Cholesteric Liquid Crystal Layer Research Articles

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.

Features of magneto-optics of dichroic cholesteric liquid crystals

In this work, magneto-optical properties of a dichroic cholesteric liquid-crystal layer are theoretically investigated at large values of the magneto-optical parameter. Features of all solutions of the dispersion equation are studied in detail. Peculiarities of the reflection, transmission, absorption spectra and the influence of dielectric boundaries on them are investigated. Specific properties of the localization of light and magnetically induced transparency in dichroic cholesteric liquid crystals are considered. The study of the light localization features showed that the presence of an external magnetic field, as well as the presence of dielectric boundaries, led to the appearance of oscillations in the dependence of the intensity of the layer-confined energy on the coordinate of the axis directed along the cholesteric axis. A strong influence of the refractive index of isotropic half-spaces adjacent to a dichroic cholesteric liquid crystal layer on the optics of the layer under consideration is shown. In particular, magnetically induced transparency and diffraction transmission appear only at certain intervals of the refractive index of isotropic half-spaces.

Development an optical sensor using lyotropic cholesteric liquid crystals for the detection of toxic gases

In the current study, a lyotropic cholesteric liquid crystal (ChLC) based sensor for the identification of vapors of polar (toluene and phenol) and apolar (1,2 dicholoropropane) toxic gases was investigated. The lyotropic ChLC sample including cholesteryl oleyl carbonate, cholesteryl pelargonate, and cholesteryl benzoate was supported on the chemically modified glass surfaces as an optical sensor for the detecting of these toxic gases vapors. The glass surfaces were modified by coating Dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP). The optical signal generated by the incorporation of different toxic gases vapors in the lyotropic ChLC layers which disturbs the pitch length. These toxic gases were evaporated at different temperatures and the exposure time was differentiated. Increasing solvent evaporation temperature lead a shift in the wavelength maximum to smaller wavelengths which can be observed by a naked eye.

Transparent UV-blocking photonic film based on reflection of cholesteric liquid crystals

An optically transparent bilayer cholesteric liquid crystal (CLC) solid film capable of almost completely rejecting ultraviolet (UV)-A light was fabricated with a bilayer CLC film comprising double handed CLC layers using UV-curing reactive mesogen mixtures (RMMs) doped with LC756 and S811. The concentrations of S811 and LC756 in RMMs and the film thickness were optimized to achieve more than 98% UV blocking (in the UV-A region) and more than 95% transparency (in the visible region). The proposed approach provides a simple and reliable fabrication process that can be employed in practical applications such as smart windows, eyeglasses, and lenses.

Nonreciprocal localization of light in magnetoactive cholesteric liquid crystals in an external magnetic field

The optical properties of cholesteric liquid crystals (CLCs) exhibiting magneto-optical activity in the presence of an external static magnetic field have been investigated. The boundary-value problem of light transmission through a finite layer of a magnetoactive CLC at normal incidence of light is considered in the limiting case of the absence of local linear birefringence in the medium. That is a CLC with parameters Re∆=0,Im∆≠0, where ∆=ε1−ε2/2, andε1,ε2 are the principal values of the local dielectric tensor, is considered. In this case, in the absence of an external magnetic field, all linearly polarized incident waves undergo practically the same reflection in the photonic band gap (PBG). An external magnetic field removes this degeneracy. For the first time, the features of the localization of light in a layer of a magnetoactive CLC are investigated, and it is shown that an external magnetic field leads to a change in the localized light energy in the CLC layer and nonreciprocal accumulation of light energy takes place.

Magnetically induced transparency and absorption in cholesteric liquid crystals

We report the discovery, theoretically, of new effects, namely, the effects of magnetically induced transparency/absorption. The effects are observed in a magnetically active cholesteric liquid crystal layer. Changing the external magnetic field and absorption, one can tune the frequency and the linewidth of the transparency/absorption band. We have shown that both effects can be simultaneously observed for two eigenmodes of a cholesteric liquid crystal (CLC) layer. Both effects are nonreciprocal and tunable. They can be observed in the case of the absence of dispersion of the dielectric and the magnetic tensor components, as well as in the case of dispersion dependences of optical parameters of the CLC layer. Comparison of these effects show that while the effect of magnetically induced transparency is observed at wide intervals of variation of the parameters of the CLC layer, the effect of magnetically induced absorption can observe only in very small intervals of these parameters.

Multi-stable cholesteric liquid crystal windows with four optical states

ABSTRACT An electrically switchable multi-stable liquid crystal device is demonstrated based on a non-uniform cholesteric liquid crystal (CLC) structure in a single cell. The non-uniform CLC structure consists of a polymer stabilised CLC layer and a non-polymer stabilised CLC layer. By taking advantage of the difference in driving electric field strength of the two layers, the device can be switched electrically to four different states: the reflecting state, the reflecting and scattering co-existing state, the scattering-only state and the transparent state. The optical properties of the four states and the transition conditions between the states were elucidated. Moreover, all states with reflecting and/or scattering properties are stable for an extended period of time after switching off the electric field. Thanks to its multi-stability, this device can be used for energy-saving smart windows of automobiles, buildings and in architectural aesthetics. An electrically switchable multi-stable liquid crystal device is demonstrated based on a non-uniform cholesteric liquid crystal (CLC) structure in a single cell. The device can be switched electrically to four different states. All states with reflecting and/or scattering properties are stable for an extended period of time after switching off the electric field. Thanks to its multi-stability, this device can be used for energy-saving smart windows of automobiles, buildings and in architectural aesthetics.