Cholesteric Liquid Crystal Layer Research Articles

Local Orientation Transitions to a Lying Helix State in Negative Dielectric Anisotropy Cholesteric Liquid Crystal

Orientation transitions in a cholesteric liquid crystal (CLC) layer with negative dielectric anisotropy, under the influence of a non-uniform spatially periodic electric field created using a planar system of interdigitated electrodes, were studied experimentally and numerically. In the interelectrode space, transitions are observed from a planar Grandjean texture, with the helix axis perpendicular to the layer plane, to states with a lying helix, when the helix axis is parallel to the layer plane and perpendicular to the electrode stripes. It was found that the relaxation time of the induced state in the Grandjean zones, corresponding to two or more half-turns of the helix, significantly exceeded the relaxation time for the first Grandjean zone with one half-turn. An analysis of experimentally observed and numerically simulated textures shows that slow relaxation to the initial state in the second Grandjean zone, as well as in higher-order zones, is associated with the formation of local topologically equivalent states. In these states, the helix has a reduced integer number of helix half-turns throughout the layer thickness or unwound into the planar alignment state.

Color-Tunable Lead Halide Perovskite Single-Mode Chiral Microlasers with Exceptionally High glum.

Chiral microlasers hold great promise for optoelectronics from integrated photonic devices to high-density quantum information processing. Despite significant progress in lead-halide perovskite emitters, chiral lasing with high dissymmetry factors (glum) has not yet been realized. Here, we demonstrate chiral single-mode microlasers with exceptional stability and tunable emission across the visible range by combining CsPbClxBr3-x perovskite microrods (MRs) with a cholesteric liquid crystal (CLC) layer. The MRs lase via a whispering gallery mode (WGM) microcavity and confer chirality through the encapsulated CLC layer, thus exhibiting circularly polarized lasing with dissymmetry factors reaching 1.62. Importantly, we demonstrate wavelength-tunable high dissymmetry chiral lasers in a broad spectral range by tuning the halide composition and using CLC layers with the desired photonic bandgap (PBG). This facile approach to generate chiral lasing not only is applicable to semiconductor nano- and microcrystals but also paves the way for potential integration into nanoscale photonic devices.

Photonic defect modes in cholesteric liquid crystal resonators with embedded isotropic layers

Photonic defect modes are explored as a viable alternative to standard photonic band edge modes in photonic crystal applications, especially due to their typically high Q-factors and local density of states. For example, they can be used in nonlinearity enhancement, lasing, and cavity quantum electrodynamics. However, they are strongly dependent on any structural change and need to be well-controlled to ensure the desired resonance frequency. Here, we present a study of the photonic defect modes that appear in a structure where a layer of isotropic material is embedded between two layers of cholesteric liquid crystal (CLC), using full electrodynamics numerical simulations. We present typical transmission spectra and electric field profiles of selected defect modes and then analyze the influence of geometrical and material parameters on the eigenfrequencies and Q-factors of the modes within and around the photonic bandgap, including refractive indices and thicknesses of isotropic and liquid crystal layers, and different anchoring orientations at the boundaries of the isotropic defect layer. Additionally, a connection of such defect modes to previously extensively analyzed twist defect modes is given. Eigenmodes in asymmetric resonators are also presented, where CLC layers surrounding the intermediate isotropic layer are not equally thick, enabling biasing of specific directional light emission. More generally, this work aims to contribute to the understanding and design capability in topological soft matter photonics where defect mode lasing could be realized in CLC geometries with different singular and solitonic topological defect structures.

Magnetically induced tunable exceptional and Dirac points

The photonic properties of cholesteric liquid crystals (CLCs) in an external magnetic field parallel to the helix axis are theoretically investigated for the first time in the short-wavelength band of the spectrum, where exceptional points (EPs) are found. It was shown that at the EP the real parts of two of four wave numbers touch each other or intersect at the absence of the dielectric anisotropy, and their imaginary parts intersect. At the EP the reflection, transmission and absorption show the same magnitudes for two eigenmodes of CLC layer. At anisotropic absorption near the EP there appear points where the magnetically induced transmittance/absorption are observed. The tunability of the EP was shown, i.e. we can change its wavelength by changing the magnitude of the external magnetic field. It was demonstrated the resonant enhancement of the chirality near the EP. Then it was shown that near the EP the two eigenmodes of CLC layer become strongly non-orthogonal.

A double-layer light shutter consisting of polymer dispersed liquid crystal and azo dye/quantum dot

We have developed a double-layer liquid crystal light shutter and investigated its temperature dependent morphological, electro-optical, and dielectric properties. The optical layers of the shutter are in direct contact, reducing optical losses. The shutter comprises a polymer dispersed liquid crystal film adjacent to an azo dye methyl red and CdSeS/ZnS quantum dot doped cholesteric liquid crystal layer. This novel double-layer light shutter device is switchable between opaque and transparent states at room temperature, and it exhibits low threshold and saturation voltages. Both the threshold (Vth) and saturation (Vsat) voltages are found to decrease with increasing temperature. The electrical characteristics of the shutter, namely the real part of dielectric constant (ε′) and the ac conductivity (σac) also increase with temperature in the low frequency region. Our results show that this double-layer light shutter has interesting potential for simple electro-optical devices and smart window applications.

P‐159: Enhanced Color Conversion Efficiency of Quantum Dot Displays based on Organic‐inorganic Composite as Scatterers and Cholesteric Liquid Crystal Layer as Reflectors

In quantum dot organic light emitting diodes (QD‐OLEDs), pixelized quantum dots convert blue OLED light to green and red. However, blue light leakage occurs due to QD layer's efficiency. Additionally, external light can cause green and red appearance even in black state. We propose using liquid crystal droplets for improved color conversion and nozzle unclogging. Further, a cholesteric LC layer is added to selectively reflect external UV and blue light.

Doped nanofibers achieve tunable broadband in cholesteric phase liquid crystals

ABSTRACT In this study, polymer-stabilised cholesterol-based liquid crystal (PSLC) broadband reflective films were prepared by doping UV absorbers and chiral compounds in bilayer poly (methyl methacrylate) (PMMA) nanofiber films. Broadband reflection was achieved by diffusion of the UV absorber and chiral compounds in the bilayer nanofibers into the liquid crystal layer, which resulted in the formation of an inhomogeneous pitch distribution in the liquid crystal layer. To attain an optimal reflective effect, various factors such as polymer monomer concentration, concentration of chiral compounds in the spun film, diffusion temperature, diffusion time, and intensity of polymerised light were investigated. The effects of these factors on the reflective bandwidth were explored. The method significantly broadened the reflective bandwidth of the cholesteric phase liquid crystal layer, establishing the foundation for studying the combination of nanofiber films with cholesteric phase liquid crystals to form an adjustable pitch.

Bioinspired Confined Assembly of Cellulosic Cholesteric Liquid Crystal Bubbles.

Construction of biomimetic models for structural color evolution not only gives new photonic phenomena but also provide cues for biological morphogenesis. Here, a novel confined self-assembly method is proposed for the generation of hydroxypropyl cellulose (HPC)-based cholesteric liquid crystals (CLCs) microbubbles. The assembly process relies on the combination of droplet microfluidics, solvent extraction, and a volume confined environment. The as-prepared HPC structural color microbubbles have a transparent shell, an orderly arranged cholesteric liquid crystal (CLC)middle layer, and an innermost bubble core. The size of the microbubble, shell thickness, and the color of the CLC layer can be adjusted by altering the microfluidic parameters. Intriguingly, benefited from the compartmentalization effect provided by droplet microfluidics, microbubbles with multiple cores of different color combinations are generated under precise control. The self-assembled CLCs microbubbles have bright structural color, suspending ability, and good temperature-sensitive characteristics, making them ideal underwater sensors. The present confined assembly approach will shed light on creating novel photonic structures and the HPC microbubble will find widespread applications in multifunctional sensing, optical display, and other related fields are believed.

Electrically Tunable Two-Color Cholesteric Laser.

Two-color lasing emission from an asymmetric structure, consisting of two dye-doped cholesteric liquid crystal (DD-CLC) layers separated by a transparent interlayer, is demonstrated. The DD-CLC mixtures have different reflection bands with long-wavelength band edges located at the green and red wavelengths of the visible spectrum, respectively. For the laser action, the CLC hosts provide the feedback, and the fluorescent laser dyes represent the active medium. When the stacked structure is optically pumped above the threshold, two simultaneous laser lines separated by 123 nm are observed at the long-wavelength band edges of the DD-CLC mixtures. The influence of an electric field on lasing behavior is also analyzed and discussed in terms of the reflection spectrum and laser action. The results show a reversible tuning of the reflection band, accompanied by a modification of the lasing characteristics under the application of an external field. Above a specific threshold voltage, one of the emission lines is suppressed and the other is conserved. With a further increase in the voltage, both laser emissions are entirely inhibited. The investigated structure demonstrates a simple technique to obtain an electrically tunable multi-wavelength laser, which might pave the way for a new generation of organic laser sources.

Towards Chiral Lasing from All-Solution Processed Flexible Perovskite Nanocrystal-Liquid Crystal Membrane.

Circularly polarized (CP) coherent light sources are of great potential for various advanced optical applications spanning from display/imaging to data processing/encryption and quantum communication. Here we report the first demonstration of CP amplified spontaneous emission (ASE)/lasing from a free-standing and flexible membrane device consisting of perovskite nanocrystals (PNCs) and cholesteric liquid crystal layers sandwiched within a Fabry-Perot cavity architecture. The chiral liquid crystal cavity enables the generation of CP light from the device. The device is completely solution-processable, and displays CP ASE with record dissymmetry factor (glum ) as high as 1.4, which is 3 orders of magnitude higher as compared with glum of CP luminescence of chiral ligand-capped colloidal PNCs. The device exhibits ultra-flexibility as the ASE intensity remains unchanged after repeated 100 bending cycles and it is stable for more than 3 months with 80% of its original intensity. Furthermore, the ultra-flexibility enabled the generation of ASE from various objects of different geometric surfaces covered with the flexible perovskite membrane device. This work not only demonstrates the first CP ASE from PNCs membrane with extremely high glum but also opens the door toward the fabrication of ultra-flexible, extremely stable, and all solution-processable perovskite chiral laser devices. This article is protected by copyright. All rights reserved.

Linear and circular dichroisms of cholesteric liquid crystals in a longitudinal external magnetic field

ABSTRACT We investigated the properties of linear and circular dichroisms of cholesteric liquid crystal layer in external static magnetic field directed along helix axis. We showed that when the direction of light propagation and the direction of external magnetic field coincide, increasing the latter magnitude leads to a decrease in the linear dichroism, whereas when these directions are opposite, increasing the external magnetic field ( B e x t ) magnitude leads an increase in the linear dichroism. The presence of external magnetic field leads to the shift of circular dichroism curves both along wavelength axis and along dichroism axis. Then, we investigated the peculiarities of dichroism dependencies on B e x t for different wavelengths of incident light. We compared the dependence of circular dichroism on B e x t for a magnetically active cholesteric liquid crystal layer and an isotropic magnetically active layer and showed that while these dependencies are similar for wavelengths away from the photonic band gap, diffraction significantly affects these dependencies for wavelengths near and inside the photonic band gap. We investigated the evolution of spectra of linear and circular dichroisms at the change in B e x t , too. The results obtained can find application in the design of innovative polarised optoelectronic devices.

80‐2: Liquid Crystal Surface Relief Diffractive Lens for Presbyopia

We demonstrated a two‐step tunable liquid crystal (LC) surface relief diffractive lens for presbyopia. The surface relief diffractive structure is fabricated by nano‐imprint lithography (NIL) roll‐to‐stage process. The LC lens is polarization‐independent with only one cholesteric liquid crystal (CLC) layer. The LC lens can be fabricated by LC one‐drop fill (ODF) process. This is useful for presbyopia vision correction. In this paper, we realized two‐step switched liquid crystal lens with optical power 0 and +2 diopters.

Transparent Multicolor Electrochromic Displays with Ingenious Hues Adjustment by Integrating Cholesteric Liquid Crystal with Viologen Gel

AbstractThe colorization of electrochromic displays (ECDs) has become an intriguing hotspot with fast‐growing demands for the development of displays. Reported colorful inorganic ECDs incorporating optical nanocavity or metal‐dielectric stacks sacrifice transparency and have intrinsic structural color, thus confining plenty of applications as transparent displays. Additionally, most transparent ECDs adopt plane alignments of EC materials that can only show one or several kinds of colors under electrical stimuli. Herein, vertically stacked viologen‐based ECDs integrating cholesteric liquid crystal (CLC) are proposed. By applying voltage, rotation, and changing chiral dopants’ concentration, the combined ECDs show enlarged color palette and multi‐mode color modulation. Moreover, red, blue, and green viologen gels are also synthesized by producing dioctyl‐, diethyl‐, and diphenyl‐substituted dipyridyl, which exhibit outstanding performance in terms of high optical variation, fast response speed, long cyclability, etc. Furthermore, the palette area of the CLC layers is dramatically expanded about 5 times more than reported by introducing a birefringent film. The vertically stacked ECDs integrate merits of both CLCs (high color purity and rich hues) and viologens (large variation and low voltage response), which will likely encourage attempts at three‐dimensional colorful EC displays and broaden its applications.

Spectral manifestation of optical Tamm states in a metal-cholesteric liquid crystals stack.

The reflection spectrum of linearly polarized light by a system consisting of a metal film and two adjacent sequentially located cholesteric liquid crystals (CLCs) with opposite helical twists is theoretically studied. The system contains a dielectric index-matching layer (DIML) between the metal film and the CLC layers. It is shown that in such a system the excitation of optical Tamm states (OTSs) by linearly polarized light is possible. The influence of the CLC pitch, refractive indices, and thicknesses of the DIML and metal film on the OTS manifestation in the reflection spectrum of the system is studied. The strong influence of the DIML thickness on the OTS wavelength and the appearance of multiple OTSs with an increase in the DIML thickness is noted.

LCP /TLC based composite multi-dimensional polarization-dependent anti-counterfeiting device

Modern anti-counterfeiting technology can effectively suppress and combat forgery and counterfeiting behaviors, which is of great significance in information security, national defense and economy. However, the realization of multi-dimensional, integrated, difficult-to-copy and easy-to-detect optical anti-counterfeiting devices is still a challenge. In this paper, a multi-dimensional and polarization-dependent anti-counterfeiting device with structure color is designed, which is composed of patterned liquid crystal polymer (LCP) nematic layer and thermotropic cholesteric liquid crystal (TLC) layer. It has the advantages of displaying and hiding polarization states, wide color tuning range, convenient operation, high integration and security. For incident light with a specific polarization state, the patterned nematic phase LCP layer can carry out regionalized phase editing and polarization state modulation, while the TLC layer can selectively reflect the incident light. Therefore, a patterned structural color security label is subtly realized. The anti-counterfeiting device can realize the display, hiding, color adjustment and image/background conversion of patterns by adjusting the polarization direction of incident light. In addition, the TLC layer in the device can meet the application requirements of the anti-counterfeit device at different environmental temperatures through the flexible design of the system weight ratio. Furthermore, the device can be easily heated by body temperature, realize dynamic real-time wide-spectrum color modulation and reversible pattern erasure, and further enhance its security dimension and security. The multi-polarization-type anti-counterfeiting device has three-dimensional anti-counterfeiting efficacy. The first dimensional anti-counterfeiting efficacy is achieved by the thermochromic liquid crystal layer. The thermochromic liquid crystal layer has no reflection color outside the operating temperature range of TLC material, and the entire device displays black background. The second and the third dimensional anti-counterfeiting efficacy are related to the polarization state of the incident light and the linear polarization direction, respectively. Only when the incident light is linearly polarized light and its polarization direction makes an angle of 45° or –45° with respect to the optical axis of the liquid crystal, will the device show the designed pattern. Consequently, our proposed anti-counterfeiting device is expected to provide a new idea for developing the anti-counterfeiting field.

Spiral Pitch Control in Cholesteric Liquid Crystal Layers with Hybrid Boundary Conditions

The optical spectra of the cholesteric liquid crystal (CLC) layers under conditions of hybrid anchoring show a short-wave shift under a pulsed electric field. This behavior is anomalous because it is associated with a decrease in the pitch of the cholesteric spiral, which is atypical at conditions when the electric field is perpendicular to the axis of the CLC spiral. An analytical model of the phenomenon is discussed, according to which the spiral pitch under hybrid boundary conditions can be greater than the natural pitch in an unlimited volume of CLC. An in-plane electric field, being localized near the homeotropic-alignment surface, can be treated as effectively influencing the azimuthal anchoring and leading to a variety of metastable states with both increased and decreased pitch. These metastable states with local minima of free energy prevent the spiral from unwinding, and corresponding bands of selective reflection can even be shifted to the short-wave region of the spectrum. The observed effect is also studied numerically. It is shown by numerical simulations that the localized electric field from short-pitch electrodes can also modify zenithal anchoring, which should allow for defect-free controlling of the spiral pitch and spectral stop-band location.

Surface plasmon excitation on a grating assisted by a cholesteric liquid crystal layer.

While surface plasmon excitation assisted by nematic liquid crystal layers has been comprehensively studied in different aspects, application of cholesteric structures for surface plasmon excitation remains an unexplored area. Moreover, structures including cholesteric layers and metal grating for surface plasmon excitation have never been considered, to the best of our knowledge. We studied theoretically and experimentally such structures focusing on different regimes of propagation of normal modes in the cholesteric liquid crystal layer. The application of such structures for sensing is accordingly discussed.

Asymmetric absorption in asymmetric dielectric Fabry-Perot resonator with cholesteric liquid crystal layer inside

Highly efficient on-chip asymmetric absorption devices are indispensable for all-optical computing, information processing, etc., but the asymmetric absorption of light has rarely been investigated. Currently, on-chip asymmetric absorption devices still have some limitations, such as low contrast of absorption, narrow working bandwidth, and polarization selectivity, complexity, and expensive cost, which fail to meet the requirements of practical applications. In order to solve above-mentioned problems, we propose to design an on-chip asymmetric absorption device based on the asymmetric dielectric Fabry-Perot resonator with the cholesteric liquid crystal layer inside. Our results show that in some wavelength regions, the proposed structure have asymmetric absorption for both right circular polarization and left circular polarization modes with forward absorption up to 0.85 and 0.78, respectively and backward ones down 0.05 and 0.3 correspondingly. And the contrast ratio is of ∼1 in the first case and ∼0.45 in the second case. From the results we have presented, it follows that there are even better results for these parameters. Our solution provides a new strategy and platform for an on-chip reciprocal asymmetric absorption device that meets the requirements of practical applications.

Research on a novel temperature indicating device based on Bragg reflection waveguide of planar texture cholesteric liquid crystal layer

In this paper, a temperature indicating device with double-layer planar texture cholesteric liquid crystal (PTCLC) is designed, which has great advantages over temperature card. It works based on the temperature dependence of the cholesteric liquid crystal helical pitch and the Bragg Reflection of PTCLC. A white light source is placed on one side of the waveguide element of the temperature indicating device, and only light of a particular wavelength can form guided wave due to the Bragg Reflection. The feasibility of the temperature indicating device was verified by measuring the Bragg reflectance spectrum and chromaticity coordinates at different temperatures.

Optics of helical periodical media with giant magneto-optical activity

The features of the optical properties of cholesteric liquid crystals (CLC) with enormous magneto-optical activity were investigated. It is shown that the system under consideration has the property of nonreciprocal reflection / transmission, absorption and light energy localization. It was shown that this system can work as an almost ideal tunable polarization plane rotator. An external magnetic field leads to a change (tune) of the localized light energy in the CLC layer over a wide range and this property can also find important practical applications.