Cholesteric Liquid Crystal Device Research Articles

Broadband wavelength designable achromatic grating based on a cholesteric liquid crystal template

Liquid crystal (LC) gratings have played important roles in light field control due to the advantages of being lightweight, low cost, having no moving parts, and low power consumption. However, the chromatic aberration limits the bandwidth of the LC device and affects the efficiency of the grating. To solve the chromatic aberration issue, a broadband wavelength designable achromatic grating is proposed. Different grating structures are integrated into a single-layer templated cholesteric liquid crystal (CLC) device, and the achromatic diffraction wavelength of the grating can be freely designed from the visible spectral region to the infrared range within the Bragg reflection band of the CLCs. The diffraction intensity of different orders can be changed with the electric field applied to meet the need for dynamic modulation. This grating shows suitable potential applications in optical communication and displays.

Quickly Formed Continuously Tuneable Laser in the Full Visible Spectrum Using Cholesteric Liquid Crystal Cells Based on the Electrothermal Effect

AbstractWavelength‐tuneable lasers can play a crucial role across many fields, and their applications continue to expand with advancements. Cholesteric liquid crystal (CLC) devices are developed, enabling good wavelength‐tuneable lasers. In CLC devices, pitch control is required to tune the desired wavelength. However, pitch control takes a long time and is difficult, which limits the speed and range of laser wavelength tuning. Here, by applying a voltage once and forming an electric field gradient along the wedge direction in the CLC wedge cell, we could realize CLC pitch gradient arrays that achieve continuous wavelength tuning over 500 nm in only 10 min through the electrothermal effect while continuously generating laser lines in the full visible spectrum. The laser wavelength could be rapidly tuned over the full visible spectrum by moving within the array in the wedge cell. The study of CLC cells for electric field application in real time is able to observe the dynamic process of pitch change directly. This new laser device and strategy has potential in many fields that require active laser tuning and is expected to lead to innovative changes in the development of future practical laser devices in the optical industry.

Low driving voltage reverse-mode polymer-stabilised cholesteric liquid crystal devices using small phenylacetylene molecule

ABSTRACT Polymer-stabilised liquid crystal devices have found widespread use in intelligent displays and dimming glass applications, but their high driving voltage induces high power consumption and needs to be reduced. In this paper, we use several small phenylacetylene molecule materials with linear structures to reduce the driving voltage and obtain a high contrast ratio in reverse-mode polymer-stabilised cholesteric liquid crystals (RPSCLC). In particular, the drive voltage of the 8-micron-thick sample was reduced by nearly 40%, and the 20-micron-thick sample was reduced by nearly 45%. Additionally, we explore the physical mechanism through which various small phenylacetylene molecule materials improve the electro-optical properties, based on polymer network morphology effects. The results will be of great value for enhancing the use of RPSCLC devices.

Influence of different small styryl molecules on electro-optical characteristics of reverse-mode polymer stabilised cholesteric liquid crystal devices

ABSTRACT Polymer stabilised liquid crystal (PSLC) is widely used in various applications such as smart windows due to its fast response speed and wide viewing angle. However, there are still limitations in terms of high driving voltage and poor contrast ratio. Therefore, there is a need to explore methods to enhance the electro-optical performance of PSLC devices. In this paper, the influence of a few types of styryl molecules on the electro-optical characteristics of PSLC was investigated. The experimental results demonstrated that the addition of an appropriate amount of small styryl molecules significantly reduced the driving voltage and improved the contrast ratio. For an 8-micron-thick sample, the threshold voltage and saturation voltage were approximately 5.3 V and 6.5 V, respectively, resulting in a contrast ratio of approximately 14.4. For a 20-micron-thick sample, the threshold voltage, saturation voltage, and contrast ratio were approximately 10.2 V, 14.5 V, and 109, respectively. Combined with the effects of different small styryl molecules on the morphology of polymer networks, the physical mechanism of different small styryl molecules to improve electro-optical properties was studied. The findings will contribute to the advancement of RPSCLC devices and their practical applications.

Recyclable Cholesteric Phase Liquid Crystal Device for Detecting Storage Temperature Failure.

The planar state of a cholesteric liquid crystal (CLC) often exhibits oily streak defects, which negatively impact the characteristics of precision optics, including transmission and selective reflection. In this paper, we introduced polymerizable monomers into liquid crystals and examined the effects of monomer concentration, polymerization light intensity, and chiral dopant concentration on oily streak defects in CLC. With the proposed method of heating cholesteric liquid crystals to the isotropic phase followed by rapid cooling, the oil streak defects presented in the liquid crystal can be successfully eliminated. Furthermore, a stable focal conic state can be obtained by a slow cooling process. Two stable states with different optical properties can be obtained based on the cholesteric liquid crystal at different cooling rates, which makes it possible to detect whether the stored procedure of temperature-sensitive material is qualified. These findings have widespread applications in devices that require a planar state without oily streak defects and temperature-sensitive detection devices.

Energy saving, transparency changing thermochromism in dye-doped cholesteric liquid crystals for smart windows

An electrically switchable smart window is made from a dye-doped Cholesteric liquid crystal (CLC). In this work, the impact of coumarin dye (0.3 and 1 wt%) on the structural and electro-optical properties of CLCs that reflect IR and blue light (Blue) was explored. The results suggest that high transmittance in Visible and NIR useful to preserve interior transmittance as well as heat suited for all seasons. Temperature-dependent microscopic studies confirmed the homogenous solubility of the dye in CLCs. The fabricated smart window was observed to show three switchable modes, comprising planar (P), focal-conic (FC), and homeotropic (H) states, depending on the applied voltages (1 kHz). The structural observations from microscopic studies and the transmission spectra of pure and dye-doped composites with and without the application of voltage are consistent. The visual transmittance of dye doped CLC increased from 84% in the planar state (OFF state) to 18% in the focal conic state (3 V/µm) and 92% in the homeotropic state (5 V/µm) with a short response time (<1ms). Using dye and CLCs, we demonstrated the functionality of the device as smart windows for energy saving, privacy windows as well as for authenticated/unauthenticated QR codes. To investigate the impact of both pure CLC and dye-doped CLC devices on interior temperature management, a solar simulator device was used. The findings demonstrate that the device can be used as a smart window to regulate the internal temperature throughout the year, cutting down on the expense of operating artificial lighting, heating, and cooling systems for buildings. In comparison to pure CLCs, coumarin dye-doped CLCs showed high transmittance in the transparent and homeotropic state and a low transmittance in the scattering state when an electric field is applied and it also exhibited a good stability over repeated switching cycles.

Investigating the Electro-Optic Response of Steroid Doped Liquid Crystal Devices

Nature is highly efficient at producing chiral compounds that are enantiomerically pure. The inherent chirality of naturally occurring biomolecules means that many have the potential to be used as chiral dopants for cholesteric liquid crystal (CLC) systems. Though many biomolecules have been identified as chiral dopants, many remain yet to be probed for their ability to function as chiral dopants. Here, 10 naturally occurring biomolecules comprised of steroids and bile acids were tested as chiral dopants for CLCs. Progesterone was identified as having high miscibility with nematic liquid crystals and was used in responsive liquid crystal devices. Progesterone-doped CLC devices were fabricated to exhibit either normal mode or reverse mode switchable behavior. Polymer stabilized CLCs (PSCLC) devices exhibiting dynamic electro-optic red- and blue-tuning behaviors were also fabricated. Furthermore, immiscible lithocholic acid was synthetically modified to afford two derivatives that were miscible at 10 wt. % in nematic liquid crystals. The two lithocholic acid derivatives were used as chiral dopants and incorporated into polymer stabilized CLCs which exhibited blue tuning behavior.

A bistable ion-doped cholesteric liquid crystal smart window with a small amount of polymer

Bistable liquid crystal devices, which require electricity only at the switching processes of the two states, have advantages in energy saving and safety compared with monostable liquid crystal devices. Due to electrohydrodynamic and dielectric effects, the ion-doped cholesteric liquid crystal can switch between a focal conic state and a planar state with different frequencies of the driving voltage. However, the cholesteric liquid crystal that self-assembles from the dynamic scattering state always has a poor focal conic state, resulting in a low contrast ratio. This paper presents a bistable polymer-stabilized ion-doped cholesteric liquid crystal device. The introduction of a small amount of polymer network can greatly enhance the light scattering capabilities of the focal conic state, endowing the device with excellent electro-optic performance. The electro-optical properties of the device are investigated with the various content of monomer and chiral dopant, and rotational viscosity of liquid crystal. Liquid crystal with a low rotational viscosity coefficient is beneficial to the formation of a good polymer network. The pitch induced by chiral dopant impacts the formation process of the polymer network, which subsequently affects the electro-optical properties. The bistable device has a low driving voltage and a high contrast ratio, making it ideal for use in smart windows.

Colorable Light-Scattering Device Based on Polymer-Stabilized Ion-Doped Cholesteric Liquid Crystal and an Electrochromatic Layer

Bistable polymer-stabilized cholesteric liquid crystal (LC) devices have been extensively researched due to their energy-saving benefits. Compared to devices with merely transparent and light-scattering states, LC devices with controlled light absorption or changeable color functions are unquestionably more intriguing. In this paper, a polymer-stabilized ion-doped cholesteric LC and an electrochromic layer are used to fabricate a colorable device which can show four operating states: transparent, light-scattering, colored transparent, and colored light-scattering. The working principle and fabrication strategy are explained in detail. Based on the dielectric response of LC, the electrohydrodynamic effect of ion-doped LC, and the redox reaction of electrochromic materials, the transparent or light-scattering state and the colored or colorless state of the device can be regulated by controlling the alternating frequency and the direction of the electric field. The display performance related to the monomer, chiral dopant, and electrochromic layer is investigated. The content of monomer and chiral dopant affects the polymer network and pitch of cholesteric LC, which then affects the driving voltages and contrast ratio. The thickness of the electrochromic layer has a significant impact on the transmittance of the device's coloring and fading states. The sample with excellent operating states is obtained by optimizing the material and the construction, which can be widely applied in smart windows and energy-saving display devices.

High-Performance and Low-Cost Overhead Projector Sheet-Based Triboelectric Nanogenerator for Self-Powered Cholesteric Liquid Crystal, Electroluminescence, and Portable Electronic Devices

High-Performance and Low-Cost Overhead Projector Sheet-Based Triboelectric Nanogenerator for Self-Powered Cholesteric Liquid Crystal, Electroluminescence, and Portable Electronic Devices

Polymer beads dispersed liquid crystal devices (PBLCD) achieved by predesigned radially constructed polymeric particles

AbstractIn this article, we demonstrate a novel design of liquid crystal (LC) displays via predesigned radially constructed polydopamine (PDA) particles. The synthesized PDA particles were modified with dimethyloctadecyl‐3‐(trimethoxysilyl)propyl ammonium chloride (DMOAP) showing radial constructions. Both PDA particles and surface‐modified PDA particles were used to disturb the order of the LC arrangement, showing an opaque appearance. Based on the results, the radially constructed PDA particles efficiently enhanced the performance of cholesteric liquid crystal (CLC) devices. Adding 0.1 wt% DMOAP‐modified PDA particles into the CLC device shows a higher contrast of 82.2%, a lower threshold voltage of 2.4 V and a fast response time of 7.7 ms. Without adding polymer beads, no any display contrast was observed. The results suggest that the synthesized surface‐modified PDA particles disturb the alignment of LCs dispersed in LC cells, leading to a great improvement in the contrast of the CLC devices. In addition, a prepared traditional polymer stabilized CLC (PSCLC) cell shows only 2.8% contrast. The best performance, with a high contrast of 93.8%, a low threshold voltage of 10.4 V and a fast response time of 10.2 ms, was achieved by adding 0.1 wt% modified PDA particles into the CLC device.

Rheological Properties of Cholesteric Liquid Crystal with Visible Reflection from an Etherified Hydroxypropyl Cellulose Derivative.

Optical properties of hydroxypropyl cellulose (HPC) derivatives have been widely investigated for their ability to exhibit cholesteric liquid crystal (CLC) phase. However, there are only a limited number of studies on their rheological properties even though they are quite important for the applications of such HPC derivatives to the versatile CLC photonic devices. In this article, we report on the optical and rheological properties of an HPC derivative possessing pentyl ether groups in the side-chains. The etherified HPC derivative exhibited thermotropic CLC phase with light reflection in the temperature range between 25 °C and 120 °C. After the HPC derivative was heated once at isotropic phase, followed by being cooled to the CLC phase, the reflection peak could not be observed, even at the CLC phase. At this stage, the HPC derivative exhibited solid-like rheological responses compared to that of sheared at a constant shear rate of 1.0 s−1. Such differences in the optical and rheological properties of the HPC derivative can be ascribed to the difference in CLC orientation state. From the rheological results, the etherified HPC derivative showed liquid-like behavior rather than the esterified HPC derivatives. This evidence provides a promising clue for fabricating high-quality CLC devices by the facile CLC orientation.

High-Performance Triboelectric Nanogenerator Based on Zno Nanosheets for Self-Powered Cholesteric Liquid Crystal, Electroluminescence, and Portable Electronic Devices

High-Performance Triboelectric Nanogenerator Based on Zno Nanosheets for Self-Powered Cholesteric Liquid Crystal, Electroluminescence, and Portable Electronic Devices

Analysis of laser radiation performance of fingerprint-texture cholesteric liquid crystal device

Low-cost tunable lasers have broad application prospects in optoelectronics. In this paper, a laser device based on fingerprint-textured cholesteric liquid crystal (CLC) was designed and fabricated, in which a narrow linewidth single-mode laser output on the side of the device was realized. The planar helical structure of the “fingerprint-texture” of CLC was obtained by the combined effect of externally applied electric field and substrate surface anchoring. Under the pumping light of a Nd:YAG solid-state pulsed second harmonic laser with the wavelength of 532 nm, the lasing emission wavelength of our CLC device changes non-monotonically with gradually increasing electric field, which first shifting from 568.5 nm to 609.2 nm and then shifting from 609.2 nm to 586.2 nm. And the observed spectral tuning of the lasing emission was reproducible and reversible. The analysis shows that the output laser radiation was the result of the Bragg reflection feedback provided by the fingerprint-texture.

Enhanced Bandwidth Broadening of Infrared Reflector Based on Polymer Stabilized Cholesteric Liquid Crystals with Poly(N-vinylcarbazole) Used as Alignment Layer

Alignment layer plays a critical role on liquid crystal (LC) conformation for most LC devices. Normally, polyimide (PI) or polyvinyl alcohol (PVA), characterized by their outstanding thermal and electrical properties, have been widely applied as the alignment layer to align LC molecules. Here, we used a semi-conductive material poly(N-vinylcarbazole) (PVK) as the alignment layer to fabricate the cholesteric liquid crystal (CLC) device and the polymer-stabilized cholesteric liquid crystals (PSCLC)-based infrared (IR) reflectors. In the presence of ultraviolet (UV) irradiation, there are hole–electron pairs generated in the PVK layer, which neutralizes the impurity electrons in the LC–PVK junction, resulting in the reduction in the built-in electric field in the LC device. Therefore, the operational voltage of the CLC device switching from cholesteric texture to focal conic texture decreases from 45 V to 30 V. For the PSCLC-based IR reflectors with the PVK alignment layer, at the same applied electric field, the reflection bandwidth is enhanced from 647 to 821 nm, ranging from 685 to 1506 nm in the IR region, which makes it attractive for saving energy as a smart window.

Generation of hidden bistable images in a cholesteric liquid crystal device

ABSTRACT Generation of hidden images in a cholesteric liquid crystal device is presented. The device operation is based on the dependence of the threshold voltage for unwinding the helical molecular order in cholesteric liquid crystal (being initially ether in Grandjean or Focal conic texture) on the surface anchoring conditions of the supporting substrates. This threshold voltage, however, is different for planar and homeotropic anchoring. By patterning the substrates’ surface anchoring condition, permanently or temporarily, so-called pre-determined hidden bistable images, can be visualised and memorised by a proper electronic driving. These images are constituting of areas being either in Grandjean, Focal conic or Uniform Lying Helix (ULH) textures, that is, appearing as light reflecting, light scattering or transparent, respectively. Due to bistability of these textures (states), the visualised hidden images are bistable too. So far, the lifetime of the visualised hidden images was proven to be a decade long and being switchable to any of the other bistable states.

Polymer-stabilized bistable dual-frequency cholesteric liquid crystal devices assisted by a predesigned chiral dopant

Fabricated polymer-stabilized bistable cholesteric liquid crystal devices show a stable opaque focal conic state and a stable transparent planar state without any energy consumption once they are turned.

Effect of the introduction of mono-functional monomer on the electro-optic properties of reverse-mode polymer stabilised cholesteric liquid crystal

ABSTRACT As a kind of polymer stabilised liquid crystal (PSLC) devices, the scattering-type polymer-stabilised cholesteric liquid crystal (PSCLC) devices can be switched between the transparent state and the scattering state, which has huge application potential in the fields of smart windows, light valves, light modulators and others. The effect of polymer network texture on the electro-optical properties of PSLC. The monomers used in traditional polymer-stabilised cholesteric liquid crystals are usually bi-functional liquid crystal structure monomers. The operating voltage is high, and the hysteresis effect is also large. In this article, the mono-functional polymer monomer is added in reverse-mode polymer stabilised cholesteric liquid crystals (RPSCLC), it is found that when the weight content of the mono-functional monomer is close to that of bi-functional monomer, the large drop of the operating voltage is realised and the contrast ratio effect is increased, and the hysteresis effect can be greatly reduced. Combined with the morphology of the polymer network, the physical mechanism of the influence of mono-functional monomers on the electro-optic properties of PSCLC is revealed. The sample has the simple manufacturing process, uniform size of polymer network, low operating voltage, low hysteresis effect and high contrast ratio. The results will provide the important influence on the research of PSLC.

Temperature-dependent chirality of cholesteric liquid crystal for terahertz waves.

Recently, the terahertz (THz) chiral field control opens a new window to THz devices and their applications. In this Letter, the active manipulation for THz chiral states based on the cholesteric liquid crystal (CLC) has been demonstrated by THz time domain cross-polarization spectroscopy. The results show that the CLC has strong THz optical activity and circular dichroism (CD) effect, and the strongest THz CD of 22 dB and a polarization rotation angle of 88.4° occur around the phase transition temperature TS-N=250K. Rising to a room temperature of 300 K, the CLC turns from a chiral state to an isotropic state for THz waves with the phase transition processes of CLC molecules. Therefore, this CLC device can be performed as a thermally active THz circular polarizer, which brings potential applications in THz polarization imaging, broadband communication, and spectroscopy.

A cholesteric liquid crystal device having stable uniform lying helix structure

A cholesteric liquid crystal device having the uniform lying helix (ULH) structure shows stable and reversible characteristics and it has been demonstrated for the analogue modulation in 10 micro-second response time at room temperature. The device comprises a polymer-free chiral nematic liquid crystal mixture and the photoalignment layer is irradiated by the UV light to cause a preferred orientation of the liquid crystal. The ULH structure is obtained by cooling the liquid crystal mixture from the isotropic phase transition temperature in the presence of electric field. On the electrical and thermal cycling, no noticeable hysteresis has been observed. In addition, the tilt angle and response time have been measured in good agreement with Meyer's theory on the flexoelectric effect in liquid crystals.