Thin-film Liquid Crystal Research Articles

Optical Acetone Vapor Sensors Based on Chiral Nematic Liquid Crystals and Reactive Chiral Dopants

Accurate monitoring of exposure to organic vapors, such as acetone, is an important part of maintaining a safe working environment and adhering to long‐ and short‐term exposure limits. Here, a novel acetone vapor detection system is described based on the use of a reactive chiral dopant in a nematic liquid crystal thin film, which can accurately monitor the total exposure over a given time frame through changes in the reflection band of the film. It is found that films exposed to 1000 parts per million by volume (ppmv) of acetone vapor for 2 h exhibit a shift in reflection band of 12 nm, with smaller shifts observed at lower concentrations of acetone. Such sensors have potential applications in industrial hygiene.

Surfing the Millimeter-Wave: Multilayer Photoimageable Technology for High Performance SoP Components in Systems at Millimeter-Wave and Beyond

Ever-growing demand and emerging applications in the millimeter-wave (mmW) range for both commercial and military purposes require high-performance yet compact circuits and systems at a reasonable cost [1], [2]. Multilayer multichip module (MCM) or system-on-package (SoP) technologies are widely regarded as an excellent means to meet these advanced requirements [3]-[5]. A variety of multilayer MCM technologies-including low-temperature cofired ceramic (LTCC), liquid crystal polymer (LCP), organic, and thin film on glass-have been developed [7]-[14]. However, when conventional thick film is used, it is very difficult in applications requiring high mmW bands to realize either 1) lumped/passive components having a high self-resonance frequency (SRF), quality factor (Q), and also compactness or 2) substrate integrated waveguide (SIW) components with continuous sidewalls [10], [13].

Electric field generation of Skyrmion-like structures in a nematic liquid crystal.

Skyrmions are particle-like topological objects that are increasingly drawing attention in condensed matter physics, where they are connected to inversion symmetry breaking and chirality. Here we report the generation of stable Skyrmion-like structures in a thin nematic liquid crystal film on chemically patterned patchy surfaces. Using the interplay of material elasticity and surface boundary conditions, we use a strong electric field to quench the nematic liquid crystal from a fully aligned phase to vortex-like nematic liquid crystal structures, centered on patterned patches, which carry two different sorts of topological defects. Numerical calculations reveal that these are Skyrmion-like structures, seeded from the surface boojum topological defects and swirling towards the second confining surface. These observations, supported by numerical methods, demonstrate the possibility to generate, manipulate and study Skyrmion-like objects in nematic liquid crystals on patterned surfaces.

Self-organized arrays of dislocations in thin smectic liquid crystal films.

Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.

High-Gain Linearly Tapered Antipodal Slot Antenna on LCP Substrate at E- and W-Bands

In this letter, we present a high-gain linearly tapered antipodal slot antenna (LTASA) on a thin-film liquid crystal polymer (LCP) substrate for E- and W-band applications. The proposed LTASA consists of a microstrip (MS) line feed, an MS-to-antipodal parallel strip line (APSL) balun, a linearly flared antipodal slot line (ASL), and periodic corrugations. An ultrawideband of from 47 to 110 GHz was demonstrated by full-wave simulation and measurement. To characterize the far fields of the LTASA, we developed an optical far-field measurement system in which the RF signal captured by the LTASA will be upconverted to the optical spectrum for power detection. By utilizing the optical approach, the developed system addresses flexibility, light weight, low profile, and low interconnection loss. A high gain of $16.5 + 1 ~\hbox{dBi}$ , narrow beamwidth, low sidelobes, and low cross polarization were demonstrated for the proposed LTASA from 75 to 95 GHz.

Chiral Liquid Crystal-Based Sensor for the Detection of Low Humidity

The humidity sensing capability of a film of (S)-5-octyloxy-2-[{4-(2-methylbuthoxy)-phenylimino}-methyl]-phenol liquid crystal (LC) was studied. The experimental results show that LC thin film offers a promising perspective as a sensing material for the detection of relatively low humidity levels at even room temperature. It was observed that the operating temperature has a considerable effect on humidity sensing performance of the sensor. A comparison of kinetic models applied to the adsorption of water molecules on the LC film was evaluated for the Ritchie and Elovich kinetic models. Linear regression analysis results show that Elovich equation provides the best correlation for the water molecule adsorption processes.

Ka-band phased patch array antenna integrated with a PET-controlled phase shifter

In this article, a 4 × 4 linear-phased patch array antenna, consisting of four 1 × 4 patch subarrays and a true time-delay multiline phase shifter, is proposed on a thin film liquid crystal polymer substrate at Ka-band. The patch antenna is designed with a gain of 6 dBi at 35 GHz and a bandwidth of 23% centered at 35 GHz. To enhance the gain and symmetrize the beam patterns of the 4 × 4 array, a 1 × 4 patch subarray in the E-plane was designed and characterized. The subarray produces an enhanced gain of 11 dBi and a wide beamwidth of ±38° in the H-plane for beam steering. The proposed phase shifter comprises a 1 × 4 microstrip line power splitter and a piezoelectric transducer-controlled phase perturber. A large phase variation of up to 370° and a low insertion loss of less than 2 dB were demonstrated for the phase shifter at Ka-band. The integrated phased array attains a gain of 15.6 dBi, and a continuous true-time delay beam steering of up to 33 ± 1° from 31 to 39 GHz. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2015.

Voltage-controllable liquid crystal waveguide

This study presents an optical waveguide formed by a self-focusing pump beam in a nematic liquid crystal thin film. The pump beam is a polarized high power Gaussian laser beam. Applying a voltage near the Freedericksz transition threshold enhances the optical field of pump beam to reorientate liquid crystal molecules. The reorientation of LC molecules forms the refractive index as a Gaussian-like distribution in the pump beam region. By the assistance of applying electric field, the pump beam can form an optic waveguide by self-focusing effect. A polarized lower power probe beam near the waveguide can be coupled to the waveguide. The coupling efficiency can be controlled by applying voltage.

Polymer stabilization of electrohydrodynamic instability in non-iridescent cholesteric thin films.

A non-iridescent cholesterol liquid crystal (CLC) thin film is demonstrated by using the polymer-stabilized electrohydrodymanic (PSEHD) method. The photopolymerized cell made from a CLC/monomer mixture exhibits an optically stable gridlike pattern. The helical axis of thus-formed CLC is aligned with the hydrodynamic flow induced by a space charge motion, and the arrayed CLC grid configuration renders a wide viewing angle thanks to the limited color shift at various lines of sight. The formation of the PSEHD structure was verified with polarized optical microscopy, ascertaining that the electrohydrodymanic pattern can be photo-cured or stabilized. The PSEHD CLC is simple to fabricate and potentially suitable for applications in wide-viewing-angle or non-iridescent devices.

Negative and Positive Anisotropic Thermal Expansions in a Hexagonally Packed Columnar Discotic Liquid Crystal Thin Film

The negative thermal expansion showing contraction of unit cell dimensions on heating is an unusual phenomenon which is rarely observed in organic materials. Here we report that a highly aligned hexagonal columnar liquid crystalline phase of discotic liquid crystal, cobalt octakis(n-decylthio)porphyrazine (CoS10), in thin films shows negative thermal expansion in the transverse plane and positive thermal expansion along the column axis. This exceptional thermal expansion behavior of CoS10 at the hexagonal columnar liquid crystalline phase was understood as the synergetic effect of the increased vibrational motion of n-alkyl side chains perpendicular to the chain axes and the increased tilting of the cores with temperature.

Homeotropic orientation behavior of nematic liquid crystals induced by copper ions

A homeotropic ordering film of nematic liquid crystal (LC) induced by copper ions (Cu2+) had been developed. The Cu(ClO4)2 was directly spin-coated on the glass substrate without any other chemical modification. A homeotropic orientation of LC thin-film was generated by the interfacial chemical interaction between nitrile-containing LC and copper ions on the surface. Results showed that an appropriate density of Cu2+ could shorten the response time of orientation, but a shelf-time was prolonged. The LC film fabrication not only offered a simple process, but also presented a great repeatability to detect organophosphonates (DMMP). This study provided guidance for the design of LC films responding to organic molecules as a biosensor.

Light-driven dynamic Archimedes spirals and periodic oscillatory patterns of topological solitons in anisotropic soft matter.

Oscillatory and excitable systems commonly exhibit formation of dynamic non-equilibrium patterns. For example, rotating spiral patterns are observed in biological, chemical, and physical systems ranging from organization of slime mold cells to Belousov-Zhabotinsky reactions, and to crystal growth from nuclei with screw dislocations. Here we describe spontaneous formation of spiral waves and a large variety of other dynamic patterns in anisotropic soft matter driven by low-intensity light. The unstructured ambient or microscope light illumination of thin liquid crystal films in contact with a self-assembled azobenzene monolayer causes spontaneous formation, rich spatial organization, and dynamics of twisted domains and topological solitons accompanied by the dynamic patterning of azobenzene group orientations within the monolayer. Linearly polarized incident light interacts with the twisted liquid crystalline domains, mimicking their dynamics and yielding patterns in the polarization state of transmitted light, which can be transformed to similar dynamic patterns in its intensity and interference color. This shows that the delicate light-soft-matter interaction can yield complex self-patterning of both. We uncover underpinning physical mechanisms and discuss potential uses.

Solvent vapour mediated spontaneous healing of self-organized defects of liquid crystal films.

Ultrathin liquid crystal films showed a nematic to isotropic transition when exposed to solvent vapour for a short duration while a reverse isotropic to nematic transition was observed when the film was isolated from the solvent exposure. The phase transitions were associated with the appearance and fading of surface patterns as the solvent molecules diffused into and out of the film matrix, resulting in the destruction or restoration of the orientational order. A long-time solvent vapour exposure caused the dewetting of the film on the surface, which was demonstrated by the formation of holes and their growth in size with the progress of time. Even at this stage, withdrawal of the solvent exposure produced an array of nematic fingers, which nearly self-healed the dewetted holes. The change in contact angle due to the phase transition coupled with the imbalance of osmotic pressure across the contact line due to the differential rate of solvent evaporation from the film and the hole helped the fingers to grow towards the centre of the hole. The appearance of the fingers upon withdrawal of the solvent exposure and their disappearance upon exposure to solvent were also found to be a nearly reversible process. These findings could significantly contribute to the development of vapour sensors and self-healing surfaces using liquid crystal thin films.

Bistable polymer-dispersed cholesteric liquid crystal thin film enabled by a stepwise polymerization

By using a proper polymer network and an ion-doped cholesteric liquid crystal with negative dielectric anisotropy, bistable polymer-dispersed liquid crystal thin films were prepared by first photo-curing, and then thermally curing.

TUNABLE AMPLITUDE AND PHASE MODULATION IN TERAHERTZ REGIME USING TRANSVERSE STRATIFIED CONFIGURATION

A number of transverse stratified configurations of metal and dielectric layers are studied for modulating Terahertz radiation in amplitude and phase. Pass band flat-top response and high wide-band transmission is achieved by means of a metallic grating filled with Liquid Crystal (LC) in different configurations and with the use of either grazing angles of incidence or cuts pierced within the grating. The transverse configuration with thin LC films allows for high speed tunability with low applied voltage. A dielectric grating with non-continuous electrodes is studied showing wide pass band response suitable for phase modulation applications.

Modeling of the Liquid Crystal/Lipid Interface for Bio-Sensing Applications

The control of the orientation of liquid crystal thin films at surfactant-decorated interfaces is of primary importance in the development of liquid crystalline devices as well as many applications in biological systems. Highly detailed molecular level modeling of these interfaces is needed to help us to create liquid crystal-based sensors that respond to specific chemical and biological signals. These liquid crystal/surfactant systems have been studied extensively due, in part, to both their birefringence optical properties and orientational sensitivity to surface interactions. It is therefore critical to understand the interplay between the conformational entropy of the surfactants, the rotational entropy of the liquid crystals, the intermolecular and molecule-surface interactions, and the packing at the interfaces to be able to create design platforms for sensing applications using these systems.In the absence of liquid crystal molecules, binary DPPC/DOPC lipid monolayers undergo phase transitions from liquid-expanded to liquid-condensed phases as the lipid areal density decreases. DPPC has two fully saturated fatty acid tails, and DOPC has two monounsaturated fatty acid tails. The area per molecule of this phase transition is highly dependent on the temperature and the composition of the monolayer. In this work, we present the effect of liquid crystal nematic elasticity on liquid-expanded/liquid-condensed phase diagram.We use theoretical model predictions for the phase behavior of liquid crystal thin films in the presence of various lipid monolayer mixtures to better understand the fundamental interactions that control molecular reorganization. These results shed light on the interplay between the conformational entropy of the lipids, the penetration of the liquid crystal into the lipid region, the propagation of the interfacial orientation to the bulk phase behavior of the liquid crystal film, and the effects of liquid crystal orientation on the phase transitions of the lipids themselves.

Rigid linear particles confined on a spherical surface: phase diagram of nematic defect states.

A thin film of liquid crystal confined on a spherical surface displays topological defects, as demonstrated by experimental observation of micron-sized double-emulsion droplets. Proposals have been made in practical applications to design novel materials by using these patterned textures. Recent phenomenological models and Monte Carlo computer simulations revealed organized defect structures with different characteristics. Here, we theoretically investigate the structure of the nematic ordering resulted from the excluded-volume interaction between rigid molecules confined on a spherical surface. The free-energy model follows a basic idea originally proposed by Onsager for studying a nematic fluid made of rigid particles. We tackle this fundamental model numerically to produce high-precision free-energy branches that enable structural determination.

Charge carrier trapping in highly-ordered lyotropic chromonic liquid crystal films based on ionic perylene diimide derivatives

Charge carrier trapping in thin films of lyotropic chromonic liquid crystals (LCLCs) based on ionic perylene diimide derivative and in chemically-similar neutral N,N′-dipentyl-3,4,9,10-perylene-dicarboximide (PTCDI-C5) films is investigated by thermally-stimulated luminescence (TSL) technique. The LCLC films comprise elongated molecular aggregates featuring a long-range orientational order. The obtained results provide direct evidence for the improved energetic ordering (smaller effective energetic disorder) in aggregated LCLC films as compared to conventional PTCDI-C5 films. The width of the density-of-state distribution of 0.09 eV and 0.13 eV was estimated for the LCLC and PTCDI-C5 films, respectively. Relatively small effective energetic disorder in LCLC films is ascribed to formation of macroscopically larger LCLC aggregates.

Preparation and thermo-optical characteristics of a smart polymer-stabilized liquid crystal thin film based on smectic A–chiral nematic phase transition

A smart polymer stabilized liquid crystal (PSLC) thin film with temperature-controllable light transmittance was prepared based on a smectic-A (SmA)–chiral nematic (N*) phase transition, and then the effect of the composition and the preparation condition of the PSLC film on its thermo-optical (T-O) characteristics has been investigated in detail. Within the temperature range of the SmA phase, the PSLC shows a strong opaque state due to the focal conic alignment of liquid crystal (LC) molecules, while the film exhibits a transparent state result from the parallel alignment of N* phase LC molecules at a higher temperature. Importantly, the PSLC films with different temperature of phase transition and contrast ratio can be prepared by changing the composition of photo-polymerizable monomer/LC/chiral dopant. According to the competition between the polymerization of the curable monomers and the diffusion of LC molecules, the ultraviolet (UV) curing surrounding temperature and the intensity of UV irradiation play a critical role in tuning the size of the polymer network meshes, which in turn influence the contrast ratio and the switching speed of the film. Our observations are expected to pave the way for preparing smart PSLC thin films for applications in areas of smart windows, thermo-detectors and other information recording devices.

Nanorheology of liquid crystal thin films confined between interfaces with anisotropic molecular orientations

The hydrodynamic properties of confined liquids play a significant role in the transportation of fluids in microfluidics, nanofluidics, and flows in porous media so on. In this paper, the effective viscosities of 4-Cyano-4′-pentyl biphenyl (5CB) liquid crystals (LCs) confined in a slab geometry with interfaces of anisotropically oriented molecules were investigated by a quartz crystal microbalance (QCM). A novel nanocell was fabricated onto the QCM substrate chip resulting in a 520-nm-thick slab geometry for viscosity measurement. Rubbed polyimide layer was spinning coated onto the nanocell interfaces to obtain different combinations of interfacial molecular orientations of the added 5CB LCs. The resulted effective viscosity of 5CB LCs increased by ~14 % on the interface with LC molecules oriented vertically to the shearing direction. However, it decreased on the substrate interface with LC molecules parallel along the shearing direction and decreased up to ~58 % when the LCs was confined in the nanocell with both parallel oriented interfaces. A varying magnitude of ~300 % in LC effective viscosity could be obtained by tailoring the interfacial molecular orientations. We also demonstrated that the shear resistance applied on the interface is predominated by the liquid microstructure within nanometers distance from the wall, but weakly influenced by that far from the local interface. The results were analyzed in terms of LC intermolecular friction and the predominance of the interfacial molecules at nanoscale, which are of the utmost importance for fluid transportation in microfluidics, in porous media, or in boundary lubrications.