Cholesteric Research Articles

Enhanced electrically-induced bandwidth broadening via stratification of polymer stabilized cholesteric liquid crystals

Enhanced electrically-induced bandwidth broadening via stratification of polymer stabilized cholesteric liquid crystals

Temperature‐Tunable Cholesteric Liquid Crystal Optical Combiners for Extended Reality Applications

Recent advancements in extended reality (XR) showcase their potential ability to enhance the user experience of traditional displays such as liquid crystal displays and organic light‐emitting diode screens. To achieve this goal, the upcoming generation of head‐mounted displays (HMDs) necessitates a seamless transition between different XR modes, including augmented reality (AR) and virtual reality (VR) modes. Herein, an innovative cholesteric liquid crystal‐ (CLC‐) based optical combiner for HMDs is introduced. This approach enables the display device to switch between AR, VR, and transparent modes via temperature modulation. Since CLC materials demonstrate varying optical properties at different temperatures, a real‐time temperature monitoring system that is paired with an external controller is integrated. This allows for dynamic temperature adjustments of the optical combiner and enables smooth transitions between display modes. Furthermore, the Berreman 4 × 4 matrix method is used to simulate the optical properties of the optical combiner, finding strong agreement with the experimental results. The performance of the tunable optical combiners using 450, 532, and 635 nm laser sources is explored. The findings demonstrate that CLC‐based optical combiners can effectively switch between the three distinct modes at corresponding temperatures, paving the way for versatile applications in future HMDs.

Direct Ink Writing of Cephalopod Skin‐Like Core‐Shell Fibers From Cholesteric Liquid Crystal Elastomers and Dyed Solutions

AbstractDirect ink writing (DIW) of core‐shell structures allows for patterning hollow or composite structures for shape morphing and color displays. Cholesteric liquid crystal elastomers (CLCEs) with liquid crystal mesogens assembled in a helix superstructure are attractive for generating tunable iridescent structural colors. Here, by fine‐tuning the rheology of the core and shell materials, respectively, this study creates droplets or a continuous filament in the core from the precursors of polydimethylsiloxane (PDMS) or poly(vinyl alcohol), whereas CLCE forms the outer shell. By introducing a dye in the droplets, the skin structures of cephalopods, consisting of chromatophores and iridocytes, are mimicked for enhanced color saturation, lightness, and camouflage. After removal of the core material, a CLCE hollow fiber is obtained, which can switch colors upon mechanical stretching and pneumatic actuation, much like papilla along with iridocytes. Further, liquid crystal mesogens assembled in the bulk of the fiber are in polydomain. Thus, the skin appears opalescent at room temperature, much like how leucophores enhance reflectins. Upon heating above the nematic to isotropic transition temperature, the skin becomes transparent. Lastly, a cephalopod model is constructed, where different parts of the model can change colors independently based on different mechanisms.

Overlooked Ionic Contribution of a Chiral Dopant in Cholesteric Liquid Crystals.

This study focuses on the ionic contribution by a chiral dopant added into a nematic host for preparing cholesteric liquid crystals (CLCs). Chiral structures were designated by individually incorporating two enantiomers, R5011 and S5011, into the nematic E44 to construct right- and left-handed CLCs, respectively. Characterized by the space-charge polarization, the dielectric spectra of the CLCs were investigated in the low-frequency regime, where f ≤ 1 kHz. The role of the individual chiral dopant, R5011 or S5011, at concentrations of 0-4.0 wt.% in altering the ionic properties of the CLC material was analyzed by deducing the electrical conductivity, ion density, and ion diffusivity. Regardless of the cell structure to be antiparallel or twisted by 90°, a significant ionic response was observed in the right-handed CLCs in comparison with the left-handed counterparts, suggesting that excess ions originating from our R5011 were introduced into the mesogenic mixtures. This work alarms the potential contribution of notorious impurity ions by a chiral dopant, which is often ignored in fabricating CLCs for electro-optical applications.

Circularly Polarized Organic Ultralong Room‐Temperature Phosphorescence: Generation, Enhancement, and Application

AbstractCircularly polarized luminescent (CPL) materials have garnered tremendous attention owing to their expanded optical properties beyond emission wavelength and intensity. Among these, the emerging circularly polarized organic ultralong room‐temperature phosphorescence (CP‐OURTP) materialsdemonstrating elegant and distinct features are of significant importance for their extended emission lifetime, which represent a novel frontier in research with promising scientific and technological applications across diverse fields. This review systematically outlines the traditional strategies to achieve CP‐OURTP including organic crystals, copolymerization, host–guest doping, a combination of the copolymerization and host–guest doping, spinning and twisting technology, and supramolecular polymer assembly. Importantly, the recent significant progress of CP‐OURTP in the chiral soft materials, such as liquid crystals (LCs) involving lyotropic LCs (cellulose nanocrystals, CNCs) and chiral thermotropic LCs (cholesteric LCs and chiral LC elastomers), is showcased. Finally, the practical applications of CP‐OURTP materials are summarized, and the review concludes with the perspectives on the current challenges and future opportunities for CP‐OURTP materials. This review aims to inspire the further innovations in the fabrication of advanced CP‐OURTP materials and enrich their promising applications.

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.

Enhanced Laser Emission by Incorporating Helically Twisted Silver Nanoflakes within Polymer-Stabilized Cholesteric Liquid Crystals

Enhanced Laser Emission by Incorporating Helically Twisted Silver Nanoflakes within Polymer-Stabilized Cholesteric Liquid Crystals

Propagation of periodic director and flow patterns in a cholesteric liquid crystal under electroconvection

The electroconvection of liquid crystals is a typical example of a dissipative structure generated by complicated interactions between three factors: convective flow, structural deformation, and the migration of charge carriers. In this study, we found that the periodic structural deformation of a cholesteric liquid crystal propagates in space, like a wave, under an alternating-current electric field. The existence of convection and charge carriers was confirmed by flow-field measurements and dielectric relaxation spectroscopy. Given that the wave phenomenon results from electroconvection, we suggest a possible model for describing the mechanism of wave generation. The validity of the model was examined using the Onsager variational principle. Consequently, it was suggested that wave generation can be described by four effects: the electrostatic potential, mixing entropy, anisotropic friction due to charge migration, and viscous dissipation of the liquid crystal.

Multi-stage and multi-colour liquid crystal reflections using a chiral triptycene photoswitchable dopant.

The photomodulation of the helical pitch of cholesteric liquid crystals results in dynamic and coloured canvases that can potentially be used in applications ranging from energy-efficient displays to colour filters, anti-counterfeiting tags and liquid crystal (LC) lasers. Here we report on the analysis of a series of photoswitchable chiral dopants that combine the large geometrical change and bistability of hydrazone switches with the efficient helical pitch induction of the chiral motif, triptycene. We elucidate the effects that conformational flexibility, dispersion forces and π-π interactions have on the chirality transfer ability of the dopant. We then use the irradiation time with visible light (442 nm) combined with a simple digital light processing microscope projection set-up to draw numerous stable multi-coloured images on an LC canvas, showcasing the fine control this dopant yields over the LC assembly.

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.

Polymer-stabilized cholesteric liquid crystals with multi-state reversibility for applications in advanced smart adjustable anti-counterfeiting and energy-saving displays

The creation of an anti-counterfeiting material with reflection color, fluorescence, infrared imaging, and electrical property information will increase the level of anti-counterfeiting. However, the creation of such a material has proved to be extremely challenging. In this study, twin anti-counterfeiting labels with intelligently tunable patterns were obtained through a strategy of PMMA loaded chiral molecules via diffusing on the micrometer scale. In order to enhance anti-counterfeiting ability, the multi-mode intelligently modulated advanced anti-counterfeiting material was obtained through the strategy of introducing photo-isomerizing dithienylethene and infrared-shielding Cs0.33WO3 nanoparticles. This material can be reversibly converted into fluorescent, reflected colors, infrared, and electrical characteristic signals. The encrypted information is displayed with red fluorescence under UV light irradiation. Furthermore, the encrypted information can be reversibly displayed and erased under repeated stimulation by voltage and pressure, heating and cooling, UV, visible and near infrared (NIR) light. The aim of our work is to create a simple method to prepare complex patterns of structural and fluorescent color anti-counterfeiting films with the improved anti-counterfeiting capability. Based on this, the strategy of co-doping fluorescent molecules and photochromic molecules in liquid crystals combined with etching technology has realized a multiple anti-counterfeiting mode with a higher level of security. The advanced multiple anti-counterfeiting mode can output encrypted information of multiple states which will greatly improve the level of anti-counterfeiting. In terms of display, we obtained billboards with complex patterns, price intelligently tunable and full-color display by diffusion S5011 in liquid crystals, thus extending their application in the field of electronic shelf labels.

A compact, highly sensitive optical fiber temperature sensor based on a cholesteric liquid crystal polymer film

A compact, highly sensitive optical fiber temperature sensor based on a cholesteric liquid crystal polymer film

Cholesteric Liquid Crystal Mirror-Based Smart Window Controlled with Ambient Temperature

In this paper, the authors demonstrate a small prototype of a smart window based on the thermo-optical properties of cholesterol liquid crystals. Due to its polymer-free design, the manufactured smart window is transparent and can reflect certain portions of visible or infrared light without requiring an external power source, and thus is easier to install and operate. The proposed smart window technology based on a cholesteric liquid crystal mirror will reduce energy consumption costs by reflecting excess sunlight and heat transfer, increasing comfort for residents of buildings and structures.

Enhanced cone-shaped lasing from cholesteric liquid crystals.

Strong conical lasing in the violet-blue range of the spectrum from dye doped cholesteric liquid crystals is obtained. In contrast to the previously reported cases, the cone laser emission is sharply increased at the local maximums of the luminescence peaks of the dyes. Besides that, for obtaining the cone lasing strong focusing of the pumping beam is not required. Extremely low pump density is achieved for the cholesteric liquid crystal lasers. It is important that the cone lasing can be observed without simultaneous manifestation of any other laser emissions.

Tunable Helix Inversion and Circularly Polarized Luminescence in Cholesteric Liquid Crystal Copolymers with Room-Temperature Phosphorescence

Tunable Helix Inversion and Circularly Polarized Luminescence in Cholesteric Liquid Crystal Copolymers with Room-Temperature Phosphorescence

Sign Inversion of Circularly Polarized Luminescence in Cholesteric Liquid Crystals Induced by Mercury Ions through Binaphthyl Dopants' Conjugation Control.

Stimuli-responsive circularly polarized luminescence (CPL) materials based on cholesteric liquid crystal (CLC) platforms show great promise for applications in information encryption and anticounterfeiting. In this study, we constructed a mercury ion-responsive CPL system in CLCs by controlling the conjugation degree of axially chiral binaphthyl derivatives. Two chiral binaphthyl derivatives (R/S-1 and R/S-2) were initially used as chiral dopants to demonstrate that CPL inversion (glum values from 0.5/-0.44 to -0.53/0.48) in CLCs could be achieved by modulating the conjugation degree of the chiral binaphthyls. Based on this concept, the thioacetal binaphthyl R-2S was developed and used as a mercury-responsive chiral dopant in CLCs. Under Hg ion treatment, the CPL sign inverted (glum value changed from 0.22 to -0.29) due to the transformation of the thioacetal into an aldehyde group. Additionally, the mercury ion-responsive CPL material was applied in information encryption.

A Multiresponsive Ferrocene-Based Chiral Overcrowded Alkene Twisting Liquid Crystals.

The reversible modulation of chirality has gained significant attention not only for fundamental stereochemical studies but also for numerous applications ranging from liquid crystals (LCs) to molecular motors and machines. This requires the construction of switchable molecules with (multiple) chiral elements in a highly enantioselective manner, which is often a significant synthetic challenge. Here, we show that the dimerization of an easily accessible enantiopure planar chiral ferrocene-indanone building block affords a multi-stimuli-responsive dimer (FcD) with pre-determined double bond geometry, helical chirality, and relative orientation of the two ferrocene motifs in high yield. This intrinsically planar chiral switch can not only undergo thermal or photochemical E/Z isomerization but can also be reversibly and quantitatively oxidized to both a monocationic and a dicationic state which is associated with significant changes in its (chir)optical properties. Specifically, FcD acts as a chiral dopant for cholesteric LCs with a helical twisting power (HTP) of 13 µm-1 which, upon oxidation, drops to near zero, resulting in an unprecedently large redox-tuning of the LC reflection color by up to 84 nm. Due to the straightforward stereoselective synthesis, FcD, and related chiral switches, are envisioned to be powerful building blocks for multi-stimuli-responsive molecular machines and in LC-based materials.

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.

Reverse Mode Polymer Stabilized Cholesteric Liquid Crystal Flexible Films with Excellent Bending Resistance.

The reverse-mode smart windows, which usually fabricated by polymer stabilized liquid crystal (PSLC), are more practical for scenarios where high transparency is a priority for most of the time. However, the polymer stabilized cholesteric liquid crystal (PSCLC) film exhibits poor spacing stability due to the mobility of CLC molecules during the bending deformation. In this work, a reverse-mode PSCLC flexible film with excellent bending resistance was fabricated by the construction of polymer spacer columns. The effect of the concentration of the polymerizable monomer C6M and chiral dopant R811 on the electro-optical properties and polymer microstructure of the film were studied. The sample B2 containing 3 wt% of C6M and 3 wt% R811 presented the best electro-optical performance. The electrical switch between transparent and opaque state of the flexible PSCLC film after bending not only indicated the excellent electro-optical switching performance, but also demonstrated the outstanding bending resistance of the sample with polymer spacer columns, which makes the PSCLC film containing polymer spacer columns have a great potential to be applied in the field of flexible devices.

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.