Nematic Layer Research Articles

Optically Ambidextrous Reflection Circularly Polarized Luminescence Film with High Dissymmetry Factor and On‐Demand Handedness

AbstractThe self‐assembled cellulose nanocrystal (CNC) film has a left‐handed layered structure, which makes the reflection of right‐handed circularly polarized (CP) light a challenge. Herein, a nematic phase layer is designed and inserted into the chiral organization, to work as a half‐wave retarder and make the ambidextrous CP light reflection. The maximum reflectivity exceeds 80%, which breaks the 50% limitations of single‐direction light reflection and is the current maximum in all the reported CNC‐based films. This “sandwich‐like” structure displays different optical properties on both sides, displayed as the chromatism and the inversion of the circular dichroism signals. The dual CP light reflection and direction‐dependent optical phenomena are reserved in the synthesized circularly polarized luminescence (CPL) film, with the dissymmetry factor (glum) of −0.4245. However, this luminescent intensity and single‐direction emission are not enough in advanced optical systems. This work designs a triple CPL amplification path and develops the handedness inversion strategy, with the glum of −1.0551 and 0.4082. Then, the dual‐directional CPL emission films are designed, where the chiral optics can be switched on‐demand. Finally, the photonic crystal films are applied in the anti‐counterfeit and chiral superstructure induction.

Fine-Tuning the Microstructure and Photophysical Characteristics of Fluorescent Conjugated Copolymers Using Photoalignment and Liquid-Crystal Ordering.

Replicating the microstructural basis and the near 100% excitation energy transfer efficiency in naturally occurring light-harvesting complexes (LHCs) remains challenging in synthetic energy-harvesting devices. Biological photosynthesis regulates active ensembles of light-absorbing and funneling chlorophylls in proteins in response to fluctuating sunlight. Here, use of long-range liquid crystal (LC) ordering to tailor chain orientation and packing structure in liquid crystalline conjugated polymer (LCCP) layers for bio-mimicry of certain structural basis and light-harvesting properties of LHCs is reported. It is found that long-range orientational ordering in an LC phase of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) copolymer stabilizes a small fraction of randomly-oriented F8BT nanocrystals dispersed in an amorphous matrix of F8BT chains, resembling a self-doped host-guest system whereby excitation energy funneling and photoluminescence quantum efficiencies are enhanced significantly by triggering 3D donor-to-acceptor Förster resonance energy transfer (FRET) and dominant intrachain emission in the nano-crystal acceptor. Further, photoalignment of nematic F8BT layers is combined with LC orientational ordering to fabricate large-area-extended monodomains exhibiting >60% crystallinity and ≈20nm-long interchain packing order. Remarkably, these monodomains demonstrate strong linearly polarized emission, whilst also promoting a new band-edge absorption species and an extra emissive interchain excited state as compared to the non-aligned films.

Modeling of laser generation in a Fabry–Pérot-Tamm structure with a nematic liquid crystal layer

In the presented work, the possibility of controlling laser generation using a nematic liquid crystal (NLC) in a hybrid layered structure consisting of a thin metal layer (Ag), a layer of NLC doped with a light-absorbing dye, and a distributed Bragg reflector (DBR) with a rectangular refractive index profile is theoretically studied. Spectral dependencies of the reflection, transmission, and absorption coefficients of light as well as the localization coefficient of the light field in NLC within the photonic bandgap of the DBR are obtained. Narrow dips in the reflection coefficient and peaks in the transmission coefficient are achieved due to the excitation of plasmons at the Ag-NLC interface. The dependence of the spectral position and magnitude of the plasmonic dips/peaks and the enhancement of the light field in the NLC medium on the thickness and orientation of the NLC layer as well as the impact of a light-absorbing dye doping are investigated. Theoretical calculations of the temporal dependencies of luminescence pulses for pumping pulses of different power settings (below, above, and at the threshold of laser generation) and different values of light absorption in the dye-doped NLC medium are performed, taking into account the peculiarities of the optical properties of the dye-doped NLC.

First Order Alignment Transition in an Interfaced Active Nematic Fluid.

We investigate experimentally the dynamic phase transition of a two-dimensional active nematic layer interfaced with a passive liquid crystal. Under a temperature ramp that leads to the transition of the passive liquid into a highly anisotropic lamellar smectic-A phase, and in the presence of a magnetic field, the coupled active nematic reorganizes its flow and orientational patterns from the turbulent into a quasilaminar regime aligned perpendicularly to the field. Remarkably, while the phase transition of the passive fluid is known to be continuous, or second order, our observations reveal intermittent dynamics of the order parameter and the coexistence of aligned and turbulent regions in the active nematic, a signature of discontinuous, or first order, phase transitions, similar to what is known to occur in relation to flocking in dry active matter. Our results suggest that alignment transitions in active systems are intrinsically discontinuous, regardless of the symmetry and momentum-damping mechanisms.

Electro-optic characteristics of chiral nematic layers deformed by in-plane electric field

ABSTRACT Deformations of twisted and super-twisted layers composed of chiral and nonchiral nematic liquid crystals induced by in-plane electric field were studied numerically. The right-hand twisted structures with twist angles equal to 90°, 120°, 150°, 180°, 210°, 240° and 270° placed between crossed polarisers were considered. The optical transmission due to electro-optic effect which occurred in such systems was studied as a function of applied electric field strength. Steep and linear electro-optic characteristics were found in the case of nonchiral nematic confined in cells twisted by 90°. Such characteristics were favoured by large twist elastic constant, large dielectric anisotropy and weak azimuthal anchoring. In the case of super-twisted layers, useful characteristics were found for twist angles 210°, 240° and 270°, small twist elastic constant, strong azimuthal anchoring and large dielectric anisotropy.

Optical transmission modulation and spectral properties of self-organized nematic domains

The modulation and spectral characteristics of self-organized ensembles of nematic domains with the disclination lines non-aligned and aligned in a magnetic field have been studied using an electric field. The obtained experimental voltage dependences of the light transmission agree well with the dependences calculated using the model considerations. These dependences have been compared with the electro-optical characteristics of a homogeneous planar nematic layer. The amplitude component of the optical transmission determined by light scattering and the oscillating component caused by the phase modulation have been examined. The effect of the spectral dispersion of transmission on the electro-optical characteristics of domain ensembles has been evaluated.

Composite color filters for wide color gamut and high optical efficiency liquid crystal displays

We propose a composite color filter to achieve the wide color gamut and high optical efficiency of liquid crystal display (LCD). The composite color filter comprises a multi-layer polarization interference filter (ML-PIF) and a color filter. Specifically, two crossed polarizers and four twisted nematic liquid crystal layers with alternating opposite twisted directions form the ML-PIF. The Jones matrix method provides the transmittance formula of the ML-PIF. In the experiment, we successfully fabricate the blue and green ML-PIFs samples using the temperature birefringence effect of liquid crystal. When composite color filters (ML-PIF with certain color filters) are used in LCD, this can successfully achieve a color gamut of 122.3% NTSC and an absolute optical efficiency improvement of 1.4%. Alternatively, the composite color filter can attain a color gamut similar to that of a high-performance color filter and an absolute optical efficiency improvement of 3.8%. This composite color filter presents a tremendous opportunity for achieving a wide color gamut and high optical efficiency.

Anomalous Spectral Shift of o-Modes in Multilayer Photonic Structure Induced by Homeotropic–Homeoplanar Transition in Chiral–Nematic Defect Layer

Anomalous Spectral Shift of o-Modes in Multilayer Photonic Structure Induced by Homeotropic–Homeoplanar Transition in Chiral–Nematic Defect Layer

Exploring the Core Parameters of CNC-Based Chiral Nematic Structures for Enhancing the Dissymmetry Factor of Right-Handed Circularly Polarized Luminescence.

The chiral nematic nanostructure formed from cellulose nanocrystal (CNC) self-assembly has shown great potential as a matrix for generating circularly polarized luminescent (CPL) light with a high dissymmetry factor. Exploring the relationship between the device composition and structure and the light dissymmetry factor is crucial to a common strategy for a strongly dissymmetric CPL light. In this study, we have compared the single-layered and double-layered CNC-based CPL devices with different luminophores, such as rhodamine 6G (R6G), methylene blue (MB), crystal violet (CV), and silicon quantum dots (Si QDs). We demonstrated that forming a double-layered structure of CNCs nanocomposites is a simple but effective pathway for enhancing the CPL dissymmetry factor for CNC-based CPL materials containing different luminophores. The |glum| values of double-layered CNC devices (dye@CNC5||CNC5) versus that of single-layered devices (dye@CNC5) are 3.25 times for Si QDs, 3.7 times for R6G, 3.1 times for MB, and 2.78 times for CV series. The different enhancement degrees of these CNC layers with a similar thickness may be due to the different pitch numbers in the chiral nematic liquid crystal layers whose photonic band gap (PBG) has been modified to match the emission wavelengths of dyes. Furthermore, the assembled CNC nanostructure has great tolerance to the addition of nanoparticles. Gold nanorods coated with the SiO2 layer (Au NR@SiO2) were added for enhancing the dissymmetry factor of MB in CNC composites (named MAS devices). When the strong longitudinal plasmonic band of the Au NR@SiO2 matched the emission wavelength of MB and the PBG of assembled CNC structures simultaneously, the increase in the glum factor and quantum yield of MAS composites was obtained. The good compatibility of the assembled CNC nanostructures makes it a universal platform for developing strong CPL light sources with a high dissymmetry factor.

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.

Dual controlled cross-shaped broadband terahertz absorber based on vanadium oxide—liquid crystal metamaterial

A low terahertz broadband tunable absorber was proposed, which composed of vanadium oxide (VO2) film and nematic liquid crystal layer. The simulation results show that the absorption is great than 90% in the frequency range of 0.458–1.1492 THz. The tuning absorber was realized by increasing the temperature to modulate the VO2 transited into metal state. Since the dielectric constant of the nematic liquid crystal can be adjusted by bias voltage, without changing the intensity of absorption, however, a shift in the bandwidth was observed. In addition, the designed structure is insensitive to incident light polarization and still remained high absorption at 60°. The tunable broadband metamaterial absorber can be used for attenuator and energy harvesting.

On‐Demand Polarization Controllable Liquid Crystal Laser

AbstractA polarization controllable band‐edge liquid crystal (LC) laser configuration is presented that provides on‐demand control of the polarization state of the laser using a four electrode in‐plane configuration to drive a nematic LC layer. By controlling the orientation of the electric field, and thus the orientation of the optic axis of the nematic LC, as well as the retardance, any laser polarization state (e.g., circular, elliptical) can, in principle, be created that is aligned along any desired direction in a plane perpendicular to the propagation direction. After calibrating the tuneable nematic device with a He‐Ne laser that has a well‐defined polarization state, control of the LC laser polarization is then demonstrated, where it is shown that different polarization states and orientations thereof can be obtained by changing the relative combination and amplitudes of the voltages applied to each of the four electrodes. It is found, however, that achieving a pure linear polarization state is challenging, which may indicate that the polarization from the LC laser is not a well‐defined circularly polarized state. These thin‐film LC lasers with full polarization control are potentially important for a variety of different applications ranging from the biomedical to the display industries.

Orientational Transition in Nematic Layer with Homeotropic-Conical Boundary Conditions Induced by Ionic Modification of Surface Anchoring

Orientational Transition in Nematic Layer with Homeotropic-Conical Boundary Conditions Induced by Ionic Modification of Surface Anchoring

Light-induced Fredericks transition in the nematic liquid crystal cell with plasmonic nanoparticles at a cell bounding substrate.

The paper presents a theoretical description of the light-induced nematic liquid crystal reorientation in a cell with gold nanoparticles deposited on the surface of one of bounding substrates. It is shown that the surface plasmon resonance in the nanoparticles significantly affects the threshold of the director reorientation. The mathematical model of a surface free-energy density of nematic cell is given, which takes into account the influence of the local electric field on the near-surface nematic layer at the substrate with gold nanoparticles. The threshold intensity of a director orientation instability is calculated and its dependence on the wavelength of incident light and the degree of filling of the surface with gold nanoparticles is analyzed. Comparison of the theoretical calculations with experimental data confirms the full adequacy of the proposed theoretical model.

Electrically switchable structural colors based on liquid-crystal-overlaid aluminum anisotropic nanoaperture arrays.

Actively tunable or reconfigurable structural colors are highly promising in future development for high resolution imaging and displaying applications. To this end, we demonstrate switchable structural colors covering the entire visible range by integrating aluminum nanoaperture arrays with nematic liquid crystals. The geometrically anisotropic design of the nanoapertures provides strong polarization-dependent coloration. By overlaying a nematic liquid crystal layer, we further demonstrate switchable ability of the structural colors by either changing the polarization of the incident light or applying an external voltage. The switchable structural colors have a fast response time of 28 ms at a driving voltage of 6.5 V. Furthermore, colorful patterns are demonstrated by coding the colors with various dimensions of nanoaperture arrays with dual switching modes. Our proposed technique in this work provides a dual-mode switchable structural colors, which is highly promising for polarimetric displays, imaging sensors, and visual cryptography.

Biaxial planar nematic layered structures of highly confined prolate hard ellipsoids

This study investigated the nematic phase structures of hard uniaxial ellipsoidal rods having a short axis of 2b, a long axis of 2a, and the elongations k=2a2b=3 and 5 in strongly confined pores via the density functional theory with Zwanzig and hyper-netted-chain approximations. By increasing the density, the number of planar nematic layers gradually rise without any phase transition. These layering phenomena occurred between two biaxial structures with different numbers of planar layers in which the layers’ spacing was approximately 2b.

Sedimentation of colloidal plate-sphere mixtures and inference of particle characteristics from stacking sequences

We investigate theoretically the effect of gravity on a plate-sphere colloidal mixture by means of an Onsager-like density functional to describe the bulk, and sedimentation path theory to incorporate gravity. We calculate the stacking diagram of the mixture for two sets of buoyant masses and different values of the sample height. Several stacking sequences appear due to the intricate interplay between gravity, the sample height, and bulk phase separation. These include the experimentally observed floating nematic sequence, which consists of a nematic layer sandwiched between two isotropic layers. The values of the thicknesses of the layers in a complex stacking sequence can be used to obtain microscopic information of the mixture. Using the thicknesses of the layers in the floating nematic sequence we are able to infer the values of the buoyant masses from the colloidal concentrations and vice versa. We also predict new phenomena that can be experimentally tested, such as a nontrivial evolution of the stacking sequence by increasing the sample height in which new layers appear either at the top or at the bottom of the sample.

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.

Active Extensile Stress Promotes 3D Director Orientations and Flows.

We use numerical simulations and linear stability analysis to study an active nematic layer where the director is allowed to point out of the plane. Our results highlight the difference between extensile and contractile systems. Contractile stress suppresses the flows perpendicular to the layer and favors in-plane orientations of the director. By contrast extensile stress promotes instabilities that can turn the director out of the plane, leaving behind a population of distinct, in-plane regions that continually elongate and divide. This supports extensile forces as a mechanism for the initial stages of layer formation in living systems, and we show that a planar drop with extensile (contractile) activity grows into three dimensions (remains in two dimensions). The results also explain the propensity of disclination lines in three dimensional active nematics to be of twist type in extensile or wedge type in contractile materials.

Controllable shifting, steering, and expanding of light beam based on multi-layer liquid-crystal cells

Shaping and steering of light beams is essential in many modern applications, ranging from optical tweezers, camera lenses, vision correction to 3D displays. However, current realisations require increasingly greater tunability and aim for lesser specificity for use in diverse applications. Here, we demonstrate tunable light beam control based on multi-layer liquid-crystal cells and external electric field, capable of extended beam shifting, steering, and expanding, using a combination of theory and full numerical modelling, both for liquid crystal orientations and the transmitted light. Specifically, by exploiting three different function-specific and tunable birefringent nematic layers, we show an effective liquid-crystal beam control device, capable of precise control of outgoing light propagation, with possible application in projectors or automotive headlamps.