aligned negative liquid crystals (LCs) by an electric field applied between the top and bottom patterned electrodes. When an electric field is applied to a patterned vertical alignment (PVA) cell, virtual walls [...] Read more.

We investigated the two-dimensional (2D) confinement effect on the switching of vertically-aligned negative liquid crystals (LCs) by an electric field applied between the top and bottom patterned electrodes. When an electric field is applied to a patterned vertical alignment (PVA) cell, virtual walls form in the middle of the gaps between and at the center of the patterned electrodes. These virtual walls formed in a PVA cell results in the turn-off time being dependent on the pitch of the patterned electrodes as well as the cell gap. We found that a short response time can be achieved by the fine patterning of pixel electrodes with little decrease in the transmittance. The obtained numerical results agree well with the model based on the 2D confinement effect of LCs. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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Graphical abstract

1772 KiB

Open AccessArticle

Photonic Bandgap–Cholesteric Device with Electrical Tunability and Optical Tristability in Its Defect Modes

by Po-Chang Wu, Chih-Yuan Hsiao and Wei Lee

Crystals 2017, 7(7), 184; https://doi.org/10.3390/cryst7070184 - 23 Jun 2017

Cited by 6 | Viewed by 4733

Abstract

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) [...] Read more.

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. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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1983 KiB

Open AccessArticle

Chiral Optical Tamm States: Temporal Coupled-Mode Theory

by Ivan V. Timofeev, Pavel S. Pankin, Stepan Ya. Vetrov, Vasily G. Arkhipkin, Wei Lee and Victor Ya. Zyryanov

Crystals 2017, 7(4), 113; https://doi.org/10.3390/cryst7040113 - 17 Apr 2017

Cited by 15 | Viewed by 5675

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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2101 KiB

Open AccessLetter

Switchable Two-Dimensional Liquid Crystal Grating in Blue Phase

by Bing-Yau Huang, Shih-Hung Lin, Ke-Chin Lin and Chie-Tong Kuo

Crystals 2017, 7(6), 182; https://doi.org/10.3390/cryst7060182 - 21 Jun 2017

Cited by 4 | Viewed by 4917

Abstract

We demonstrate a switchable two-dimensional phase grating in blue phase liquid crystal (BPLC), which is fabricated by sawtooth in-plane-switch (IPS) electrodes. They are used to generate the horizontal electric field on a single indium-tin-oxide (ITO) glass substrate and, as a result, the 1-D [...] Read more.

We demonstrate a switchable two-dimensional phase grating in blue phase liquid crystal (BPLC), which is fabricated by sawtooth in-plane-switch (IPS) electrodes. They are used to generate the horizontal electric field on a single indium-tin-oxide (ITO) glass substrate and, as a result, the 1-D and 2-D phase gratings can be mutual switched via different polarizations of incident light with an applied voltage. The first-order diffraction efficiency is up to 20% and 10% for the 1-D and 2-D phase grating at V = 150 V, respectively. Moreover, the rise and decay time is 0.9 and 1.1 ms, respectively, which is suitable for wide applications of high-speed optical manipulations. Full article

(This article belongs to the Special Issue Selected Papers from "3rd Asian Conference on Liquid Crystals, ACLC 2017")

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