Focal Conic State Research Articles

Mechanism of switching between planar and focal conic states in a bistable cholesteric-liquid-crystal reflective display

Mechanism of switching between planar and focal conic states in a bistable cholesteric-liquid-crystal reflective display

Effect of surface alignment on electric-field-induced phase transitions in blue phases

This study investigates the critical electric field thresholds required for phase transitions in the blue phases of liquid crystals (BPs) confined within vertical field switching cells. BPs are attractive for electro-optical applications due to their polarization-independent response and fast switching times; however; challenges remain regarding their limited temperature stability and previously reported high operational voltages. We employ a combined approach of polarized optical microscopy and electrical impedance analysis to identify the electric field thresholds triggering BP transitions to focal conic domains in cells with and without planar polyimide alignment layers. We show that surface alignment layers stabilize the blue phases and allow for higher applied electric fields before transitioning into the focal conic state. This suggests the possibility of a wider operational voltage range in thinner cells with alignment layers. These findings significantly improve our understanding of BPs, addressing key challenges and paving the way for their integration into advanced photonic devices, based on the CMOS technology.

Near-infrared reflecting/transmitting smart windows based on chiral nematic liquid crystals with enhanced electrically switching performance.

This paper addresses global warming concerns stemming from energy consumption, particularly in buildings, which contribute 40% to global energy use. Smart windows that reflect near-infrared radiation have emerged as a solution to reduce indoor temperatures. Chiral nematic liquid crystals (CLCs) play a crucial role in this technology. Numerous approaches have been explored for regulating indoor temperatures using liquid crystals. Despite achieving ideal transparency, rapid switching speeds, negligible power consumption, and user control over switching, reported samples often face challenges when attempting to revert from either the focal conic state or the transmitting state back to the initial reflecting state. In this work, for the first time to our knowledge, CLC cells with electrical reversibility are visually demonstrated rapidly switching between reflective and transmitting modes. Cell thickness emerged as a pivotal factor in achieving smart window reversibility, with 3µm identified as the optimal choice. Samples exhibited effective IR reflection, high visible transparency, and complete reversibility, marking a significant step toward practical smart windows to combat global warming.

Smart Switchable Window with Bistability and Tunability of Transmission

AbstractWindows are used everywhere, from buildings to vehicles. Smart switchable windows are highly sought‐after in order to reduce energy consumption. An ideal smart window should have two functions. First, it should be able to control privacy. Second, it should be able to control radiant energy flow. Here, a smart window is reported that possesses the two functions. The smart window is based on a dichroic dye‐doped cholesteric liquid crystal. It exhibits two stable states in the absence of applied voltage. One of them is the planar state that is optically absorbing without haze and can control radiant energy flow. The other state is the focal conic state that is optically scattering and can control privacy. Furthermore, when voltage is applied, the liquid crystal is switched to a heliconical state, where the transmittance can be tuned continuously. Thanks to its superior performance, this smart window has a big potential for applications in architectural and vehicle windows.

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.

ZnO nanoparticles dispersed cholesteric liquid crystal based smart window for energy saving application

Electrically tunable smart window has been fabricated by using green synthesized Zinc oxide nanoparticles (ZnO NPs) dispersed in cholesteric liquid crystals (CLC). The fabricated device is semi-transparent in the planar state, opaque in the focal conic state and fully transparent in the homeotropic state. Interestingly, switching among these states is possible by just varying the applied voltage. The proposed device can even regulate the ambient temperature by controlling the sunlight. This type of smart window is developed by doping of ZnO NPs into Chiral nematic LC mixture, interestingly, as the concentration of ZnO NPs increases the transmittance decreases in Focal conic and it increases in Homeotropic state. The transmittance of NPs doped CLC is decreased from ∼87 to 4 % in the focal conic state (2.4 V/μm) and increased 4 % to ∼ 90 % in the homeotropic state (7 V/μm) with a short response time (< 1 ms). The versatility exhibited by the device showed good stability even after repeatedly turning the electric field on and then off for 170 s. Quantitatively, after 20 cycles of repeated switching, the spectral transmittance in each state remained essentially constant. We have demonstrated the operation of privacy windows alongside smart windows by exploiting the capabilities of NPs and CLCs to reduce reliance on artificial lighting, heating and cooling building operation costs. The indoor temperature-regulation effects of Pure CLC and ZnO doped CLC devices were investigated using a solar simulator device. In order to regulate interior temperature and to conserve energy, it is anticipated that this technology may be used as windows for automobiles and buildings to control temperature and save energy.

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.

Comparison of electro-optical and dielectric properties of cobalt (II) phthalocyanine doped cholesteric liquid crystals

The aim of this study is to investigate and compare the electro-optical and dielectric properties of cobalt (II) phthalocyanine (CoPc) doped cholesteric liquid crystal (CLC) composites. Experimentally, pure CLC and CoPc (at different concentrations) doped CLC composites were produced. The wavelength-dependent transmittance behaviour of the samples was observed in Planar (P)-state without voltage and Focal Conic (FC)-state at low frequency with applied voltage. The P and FC states correspond to the transmitted and scattered of the incident light, respectively. Accordingly, the samples were transparent in P-state and opaque in FC-state. Furthermore, dielectric characteristic of pure CLC and CoPc doped CLC composites were investigated depending on the frequency. It was observed that dielectric constants and dielectric anisotropy changed with CoPc concentration in the low frequency region. Based on the electro-optical and dielectric properties, it is thought that produced CoPc doped CLC composites will be beneficial for optoelectronic devices.

Dual-Function Smart Windows Using Polymer Stabilized Cholesteric Liquid Crystal Driven with Interdigitated Electrodes.

In this study, we present an electrically switchable window that can dynamically transmit both visible light and infrared (IR) light. The window is based on polymer stabilized cholesteric liquid crystals (PSCLCs), which are placed between a top plate electrode substrate and a bottom interdigitated electrode substrate. By applying a vertical alternating current electric field between the top and bottom substrates, the transmittance of the entire visible light can be adjusted. The cholesteric liquid crystals (CLC) texture will switch to a scattering focal conic state. The corresponding transmittance decreases from 90% to less than 15% in the whole visible region. The reflection bandwidth in the IR region can be tuned by applying an in-plane interdigital direct current (DC) electric field. The non-uniform distribution of the in-plane electric field will lead to helix pitch distortion of the CLC, resulting in a broadband reflection. The IR reflection bandwidth can be dynamically adjusted from 158 to 478 nm. The electric field strength can be varied to regulate both the transmittance in the visible range and the IR reflection bandwidth. After removing the electric field, both features can be restored to their initial states. This appealing feature of the window enables on-demand indoor light and heat management, making it a promising addition to the current smart windows available. This technology has considerable potential for practical applications in green buildings and automobiles.

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.

A bistable cholesteric liquid crystal film stabilized by a liquid-crystalline epoxy/thiol compound-based polymer.

Bistable cholesteric liquid crystals have promising application prospects in various fields, such as smart windows and displays. However, the long-term stability of two individual states is not easy to achieve, hindering their practical use. In this research, the bistable feature was enhanced by constructing a microsphere-type polymer with a liquid-crystalline epoxy/thiol monomer in negative dielectric anisotropic cholesteric liquid crystals. Spectroscopic and optical examinations revealed that either the transparent planar state or the opaque focal conic state can be maintained without the aid of an external field. Moreover, they can be switched to each other by applying a high- or low-frequency electric field. Further, factors such as the chemical structure of thiol compounds, curing temperature and curing time were investigated to explore their influences on the micro morphology of the polymer and thereby the electro-optical properties. In addition, the frequency-dependent driving scheme was analysed. Finally, bistable switching was demonstrated using an optimized sample. This energy-efficient bistable film shines light on future applications in smart windows, photonic paper and other electro-optical devices.

Stable hazy states by electrohydrodynamic convection of ultraviolet-treated chiral nematic liquid crystal

In this paper, we show that electrohydrodynamic convection of ultraviolet (UV)-treated chiral nematic liquid crystals (CNLCs) generates stable foggy states. UV-irradiation produces relatively heavier and larger molecular ions at CNLC mixture, which activates electroconvection, and leads to easily dynamic scattering mode (DSM) with strong turbulence. Chirality of LC combined with the DSM builds two stable states (planar state and focal conic state) more strongly, unlike the conventional method generating the two states in the CNLCs. The molecular ion effect enables state conversion to a much lower switching voltage than that of the conventional cholesteric devices. The focal conic state connected to DSM2 exhibits a stable deep hazy state, the mixed state combined with DSM1 displays a stable weak hazy state, and the planar state activated in a particular frequency region creates a stable and very clear transparent state due to the chiral pitch of the infrared region. Therefore, a smart window with the tri-stable-hazy state specialized in low power consumption can be realized. The tri-stable-hazy state device also generates various hazy states according to the applied voltage; thus, it can provide a proper indoor temperature for users. Furthermore, a stable planar state with a chiral pitch designed in the infrared region may save energy through selective reflection of infrared rays from the sunlight in the summer. The proposed technology is suitable for commercial use in the next-generation smart window.

Electrically induced bistable switching of stop band in chiral nematic photonic crystal

Chiral nematic liquid crystals (CLCs) are highly promising tunable photonic materials that own various states and exhibit different optical properties in each state. The natural bistability between planar and focal conic states is widely exploited in power-saving applications. This work proposes another bistable electrical switching mode that involves two planar states that are realized by pitch jumps in polymer-stabilized chiral nematic liquid crystals (PSCLCs) with negative anisotropy. Under an appropriate polymerization condition, a polymer network can be formed throughout the CLC cell; it not only eliminates the original pitch jump behavior that is associated with thermal hysteresis in CLCs but also provides a balance between two planar states in the neighborhood of a pitch jump. The helical pitch of the PSCLC can be reversibly switched from the long pitch to the short pitch by applying pulse direct current (DC) voltages. The switching mechanism involves field-induced distortion in the polymer network, which generates the mechanical force that causes the pitch jump transition. Both planar states with different pitches can shift the wavelength of Bragg reflection, which depends strongly on the thickness of the PSCLC cells. A tunable laser that is based on a dye-doped PSCLC cell is fabricated and its wavelength-tuning ability is demonstrated.

Temperature Dependent Morphological and Electro-Optical Characteristics of Dye Doped Cholesteric Liquid Crystal

Cholesteric liquid crystals (CLCs) have gain remarkable attention due to its property of stabilizing the planar (P) and focal conic (FC) states. This paper presents the temperature dependent behavior of cholesteric liquid crystal (CLC) mixed with appropriate amount of dye. The pitch of sample corresponds to visible region (~1 µm) was adjusted by adding appropriate concentration of chiral dopant. The phase transition and morphological behavior was recorded using polarized optical microscope (POM) from planar state to isotropic state by heating the sample using temperature-controller. Experimental results indicated that increase in temperature impacts the morphology of CLC and strongly affects its electro-optical (E-O) properties. Further, POM studies showed that at room temperature dye doped CLC cell has initial transparent state (planar texture) however, when driving voltage was high (~27V) with lower frequency of 1 kHz, P state switched to FC state. On further increasing the temperature, minor distortion was observed in P state in comparison to P state of dye doped CLC cell at room temperature, however, at higher (~ 45˚C) temperature, low driving voltage was required to achieve the stable FC state. Further, FC state was switched to stable P state on applying high frequency (50 kHz) voltage. Afterward, above 45˚C temperature, the textures showed the sustained FC state of dye doped CLC cell with no switching to P state on applying high frequency (50 kHz) voltage. Thus, temperature dependent dye doped CLC cell showed the bi-stability feature upto 45˚C temperature, however, above this temperature showed the fixed FC state.

Performance augmentation of bistable cholesteric liquid crystal light shutter- effect of dichroic dye on morphological and electro-optical characteristics

In this article, bistable cholesteric liquid crystal (CLC) light shutters have been fabricated and the effect of dichroic dye on their morphological and electro-optical properties was studied. Experimentally, two different sample cells were prepared for pure CLC and 0.5 wt% dichroic dye (azo orange 3) doped CLC. The morphological behaviour of bistable CLC light shutters observed in transmissive and scattering modes represents planar and focal-conic (FC) states on applied voltages under high and low frequencies, respectively. The voltage-transmission and absorbance characteristics were examined and found the operating voltage lowered with the doping of dichroic dye in CLC. Moreover, dye doped CLC light shutter showed the higher stabilization rate (SR) for planar state along with better contrast ratio (CR) compared with pure CLC light shutter. In addition, phase transition studies of pure and dye doped CLCs have been discussed.

Label-Free Cholesteric Liquid Crystal Biosensing Chips for Heme Oxygenase-1 Detection within Cerebrospinal Fluid as an Effective Outcome Indicator for Spontaneous Subarachnoid Hemorrhage.

A novel device for cholesteric liquid crystal (LC; CLC)-based biosensing chips for detecting heme oxygenase (HO)-1 within the cerebrospinal fluid (CSF) was invented. In the CLC device, the reorientation of the LCs was strongly influenced by the alignment layer surface and adjacent LCs. When the substrate was coated with the alignment layer, the CLCs oriented homeotropically in a focal conic state. Once HO-1 was immobilized onto the orientation sheet-coated substrate, the CLC changed from a focal conic state to a bright planar state by disrupting the CLCs. The concentration of HO-1 within CSF was shown to be an effective outcome indicator for patients with a spontaneous subarachnoid hemorrhage. We showed that the CLC immunoassaying can be used to measure HO-1 with a lower detection limit of about 10 ng/mL. The linear range was 10 ng/mL to 1 mg/mL. An easy-to-use, rapid-detection, and label-free CLC immunoassay device is proposed.

Thermal imprint of wide-angle viewing bi-stable cholesteric liquid crystal displays.

A thermal-imprint addressable and electrically erasable bi-stable cholesteric liquid crystal (CLC) display with a wide viewing angle is demonstrated. The proposed device with a multi-domain planar state is realized by filling a negative CLC in a vertical-alignment cell. The thermal-imprint method is introduced to restore the CLC from a reflective state (multi-domain planar state) to a translucent state (focal-conic state) to display images, and an electric field is used to erase the device back to totally reflective mode. This CLC display is bi-stable and does not require a complex driving circuit. Together with the features of a large viewing angle and less color shift, this device shows great potential for update-on-demand applications.

Bistable Color‐Tunable Liquid Crystal Fiber by the Method of Mechanical Drawing with Synchronous Ultraviolet Polymerization

AbstractA color‐tunable fiber is demonstrated to reflect colors by switching the electric field on a chiral liquid crystal (CLC) with different helical twisting powers. With the purpose of bistability and low power consumption, the mechanism of the CLC fiber proposed in this study is an electric‐field‐induced phase change. To maintain the balance of wearable flexibility and mechanical stability, the liquid crystal fiber is constructed by the method of mechanical drawing with synchronous ultraviolet polymerization. To be a typical 1D structure, the central core of the CLC fiber is made of an aluminum wire, which is used to load the driving voltage as well as a mechanical support. In addition, there are three claddings surrounding the central core: the color‐tunable cladding filling with CLC material, the polymer cladding to anchor the liquid crystal, and the outer transparent electrode cladding. Bistable color change is realized by switching two given voltages corresponding to the planar state (P‐state) and focal conic state (FC‐state). Furthermore, to knit N fibers together, 2N colors can be realized by applying electric voltages to the fiber bundle.

Triboelectric-optical responsive cholesteric liquid crystals for self-powered smart window, E-paper display and optical switch

Intelligent responsive devices are crucial for a variety of applications ranging from smart electronics to robotics. Electro-responsive cholesteric liquid crystals (CLC) have been widely applied in display panels, smart windows, and so on. In this work, we realize the mechanical stimuli-triggered optical responses of the CLC by integrating it with a triboelectric nanogenerator (TENG), which converts the mechanical motion into alternating current electricity and then tunes the different optical responses of the CLC. When the voltage applied on the CLC is relatively low (15–40 V), the TENG drives the switching between the bistable planar state and focal conic state of the CLC, which shows potential applications in self-powered smart windows or E-paper displays. When the voltage supplied by the TENG is larger than 60 V, a self-powered optical switch is demonstrated by utilizing the transformation between focal conic state and instantons homeotropic state of the CLC. This triboelectric-optical responsive device consumes no extra electric power and suggests a great potential for future smart electronics.

Sensitive, Color-Indicating and Labeling-Free Multi-Detection Cholesteric Liquid Crystal Biosensing Chips for Detecting Albumin.

A novel device for cholesteric liquid crystal (CLC)-based microfluidic chips, accommodated in a polydimethylsiloxane material, was invented. In this device, the reorientation of the CLCs was consistently influenced by the surface of the four channel walls and adjacent CLCs. When the inside of the microchannel was coated with the alignment layer, the CLCs oriented homeotropically in a focal conic state under cross-polarizers. Once antigens had bound onto antibodies immobilized onto the orientation sheet-coated channel walls, the light intensity of the CLC molecules converted from a focal conic state to a bright planar state caused by disrupting the CLCs. By means of utilizing pressure-propelling flow, the attachment of antigen/antibody to the CLCs should be detectable within consecutive sequences. The multi-microfluidic CLC-based chips were verified by measuring bovine serum albumin (BSA) and immune complexes of pairs of BSA antigen/antibody. We showed that the multiple microfluidic immunoassaying can be used for measuring BSA and pairs of antigen/antibody BSA with a detection limit of about 1 ng/mL. The linear range is 0.1 μg/mL–1 mg/mL. A limit of immune detection of pairs of BSA antigens/antibodies was 10 ng/mL of BSA plus 1000 ng/mL of the anti-BSA antibodies was observed. According to this innovative creation of immunoassaying, an unsophisticated multi-detection device with CLC-based labeling-free microfluidic chips is presented.