Birefringence Of Liquid Crystal Research Articles

Tunable dual-band liquid crystal based near-infrared perfect metamaterial absorber with high-loss metal

ABSTRACTIn this work, a tunable dual-band near-infrared perfect metamaterial absorber formed by combining a highly birefringent nematic liquid crystal with a nanoscale metamaterial cavity arranged in mirror symmetry is designed and numerically investigated. Electromagnetic simulations indicate that the absorbance greater than 99.4% may be achieved at 328 THz and 364 THz. Perfect absorbance results both form the use of highly lossy metal and the optimization of the metamaterial structure. In addition, absorbance of the metamaterial device can be substantially tuned both in terms of its magnitude and wavelength with the spectral tunability up to 8 THz by switching of liquid crystal alignment. The soft-matter-based metamaterial absorbers may pave a crucial role towards various active multifunctional systems working in the near-infrared range.

Multifrequency Driven Nematics

Liquid crystals act on the amplitude and the phase of a wave front under applied electric fields. Ordinary LCs are known as field induced birefringence, thus both phase and amplitude modulation strongly depend on the voltage controllable molecular tilt. In this work we present electrooptical properties of novel liquid crystal (LC) mixture with frequency tunable capabilities from 100Hz to 10 KHz at constant applied voltage. The frequency tunability of presented mixtures shown here came from composition of three different families of rodlike liquid crystals. Dielectric measurements are reported for the compounds constituting frequency-controlled birefringence liquid crystal. Characterization protocols allowing the optimum classification of different components of this mixture, paying attention to all relevant parameters such as anisotropic polarizability, dielectric anisotropy, and dipole moment are presented.

Liquid crystals for space instrumentation: optical properties of liquid crystal mixtures for polarimeters

Liquid crystal (LC) based polarization modulators will be used for the first time in space instrumentation in the ESA/NASA Solar Orbiter mission. LC birefringence dependence on temperature and wavelength is key for the design of these instruments. A complete characterization of the optical properties of five different LC mixtures (ZLI-3700-000, BL006, MLC-6025-000, MLC-6610, and MLC-6053-000) in a temperature range between 20 and 75 °C and a range of wavelengths between 480 and 650 nm is presented in this paper. In addition, the effects of the variety of harsh environmental conditions during the space mission lifetime on the LC properties is reported. Based on the results shown in this work, the most promising LC material was selected to be on board the Solar Orbiter mission.

Optically induced birefringence in dye-doped blue phase liquid crystals

ABSTRACTThis paper investigates the photoinduced change of refractive index in dye (methyl red, MR)-doped blue phase (DDBP) cells by illumination of a pump beam. Through excitation of light irradiation with proper photon energy, MR can transform from trans-state to cis-state and successively diffuse and anisotropically adsorb on the inner glass substrate of the DDBP cell along the direction perpendicular to the polarisation of the pump beam. The adsorbed MR molecules can effectively rotate the orientation of the liquid crystal (LC) molecules and thereby modulate the effective refractive index of the DDBP cell. The SEM images of the adsorbed regions of the illuminated DDBP samples were also taken for discussing the relation between the pump intensity and the photoinduced birefringence.

Study of Fluorescent Dichroic Liquid Crystal Dyes for Wearable and Flexible Device

There is growing attention and rapid development on flexible electronic devices with electronic materials and sensing technology innovations. In particular, strain sensors with high elasticity and stretchability are needed for several potential applications including human entertainment technology, human–machine interface, personal healthcare, and sports performance monitoring, etc. Significant advancements in wearable and flexible electronic and photonic technologies, are materials science of new and alternative materials and methods of coating and deposition, characterization of flexible and transparent or display electronic devices, the electronics behind new sensor development for wearables and flexible technology, and new device design concepts.The anisotropic properties of a system comprising a guest dye dissolved in an aligned liquid-crystalline host were reported, arising from the combination of the bulk liquid crystal host alignment and the anisotropic shape and absorption characteristics of the dye molecules. For many applications, it is also desirable for a guest–host mixture to exhibit a long useable lifetime in addition to the optical properties described above. Many of the proposed applications involve exposure of the systems to wearable and flexible device, polarisers, smart windows and lasers. The stability of a dye molecule may be strongly dependent on its chemical environment, so the stabilityof a guest–host system may depend on the nature of the host mixture as well as that of the dye. A series of new fluorescent dichroic dyes which included phenylacetylene substituent group to 4-site of the 1,8-dicyanonaphthene were synthesized. All dyes were characterized by nuclear magnetism and high resolution mass spectrum. The dicyanonaphthene fluorescent dichroic dyes meet the application conditions of ‘Guest-host’ display. Phenylacetylene bridge bond is often used for liquid crystal molecular design and synthesis to improve the liquid crystal birefringence because of its excellence stability, optical anisotropy.On the one hand, the addition of the phenylacetylene bond increases the molecular linearity and the aspect ratio, which is more favorable for the arrangement of the dye molecules in the main liquid crystal, thereby increasing the ordered parameters of the dye molecules in the liquid crystal; On the other hand, phenylacetylene bonds can increase the molecular conjugation system and optimize the optical properties of dye molecules. Ester groups as a class of important linking groups have been widely used in the synthesis of diamodal dyes4.The 1,8-naphthalene dinitrile is regarded as the precursor, introduced p-ethynylbenzoic acid at its 4-position and then through the acylation reaction to introduce the alkoxy chain, in order to obtain a greater degree of conjugation, in the main liquid crystal has good solubility and high dichroic dye. Acknowledgements We are grateful to the National Natural Science Foundation of China (21576043), supported by the Fundamental Research Funds for the Central Universities.

Design, Manufacture, and Analysis of Photonic Materials for Historical and Modern Visual Art: feature issue introduction

History has seen many significant contributions by artists in science and vice versa, e.g. Leonardo Da Vinci’s flying machines, and Isaac Newton’s color theory. More recently, with the advent of nanofabrication techniques, lasers, and materials, photonics research has found exciting new applications in art. The 14 papers positioned at the boundary between art and science in this feature issue capture this breadth of topics. Creative art-scientists/science-artists create new art using the colorful properties of metallic nanostructures, birefringence of liquid crystals, interference in thin films, and coherence of lasers. A noteworthy new art form that is gaining interest uses metals as the “canvas”, lasers and nanolithography as the “brush”, and resonant nanostructures as the “paint”. We hope the reader will enjoy the spectrum of topics here that illustrate the potential of cutting edge photonic research for art-related applications: from the generation of modern visual art to the preservation and tagging of precious historical art.

Geometric-Phase Microscopy for Quantitative Phase Imaging of Isotropic, Birefringent and Space-Variant Polarization Samples

We present geometric-phase microscopy allowing a multipurpose quantitative phase imaging in which the ground-truth phase is restored by quantifying the phase retardance. The method uses broadband spatially incoherent light that is polarization sensitively controlled through the geometric (Pancharatnam-Berry) phase. The assessed retardance possibly originates either in dynamic or geometric phase and measurements are customized for quantitative mapping of isotropic and birefringent samples or multi-functional geometric-phase elements. The phase restoration is based on the self-interference of polarization distinguished waves carrying sample information and providing pure reference phase, while passing through an inherently stable common-path setup. The experimental configuration allows an instantaneous (single-shot) phase restoration with guaranteed subnanometer precision and excellent ground-truth accuracy (well below 5 nm). The optical performance is demonstrated in advanced yet routinely feasible noninvasive biophotonic imaging executed in the automated manner and predestined for supervised machine learning. The experiments demonstrate measurement of cell dry mass density, cell classification based on the morphological parameters and visualization of dynamic dry mass changes. The multipurpose use of the method was demonstrated by restoring variations in the dynamic phase originating from the electrically induced birefringence of liquid crystals and by mapping the geometric phase of a space-variant polarization directed lens.

Optical retardation and Birefringence measurement in phasmidic columnar liquid crystals (PCLC)

In this work, we measure, for the first time, the birefringence of a phasmidic columnar liquid crystal (PCLC) in a hexagonal phase using two different types of textures.In the first texture, we use some types of developpable domain where the columns are organized in concentric hemispheres. We present the measurement of the birefringence Δn by observing interference fringes.We use in the second texture, a monocrystalline PCLC in homeotropic orientation between two glass plates with a thickness d. we illuminate the structure by a monoochromatique convergent light, we observe the figure of conoscopy which shows a circle of interference between ordinary and extraordinary rays.The two textures give the same value of the birefringence Δn =-0.06 for λ = 532nm.

Superior electro-optics of nano-phase encapsulated liquid crystals utilizing functionalized carbon nanotubes

Nano-size distribution of birefringent liquid crystal droplets embedded in the polymer matrix gives rise to an optically isotropic liquid crystal (OILC) phase and the device utilizing OILC composite opens the possibility of alignment layer-free, flexible liquid crystal displays with fast response time. However, few critical issues such as high operating voltages and feeble light scattering are remained to be overcome before their widespread applications. To rectify the drawbacks, a small amount of conductive and anisotropic functionalized carbon nanotubes (f-CNTs) is doped into conventional OILCs. Consequently, switchable electro-optic properties of the proposed approach reveal that driving voltage is decreased by18.7%, response time becomes fast by 27%, and feeble light leakage is reduced to around 3.8 times, compared to conventional one, which helps the proposed approach to be competitive for display and photonic applications of film type of OILCs.

Midinfrared Birefringence of Liquid Crystals, Polarimetry, and Intensity Modulators.

The Stokes parameters of transmitted light and the birefringence were measured for several liquid crystal mixtures in the midinfrared (MIR) spectral range (≈1900-5500 cm-1). A MIR intensity modulator has been fabricated and characterized. Germanium substrates show both sufficient conductivity and transparency in the MIR range to serve as transparent electrodes. The modulator makes use of the well-known principle of a twisted nematic cell, which rotates the plane of polarization of a transmitted electromagnetic wave in the field-off state and does not change the state of polarization at sufficiently high voltage. For this application, alignment layers made of poly(vinyl alcohol) and wire grid polarizers on a substrate made of 1,1,2,2-tetrafluorethen (Teflon) are used. The modulator shows high contrast ratios.

Research progress of terahertz liquid crystal materials and devices

Liquid crystal (LC) is an excellent tunable functional material which can be controlled by the external stimulus such as electric field, magnetic field and temperature. Terahertz (THz) radiation in a frequency range of 0.1−10.0 THz, has enormous advantages such as a low photon energy, sensitivity to crystal lattice vibration, magnetic spins, hydrogen bonds, intermolecular interaction, and water, and high transparency to non-conducting materials. The THz technology, therefore, has great potential in a diverse range of applications from spectroscopy, security screening to biomedical technology and high-speed wireless communication. But the development of high-performance LC based tunable THz functional devices is still in its infancy stage. The dispersion of LC refractive index induces a comparatively low birefringence in the THz regime. The lack of transparent electrodes makes the electric tuning of LCs difficult to achieve. To achieve certain modulations requires a very thick THz layer, leading to several disadvantages such as high operating voltage, slow response and poor pre-alignment. In this paper, we first present the research progress of large birefringence LCs in THz range. A room-temperature nematic LC NJU-LDn-4 with an average birefringence greater than 0.3 in a frequency range from 0.5 to 2.5 THz is shown in detail. This kind of LC can remarkably reduce the required cell gap, thus reducing the operating voltage and response time. Then we summarize varieties of conventional THz devices based on LC. Many electrodes are used for THz range. Graphene which can be used as a perfect transparent electrode material in THz band is proposed. Not only tunable transmissive but also reflective THz waveplates are introduced. The thickness of the LC layer of the reflective one can be reduced to ~10% of that needed for the same phase shift at a given frequency in a transmissive waveplate. The same tunability as that in the transmissive type just needs half the thickness. We also introduce that LC can generate THz vortex beam based on a photopatterned large birefringence LC. In the area of LC based versatile THz metamaterial devices, the adjacent units of a metasurface layer, such as a fishnet or grating, are usually connected to each other which may cause low-quality (<i>Q</i>) factor and polarization sensitivity, which is undesirable. We emphasize a graphene-assisted high-efficiency tunable THz metamaterial absorber. Few-layer porous graphene is integrated onto the surface of a metasurface layer to provide a uniform static electric field to efficiently control the LC, thereby enabling flexible metamaterial designs. The THz far-field and near-field with large modulation and fast response are realized. A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and the large-birefringence liquid crystals is also demonstrated to be able to generate broadband THz radiation and control the polarization of THz waves perfectly as well as LC based THz reflectarray. Last but not least, a temperature-supersensitive cholesteric LC used for THz detection is shown. It can not only measure the beam profiles but also detect the power values of THz waves generated from a nonlinear crystal pumped by a table-top laser. Quantitative visualization based on not only the thermochromic but also the thermal diffusion effect, can be used conveniently and effectively at room temperature. In this review, we summarize the latest progress of liquid crystal materials and components in THz and discuss the possible prospects of the combination of liquid crystal technology and THz technology. We envision that LCs will play a unique role in THz sources, THz functional devices and THz detectors.

Reconfigurable Chiral Metasurface Absorbers Based on Liquid Crystals

We propose a liquid-crystal-based reconfigurable chiral metasurface absorber and numerically investigate its chiro-optical properties. The chiral metasurface absorber consists of a metal–insulator–metal structure with the substrate, which can strongly absorb a circularly polarized wave of one spin state and reflects that of the opposite spin, resulting a strong circular dichroism (CD). A birefringent liquid crystal (LC) is exploited to serve as the insulator layer in the metal–insulator–metal structure. We could then vary the circular state of the incident light by controlling the alignment of the LC molecules, hence inversing the CD. The simulation results show that the CD will change the sign as the LC molecules are realigned from 0° to 90°. The absorption efficiency for the specific circularly polarized wave is larger than 80% and the CD is nearly 70%. The simple and compact design of our proposed chiral metasurface absorber is especially favorable for integration, and such a reconfigurable chiral absorber could find many potential applications in biological detection/sensing, polarimetric imaging, and optical communications.

High-sensitivity Fabry-Perot interferometer temperature sensor probe based on liquid crystal and the Vernier effect.

A novel high-sensitivity Fabry-Perot (FP) interferometer temperature sensor based on liquid crystals (LCs) and the Vernier effect is proposed and demonstrated in this Letter. This sensor is prepared by inserting two cleaved single-mode fibers into a section of a capillary tube to generate an FP cavity. The cavity is filled with LCs, which induces a Vernier effect due to the birefringence of LCs. The refractive indices of the ordinary and extraordinary light have different responses to the temperature changes. As a result, the temperature sensitivity is significantly improved by detecting the peak shifts of a periodic envelope. The experimental results show that the proposed sensor can provide a high-temperature sensitivity of 19.55nm/°C. This sensor offers key features and advantages of the Fabry-Perot interferometer, including low cost and good fringe visibility. Furthermore, such a sensor probe can meet different requirements of temperature sensing in various application areas by using different kinds of LCs.

Highly Oriented Liquid Crystal Semiconductor for Organic Field-Effect Transistors.

We report a mesogenic compound which introduces nematic liquid crystal (LC) ordering into the benzothienobenzothiophene (BTBT) family of LCs, creating a new class of LC semiconducting materials which respond in a facile way to anisotropic surfaces, and can, thereby, be effectively processed into highly oriented monodomains. Measurement on these domains of the electrical conductivity, with in situ monitoring of domain quality and orientation using LC birefringence textures in electroded cells, brings a new era of precision and reliability to the determination of anisotropic carrier mobility in LC semiconductors.

Liquid Crystal Enabled Dynamic Nanodevices.

Inspired by the anisotropic molecular shape and tunable alignment of liquid crystals (LCs), investigations on hybrid nanodevices which combine LCs with plasmonic metasurfaces have received great attention recently. Since LCs possess unique electro-optical properties, developing novel dynamic optical components by incorporating nematic LCs with nanostructures offers a variety of practical applications. Owing to the large birefringence of LCs, the optical properties of metamaterials can be electrically or optically modulated over a wide range. In this review article, we show different elegant designs of metasurface based nanodevices integrated into LCs and explore the tuning factors of transmittance/extinction/scattering spectra. Moreover, we review and classify substantial tunable devices enabled by LC-plasmonic interactions. These dynamically tunable optoelectronic nanodevices and components are of extreme importance, since they can enable a significant range of applications, including ultra-fast switching, modulating, sensing, imaging, and waveguiding. By integrating LCs with two dimensional metasurfaces, one can manipulate electromagnetic waves at the nanoscale with dramatically reduced sizes. Owing to their special electro-optical properties, recent efforts have demonstrated that more accurate manipulation of LC-displays can be engineered by precisely controlling the alignment of LCs inside small channels. In particular, device performance can be significantly improved by optimizing geometries and the surrounding environmental parameters.

Columnar Liquid Crystals Self-Assembled by Minimalistic Peptides for Chiral Sensing and Synthesis of Ordered Mesoporous Silica

We report the spontaneous formation of lyotropic liquid crystals from the self-assembly of a series of minimalistic peptides. The highly charged peptides could self-assemble into rigid helical nanofilaments in water and spontaneously order into hexagonal liquid-crystalline phases with interfilament separations of ≤240 A. The formation of liquid crystals from the self-assembly of such simple peptides is dictated not only by the concentration and pH but also by the amino acid sequence of the peptides. Peptides containing the rigid Phe-Phe (FF) segment showed much higher propensities to form a liquid-crystalline phase with long-range order. Moreover, because of the intrinsic chirality of the peptides, the self-assembled birefringent liquid crystals could serve as sensing elements for the visual discrimination of chiral species and as templates for the biomimetic synthesis of mesoporous silica with ordered cavities. The results offer new opportunities for the design of peptide liquid crystals that are potenti...

Investigation of several essential display features for the low birefringent nematic liquid crystal dispersed with polymer

In this investigation, we have reported some important electro-optical and dielectric parameters for pure nematic liquid crystal (NLC) and polymer-dispersed system. The used liquid crystal in the present investigation is NLC 1550C, consisting of 4′-(trans, trans-4-alkylbicyclohexyl) carbonates and 4′-(4-(trans, trans-4-alkyl)-4-cyanobicyclohexane which is low birefringent material. The dopant used here is polyaniline (PANI). Birefringence has been decreased after the dispersion of polyaniline. Fast optical response time has also been observed for the PANI dispersed system which is the key finding of this investigation, because fast optical response is necessary for any display devices. Splay elastic constant and rotational viscosity have also been calculated and we have found that both parameters are decreasing after the dispersion of polyaniline. We have also calculated threshold voltage and found to be decreased. Relative permittivity has been enhanced for PANI dispersed system. Dielectric anisotropy has also been measured and found to be increased after the dispersion of PANI into the pure NLC. Figure-of-Merit (FoM) is also a very important parameter for display; therefore, we have also calculated FoM which has been decreased for the dispersed system. The output of the present work may be useful in making or enhancing the features of liquid crystal displays and in other energy-efficient devices.

Graphene-based spatial light modulator operating at near infrared spectral range

In this paper, a near infrared operating spatial light modulator is demonstrated, assembled in a way to combine the excellent photoconductivity of Ru-doped Bi12SiO20 (BSO:Ru) crystal, strong birefringence of liquid crystal (LC) and exceptionally high transparency and conductivity of graphene. A photo-alignment method instead of mechanical rubbing is used to simplify the fabrication procedure and to prevent detachment of the graphene layer from the substrate. The proposed device operates at low driving voltage (competitive to a reference device using ITO electrodes) moreover requires much less near infrared intensity for the light modulation. It is assumed the effect is due to the photo-induced space charge exchange between BSO:Ru and graphene allowing charge redistribution and optical doping effect resulting in a modulation of graphene’s properties. The voltage-dependent transmittance and phase retardation show high contrast ratio with the response time of ∼100 ms at 1064 nm. In addition, by projecting a video image through the proposed structure the response of modulated pump light intensity is demonstrated which supports device ability to work as near infrared optically addressed spatial light modulator (OASLM). The obtained performances reveal great potentials of graphene-based electro-optic devices for near infrared applications.

A novel approach for analyzing refractive Indices and birefringence of nematic liquid crystals

In this paper, we have proposed new technique for determining the ordinary and extra ordinary refractive indices and birefringence of liquid crystals EPAP Valerate, HOBA and NOBA through image analysis method in conjunction with polarizing optical microscopy as a function of temperature by keeping the liquid crystal sample between the polarizers attached with a hot stage and a high-resolution camera. Liquid Crystals employed with image intensities is being used to find out the optical transmittance which is necessary for evaluating the refractive indices by validating the Swanepoel's method. Results obtained in this image analysis method are compared with experimentally determined values of EPAP Valerate, HOBA, NOBA performed with wedge technique and are found in good agreement with an accuracy of ±0.0001. Image analysis method is simple and suitable to determine the refractive indices of liquid crystals.

Liquid-crystal-based tunable plasmonic waveguide filters

We propose a liquid-crystal-based tunable plasmonic waveguide filter and numerically investigate its filtering properties. The filter consists of a metal-insulator-metal waveguide with a nanocavity resonator. By filling the nanocavity with birefringent liquid crystals (LCs), we could then vary the effective refractive index of the nanocavity by controlling the alignment of the LC molecules, hence making the filter tunable. The tunable filtering properties are further analyzed in details via the temporal coupled mode theory (CMT) and the finite-difference time-domain (FDTD) method. The simulation results show that the resonant wavelengths have linear redshift as the refractive index of the nanocavity increases and the coupling efficiency is more than 65% without considering the internal loss in the nanocavity and waveguides. These achieved results by the FDTD simulations can be also accurately analyzed by CMT. The compact design of our proposed plasmonic filters is especially favorable for integration, and such filters could find many important potential applications in high-density plasmonic integration circuits.