Conventional Liquid Crystal Research Articles

Ultra-wideband and high-efficiency polarization rotator based on liquid crystals

We propose a novel design of an asymmetrically anchored negative liquid crystal (LC) cell capable of linearly rotating the polarization axis of broadband light, with the rotation angle controlled by voltage. Unlike conventional LC cells that require polarization filters or retarders, our LC cell can directly modulate the polarization state of light without any additional components. The LC cell exhibits high polarization rotation efficiency over a wide wavelength range (200 nm to 2000nm), superior to the existing polarization modulators. We validate our design through theoretical calculations and numerical simulations, proving that a degree of linear polarization of over 0.95 can be achieved in the UV, visible, and NIR regions. Our design can be applied in optical communications, biological imaging, and spectroscopic technologies.

Hydrogen bonding in 2,2,2-trifluoroethanol.

2,2,2-Trifluoroethanol (TFE) is known as a membrane mimetic solvent. The IR spectrum, 1H NMR spectrum, 13C NMR spin‒lattice relaxation times (T1), and nuclear Overhauser effect (NOE) data are consistent with extensive hydrogen bonding in TFE, but do not lead to structural features of the hydrogen bonding. Hence, DFT computations were carried out. The results predict the existence of a set of H-bonded dimers and trimers. The bond lengths and dihedral angles in these complexes are obtained, together with their dissociation energies. Computations were also performed for the geometry of the two conformers of the isolated monomer. The structure of one of the dimers consists of a 7-member cyclic fragment with a free CF3CH2 side chain. One set of the trimer structures involves the OH of a third monomer H-bonding to one of the F atoms in the CF3 group of the side chain of this dimer, thereby creating three trimer isomers. A fourth trimer cluster is formed from three monomers in which three OH∙∙∙O bonds create a cyclic fragment with three CF3CH2 side chains. The high dissociation energy (with respect to three monomers) indicates the high stability of the trimer complexes. The structural features of the trimer complexes resemble the structure of a conventional liquid crystal molecule and are postulated to resemble the latter in properties and function in solution, but at a much shorter timescale because of the noncovalent bonding. This hydrogen bonding phenomenon of TFE may be related to its function as a membrane memetic solvent. Initially, IR and NMR spectroscopic methods were used. Standard procedures were followed. For the computations, a hybrid DFT method with empirical dispersion, ωB97X-D, was used. The basis set, 6-311++G**, is of triple-ζ quality, in which polarization functions and diffuse functions were added for all atoms.

Probing the impact of bent-core mesogen on the dielectric relaxations of ferroelectric liquid crystal

The modulation in dielectric relaxations of a conventional ferroelectric liquid crystal (FLC, SCE9) material by doping the bent-core mesogen (BCM) derived from the oxadiazole bisaniline moiety has been greatly investigated using frequency-temperature dependent dielectric spectroscopy. The application of probing ac voltage (i.e., oscillating voltage, 800 mV) has clearly shown the presence of partially unwound helical mode (p-UHM) along with Goldstone mode (GM). However, the temperature-dependent dielectric studies further demonstrate that p-UHM gets merged with GM at higher temperatures and hence becomes difficult to observe it separately. It appears that doping of BCM into FLC leads to the suppression of p-UHM at lower temperature, but with the increase in the concentration of dopant beyond a certain point, p-UHM mode reappears. Moreover, the significant decrement (∼9 °C) in SmC*-SmA* phase transition temperature of FLC with increase in the dopant BCM concentration up to 3 wt%. We believe that such studies would be certainly useful for the understanding of the plausible interaction between two different shaped mesogens (i.e., rod and bent-core) present in the mixture and could pave the way forward to the use of one type of mesogen in the modulation of the dielectric properties of other mesogen.

Optical properties of Heliconical Cholesterics: effects of losses and photonic density of states

ABSTRACT The investigation of both linear and nonlinear optical properties of Heliconical Cholesteric has been triggered by the easy tuning of the Bragg resonance that can be achieved either by low-frequency electric field or by light waves. However, in previous studies two factors that can strongly affect the optical behaviour of these materials have not been considered. The first one is absorption originated by a complex dielectric permittivity at optical frequency, the second one are the modifications of the photonic density of states incurred by electrical or optical tuning of the Bragg resonance. In this paper we report an extended work of simulations addressing the mentioned issues following the same calculation method presented in previous published papers. We report first the results of the simulations performed on the optical properties of Heliconical Cholesterics showing the effects due to isotropic and anisotropic losses comparing them to what obtained in conventional cholesteric liquid crystals. Then we focus on the photonic density of states near the Bragg resonance and show that its modification when applying increasing static or optical field must be taken into account when considering laser action based on these materials since tuning may remarkably affect the lasing threshold.

Memorizable Electro‐Birefringence Effect Exhibited by Transparent Liquid Crystal/Polymer Composite Materials

AbstractPolymer‐dispersed liquid crystals (PDLCs) with nano‐phase‐separated structures, in which nanometer‐sized liquid crystal (LC) domains are dispersed within a polymer matrix (nano‐PDLCs), are transparent solid materials whose optical properties can be modulated by applying an electric field (E‐field). Because the proportion of LC that can respond to an electric field is small, the specific surface area of the phase‐separated interface of nano‐PDLCs is larger than that of conventional PDLCs, resulting in higher drive voltages than those of conventional PDLCs. To lower the driving voltage of nano‐PDLCs, highly polar LCs (C3DIO) are used with a large dielectric anisotropy (>10000), and prepared nano‐PDLCs using DIO mixtures obtained by mixing them with related compounds as the host LC. Nano‐PDLCs employing DIO mixtures exhibit higher E‐field responsivity than those using conventional LC. In addition, the electro‐optical Kerr coefficient at visible wavelength is significantly high, reaching 10−8 m V−2. Furthermore, nano‐PDLCs using the DIO mixture exhibit a memory effect in which the induced birefringence remains even after the removal of the in‐plane E‐field. Memorized birefringence can be erased by heating or applying an E‐field perpendicular to the substrate surface. Nano‐PDLCs using a DIO mixture can be rewritable electro‐birefringence‐responsive materials that can memorize arbitrary birefringence values.

An Electro-Optical Kerr Device Based on 2D Boron Nitride Liquid Crystals for Solar-Blind Communications.

Achieving light modulation in the spectral range of 200-280 nm is a prerequisite for solar-blind ultraviolet communication, where current technologies are mainly based on the electro-luminescent self-modulation of the ultraviolet source. External light modulation through the electro-birefringence control of liquid crystal (LC) devices has shown success in the visible-to-infrared regions. However, the poor stability of conventional LCs against ultraviolet irradiation and their weak electro-optical response make it challenging to modulate ultraviolet light. Here, an external ultraviolet light modulator is demonstrated using two-dimensional boron nitride LC. It exhibits robust ultraviolet stability and a record-high specific electro-optical Kerr coefficient of 5.1×10⁻2 m V-2, being three orders of magnitude higher than those of other known electro-optical media that are transparent (or potentially transparent) in the ultraviolent spectral range. The sensitive response enables fabricating transmissive and stable ultraviolet-C electro-optical Kerr modulators for solar-blind ultraviolet light. An M-ary coding array with high transmission density is also demonstrated for solar-blind ultraviolet communication.

Encapsulation engineering of porous crystalline frameworks for delayed luminescence and circularly polarized luminescence.

Delayed luminescence (DF), including phosphorescence and thermally activated delayed fluorescence (TADF), and circularly polarized luminescence (CPL) exhibit common and broad application prospects in optoelectronic displays, biological imaging, and encryption. Thus, the combination of delayed luminescence and circularly polarized luminescence is attracting increasing attention. The encapsulation of guest emitters in various host matrices to form host-guest systems has been demonstrated to be an appealing strategy to further enhance and/or modulate their delayed luminescence and circularly polarized luminescence. Compared with conventional liquid crystals, polymers, and supramolecular matrices, porous crystalline frameworks (PCFs) including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolites and hydrogen-bonded organic frameworks (HOFs) can not only overcome shortcomings such as flexibility and disorder but also achieve the ordered encapsulation of guests and long-term stability of chiral structures, providing new promising host platforms for the development of DF and CPL. In this review, we provide a comprehensive and critical summary of the recent progress in host-guest photochemistry via the encapsulation engineering of guest emitters in PCFs, particularly focusing on delayed luminescence and circularly polarized luminescence. Initially, the general principle of phosphorescence, TADF and CPL, the combination of DF and CPL, and energy transfer processes between host and guests are introduced. Subsequently, we comprehensively discuss the critical factors affecting the encapsulation engineering of guest emitters in PCFs, such as pore structures, the confinement effect, charge and energy transfer between the host and guest, conformational dynamics, and aggregation model of guest emitters. Thereafter, we summarize the effective methods for the preparation of host-guest systems, especially single-crystal-to-single-crystal (SC-SC) transformation and epitaxial growth, which are distinct from conventional methods based on amorphous materials. Then, the recent advancements in host-guest systems based on PCFs for delayed luminescence and circularly polarized luminescence are highlighted. Finally, we present our personal insights into the challenges and future opportunities in this promising field.

Room-temperature ferroelectric nematic liquid crystal showing a large and diverging density.

The ferroelectric nematic phase (NF) is a recently discovered phase of matter in which the orientational order of the conventional nematic liquid crystal state is augmented with polar order. Atomistic simulations suggest that the polar NF phase would be denser than conventional nematics owing to contributions from polar order. Using an oscillating U-tube densitometer, we obtain detailed temperature-dependent density values for a selection of conventional liquid crystals with excellent agreement with earlier reports. Having demonstrated the validity of our method, we then record density as a function of temperature for M5, a novel room-temperature ferroelectric nematic material. We present the first experimental density data for a NF material as well as density data for a nematic that has not previously been reported. We find that the room-temperature NF material shows a large (>1.3 g cm-3) density at all temperatures studied, notably including phases without polar order. An increase in density at phase transitions is observed. The magnitude of the increase for the intermediate-to-ferroelectric nematic (NX-NF) transition is an order of magnitude smaller than the isotropic-nematic (I-N) transition. We then probe potential consequences that may result from an elevated density through measurement of the refractive indices (no and ne). The navg of M5 is compared with 5CB and polar smectic liquid crystals. We observe how the highly polar nature of the system counteracts the effects of an increase in density. With knowledge of experimental density, we are able to derive an approximation that yields the polar order parameter, 〈P1〉, from polarisation measurements. Present results may be typical of ferroelectric nematic materials, potentially guiding material development, and is especially relevant for informing ongoing studies into this emerging class of materials.

Janus helices: From fully attractive to hard helices.

The phase diagram of hard helices differs from its hard rods counterpart by the presence of chiral "screw" phases stemming from the characteristic helical shape, in addition to the conventional liquid crystal phases also found for rod-like particles. Using extensive Monte Carlo and Molecular Dynamics simulations, we study the effect of the addition of a short-range attractive tail representing solvent-induced interactions to a fraction of the sites forming the hard helices, ranging from a single-site attraction to fully attractive helices for a specific helical shape. Different temperature regimes exist for different fractions of the attractive sites, as assessed in terms of the relative Boyle temperatures, that are found to be rather insensitive to the specific shape of the helical particle. The temperature range probed by the present study is well above the corresponding Boyle temperatures, with the phase behaviour still mainly entropically dominated and with the existence and location of the various liquid crystal phases only marginally affected. The pressure in the equation of state is found to decrease upon increasing the fraction of attractive beads and/or on lowering the temperature at fixed volume fraction, as expected on physical grounds. All screw phases are found to be stable within the considered range of temperatures with the smectic phase becoming more stable on lowering the temperature. By contrast, the location of the transition lines do not display a simple dependence on the fraction of attractive beads in the considered range of temperatures.

Light-powered self-propelled trolley with a liquid crystal elastomer pendulum motor

Conventional liquid crystal elastomer (LCE)-based robots require complex controllers and bulky power supplies, limiting their applications in areas such as microrobots and soft robots. In this paper, a light-powered self-propelled trolley with a LCE pendulum motor is proposed, and the dynamics of its self-propelled motion is investigated theoretically. Under steady illumination, the periodic radial contraction and relaxation of LCE can trigger the trolley to move forward through the self-rotation of the LCE pendulum. The coupling of the optically-induced deformation with its motion facilitates the energy input from ambient illumination to compensate for the damping dissipation so as to maintain the sustained motion. Through theoretical modeling and numerical calculations, three typical motion states of the LCE pendulum are identified, namely static state, self-oscillation state and self-rotation state. Among them, the LCE pendulum can propel the trolley when it is self-rotating. The influences of different physical parameters on the system motion are studied quantitatively. The results show that the period of the simple pendulum and the trolley speed can be controlled by adjusting the illumination range, optical intensity, damping coefficient of pendulum and damping coefficient of trolley. The light-powered self-propelled trolley constructed in current paper does not require complex controllers or self-contained power supplies, and possesses the characteristics of simple structure, light weight and energy efficient, which is of latent applied value in microrobots, soft robots and other fields.

Smart Anti‐Counterfeiting Inks: Controlled Luminescence Polarization and Its Fixation by Photoisomerization and Subsequent Photopolymerization of Cyanostilbene‐Based Liquid Crystal Monomers

AbstractTo identify and protect authentics from counterfeits, anti‐counterfeiting materials are developed. In spite of many advances in luminescent anti‐counterfeiting materials, there is still a need for easily processible, multilevel anti‐counterfeiting materials. Here, the study reports a novel strategy for the production of smart anti‐counterfeiting inks and films using a cyanostilbene‐based liquid crystal (LC) monomer (CSRM). This smart ink is prepared by mixing CSRM and conventional nematic LC monomers. The optimized ink with a nematic mesophase is uniaxially oriented by simple shear coating for high transmittance and linearly polarized light emission. Upon subsequent irradiation with 365 nm UV light using a photomask, a phase transition to an isotropic state occurs by photoisomerization of CSRM in selected areas, resulting in unpolarized photoluminescence in those areas. Two optically different regions (UV treated and untreated) can be permanently fixed by photopolymerization under 460 nm blue light. Multilevel anti‐counterfeiting labels programmed by simple shear coating, molecular self‐assembly, selective photoisomerization, and photopolymerization fixation can provide a new insight in developing smart anti‐counterfeiting inks and films.

Geometrical Optics Analysis of Diffraction in Patterned Cholesteric Liquid Crystals

A geometrical optics method is developed to analyze the diffraction in patterned cholesteric liquid crystals. We extend the equation for the center wavelength of selective reflection in a conventional cholesteric liquid crystal to the wavelength edges of the diffraction in patterned cholesteric liquid crystals. This method can be used to determine the reflection bandwidth and its wavelength edges of patterned cholesteric liquid crystals which depends on a slant angle and an incident angle. Furthermore, we also investigate a difference in the diffraction between planar and slanted patterned cholesteric liquid crystals using the geometrical optics method.

Accurate modelling of nematic liquid crystal based inverted microstrip phase shifter

ABSTRACT In this paper, an accurate numerical modelling of nematic liquid crystal for the inverted microstrip phase shifter is presented. Four kinds of liquid crystal with different values of anisotropy have been studied using the conventional simplified liquid crystal model where the liquid crystal is usually assumed isotropic and homogeneous. The S parameters of the phase shifter based on these liquid crystals are simulated. Moreover, considering the anisotropy and inhomogeneous of liquid crystal, the accurate modelling of nematic liquid crystal BL006 applied in the phase shifter under the voltage is calculated. The distribution of the director and permittivity of liquid crystal at the steady state under different voltages is obtained. Based on the accurate modelling results of liquid crystal BL006, the phase shifter is remodelled and simulated. Compared with that of the simplified model, the maximum differential phase shift of the phase shifter based on the liquid crystal BL006 in the accurate model at the centre frequency of 20 GHz was reduced by about , and this error will increase with the increasing of the frequency. This numerical modelling method of liquid crystal also can be used in the design of other microwave devices based on nematic liquid crystal.

Temperature and frequency dependence on dielectric permittivity and electrical conductivity of recycled Liquid Crystals

The dielectric behavior and the electrical conductivity response of recycled nematic Liquid Crystals (LCs) were studied over a wide range of frequencies (0.1 to 106 Hz) and temperatures (100 to −20 °C) with a broadband dielectric spectroscopy technique. These LC mixtures were extracted from a total of 65700 End-Of-Life (EOL)-Liquid Crystals Displays (LCD) such as computers, tablets and TVs of various sizes, brands, and production years. After a stage of purification, the recycled LC blends seemed to possess similar physical properties as conventional nematic LCs. In this report, two types of anchoring of LC molecules, obtained with homeotropic or homogeneous alignment cells, were investigated. All dielectric data were analyzed with adapted models: Havriliak-Negami and Almond West formalisms for the frequency dependence of the data, as well as Arrhenius and Vogel-Fulcher-Tammann models for their temperature dependence. In this article, it is demonstrated that despite the wide variety of EOL-LCD devices used leading to a large number of LC molecules, the frequency and temperature evolution of the recorded spectra are comparable to those of commercial LC mixtures as reported in the literature. Therefore, their future reuse could be envisaged for display applications.

N-Dodecyl-ethane-1,2-diamine as amphiphilic molecular probes in liquid crystal-based sensors for detecting aluminum ions

Conventional liquid crystal (LC)-based sensors for detecting metal ions usually apply a molecular probe for target recognition and an amphiphilic molecule for LC alignment, which makes LC-based sensors require additional pre-mixing procedures such that they are not suitable for in-situ monitoring. In this work, we applied an amphiphilic molecule, i.e., N-dodecyl-ethane-1,2-diamine (NN12C) as the molecular probes in LC-based sensors for detecting aluminum ions (Al3+). At the LC/aqueous interface, NN12C is protonated and aligns at the interface to cause the homeotropic orientation of LC. The binding of Al3+ and the amino groups of NN12C changed the amphiphilicity of NN12C and disrupted the orientation of LC, resulting into a dark-to-bright transition of LC images. Based on this mechanism, the lowest detectable concentration of Al3+ was 8 μM with the detection range from 8 to 25 μM. Moreover, we demonstrated that this sensor can detect aluminum ions in drinking water. This work provides a strategy to develop the LC-based sensors to detect metal ions with simplified operating procedures.

Low aberration and fast response liquid crystal lens with optically compensated bend structures

A hole-patterned electrode (HPE) liquid crystal (LC) lens with positive (0 D – 0.7 D) and negative focal lengths (0 D - −1.1 D) is demonstrated. The lens uses an optically compensated bend (OCB) cell filled with dual-frequency LC materials and alignment films with an intermediate pretilt angle. Better reduction in tilt and coma aberration can be achieved using this simple one-layer structure with its symmetric LC orientation in an OCB cell than can be obtained with a complicated two-layer HPE LC lens. The use of dual-frequency switching together with the reduction of the backflow effect in the OCB cell also improves the response by about 7 times compared with the conventional HPE LC lens.

Label-free optical and electrical immunoassays based on lyotropic chromonic liquid crystals: Implications of real-time detection and kinetic analysis

Conventional liquid crystal (LC)-based biosensors utilize predominantly thermotropic LCs as the signal-transducing media, which are less environmentally sustainable compared with lyotropic counterparts. In this study, the nematic phase of the anionic azo dye sunset yellow (SSY), a type of lyotropic chromonic liquid crystals (LCLCs), was employed in the optical and electrical biosensing of bovine serum albumin (BSA) and the cancer biomarker CA125. The optical response observed under a polarizing optical microscope was quantified by image analysis, taking advantage of the specific absorption of SSY. The electrical response derived from the dielectric spectra of SSY provided a new alternative for quantitative bioassay based on nematic LCLCs. The limit of detection (LOD) of the optical and electrical protein assay was ∼10−11- and ∼10−10-g/ml BSA, respectively, whereas that of the optical and electrical immunoassay was 5.97 × 10−11 and 6.02 × 10−12 g/ml for CA125, respectively. Moreover, real-time monitoring and kinetic analysis, which are hardly achievable for the hydrophobic thermotropic LCs, were demonstrated by dispersing CA125 in nematic SSY and subsequently recording the optical response over time during the specific binding between CA125 and the immobilized anti-CA125 antibody. Results from this study further the potential of nematic LCLCs in biosensing, especially in dielectric and real-time detection.

Low threshold electro-optic switch and TE/TM polarization selector using liquid crystal channel waveguide

We demonstrate a Liquid Crystal (LC) core channel waveguide-based low threshold electro-optic switch by employing in-plane switching configuration and show its application for polarization selection device. The waveguide is fabricated on an indium tin oxide (ITO) coated glass substrate using the laser direct writing lithography technique. We have experimentally shown that the Freedericksz transition voltage of LC molecules is ∼ 1 V, and it relatively requires a low threshold voltage of ∼ 2 V to switch on the waveguide. The measured extinction ratio of the device is greater than 18 dB, along with a low insertion loss of ∼ 3.27 dB as compared to the conventional LC waveguides. The fabricated device is further demonstrated as a voltage-controlled polarization selector that passes either TM or TE-polarized light with a polarization extinction ratio of 28 dB and 25 dB, respectively. The proposed device exhibits the features such as low voltage control, low optical and insertion loss, high extinction ratio, and an externally controlled polarization selector, which are the key requirements in integrated photonics.

An Evaluation of Surface-Active Agent Hexadecyltrimethylammonium Bromide with Vertical Self-Alignment Properties to Align Liquid Crystals for Various Cell Gap Conditions

We evaluated hexadecyltrimethylammonium bromide (HTAB) for liquid crystals (LCs) in layered ITO cells with various cell gap conditions. HTAB is a surfactant that can self-align vertically on the surface of indium tin oxide (ITO) substrates and induce homeotropic alignment of the LC molecules. For implementing RF devices with HTAB and LCs, we should consider limitations caused by the design conditions which are different from conventional liquid crystal displays such as cell gap. We quantified the concentration of HTAB ([HTAB]) that is necessary to form and maintain a sufficiently dense vertical alignment of 5CB (4-Cyano-4′-pentylbiphenyl). The required [HTAB] for full-homeotropic alignment was increased to the cell gap until it was too large to support the transfer of the surface alignment to the LC molecules, due to the weak anchoring nature of HTAB. We also showed the phase-change characteristic of the LC mixture related to [HTAB] for the design of RF devices driven by light or heat. This study may help to guide the development of new approaches to designing efficient RF devices that use LCs.

Bistable Cholesteric Liquid Crystal Films with Excellent Electro-Optical Performance and Spacing Stability for Reflective Displays

A bistable cholesteric liquid crystal display offers the Bragg selective reflection over the incidence of light to address various needs in energy conservation, long image storage time, and no backlight; yet, most of the existing technologies still suffer from unstable cell spacing in the curved state. In this work, a bistable cholesteric liquid crystal film with polymer spacer columns for flexible displays was prepared by stepwise UV light polymerization. By investigating the effects of cross-linking agent concentration and liquid crystal content, a formulation composition with optimal electro-optical performance was determined. Then, the influences of different process conditions for mask polymerization on the morphologies of polymer spacer columns and the electro-optical performance were systematically investigated. The results show that films with polymer spacer columns exhibit better electro-optical performance and spacing stability compared to conventional bistable liquid crystal films. Therefore, the bistable liquid crystal films reported in this paper show broad application prospects, especially in flexible displays.