Electrically tunable polarisation-independent blue-phase liquid crystal binary phase grating via phase-separated composite films

A simple method for fabricating polarisation-independent blue-phase liquid crystal phase grating is demonstrated by implementing photopolymerisation-induced phase separation through a binary photomask. The dynamic focusing property of the proposed liquid crystal grating is independent of the polarisation state of incident light. The efficiency of various diffraction orders for the phase grating was measured as a function of the applied voltage. Experimental results show that the maximum diffraction efficiency reaches 36% for the ±1 order, which approaches the theoretical limit ~41%. The measured rise time is 1.4 ms and fall time is 2.2 ms. Such a tunable grating has great potential for photonic applications.     

Electrically tunable photo-aligned two-dimensional liquid crystal polarisation grating

Two-dimensional liquid crystal polarisation grating based on twist liquid crystal has been demonstrated and fabricated by polarisation holography in this paper. The maximal diffraction efficiency of the second order is up to 90%. And the two-dimensional liquid crystal polarisation grating has the properties of electrically tenability and polarisation tenability. The two-dimensional polarisation grating diffracts light into a spot array. Different direction diffraction light is with different polarisation states. The intensity of the different orders diffraction light is optically controlled.     

Electro-disclinic effect in tilted smectic phases of banana-shaped liquid crystal materials

We give the first evidence that the director tilt angle can be reduced by electric fields in the tilted smectic phase of banana-shaped molecules. In these phases the value of polarization is determined by the molecular packing and no electro-clinic effect is expected. Our studies show that high electric fields eventually induce a meta-stable phase with zero director tilt. The tilted phase slowly recovers at low fields. We propose that the field-induced quenching of the layer fluctuations is responsible for the observed effects.     

A polarisation-free blue phase liquid crystal lens with enhanced tunable focal length range

A polarisation-free blue phase liquid crystal (BPLC) lens with enhanced tunable focal length range is proposed. A matched conventional glass lens is introduced on a BPLC lens to increase the range of the tunable focal length. The focal length of the BPLC lens can be switched from positive to negative, the negative lens-like phase profile can be neutralised by the conventional glass lens to get an infinity focal length and the positive lens-like phase profile can be enhanced by the conventional glass lens to get a shorter focal length. The minimum focal length can be decreased to almost half of that after the proposed method is adopted in our simulation. Moreover, the proposed BPLC lens exhibits a good polarisation-free feature and the optical effect is relatively good.     

High-efficiency large-angle reflective composite polarization grating

ABSTRACT

We introduce and demonstrate a new structure of polarisation grating which can substantially enlarge the deflection angle. We derive the polarisation state of a beam passing through the reflective composite polarisation grating by Jones matrix and build a rigorous simulation about the deflection angle of reflective composite polarisation grating using finite-element analysis. The theoretical diffraction efficiency of the reflective composite polarisation grating can reach up to 90%. The experimental result of deflection angle of the reflective composite polarisation grating is consistent with that of simulation. The reflective composite polarisation grating could be expected to be applied in augment reality system, spectroscopy, optical telecommunications, non-mechanical beam steering and so on.     

Design of filtering tunable liquid crystal phase shifter based on coplanar waveguide and split-ring resonators

ABSTRACT

In this paper, the designing method for microwave filtering tunable liquid crystal phase shifter (LCPS) is proposed for the first time. The proposed microwave filtering tunable LCPS integrated tunable phase shifter and passband filter can simultaneously provide tunable differential phase shifts and self-embedded stable filtering function. This filtering tunable LCPS is based on the coplanar waveguide (CPW) magnetically coupled with a series of split-ring resonators (SRRs). On this basis, the equivalent circuit model is utilized to analyse the relationship between stable filtering characteristic and tunable differential phase shifts. Finally, the prototype samples are fabricated and tested. The measured results not only show that the proposed filtering tunable LCPS realizes the stable passband from 7.9 to 8.2 GHz but also indicate that the differential phase shifts and figure of merit are, respectively, more than 55° and 9.6°/dB in the passband, which provide a good verification for the theoretical concepts.     

Low voltage blue-phase liquid crystal display with insulating protrusion sandwiched between dual-layer electrodes

In order to lower the operating voltage of blue-phase liquid crystal display (BPLCD), a BPLCD with insulating protrusion, which is sandwiched between dual-layer electrodes, is proposed. There are four electrodes in this structure; thus, we investigate different driving methods to find a proper driving method. The effect of protrusion’s dielectric constant on operating voltage of the proposed BPLCD is explored under various electrodes’ parameters. As a result, the operating voltage of the proposed BPLCD with protrusion’s dielectric constant of 1,000 is 9.8 V, which is reduced by ~ 4.67× compared with that of conventional in-plane switching (IPS) BPLCD (45.8 V). Moreover, the zigzag electrode structure is adopted to reduce the large off-axis image distortion index. Besides, the azimuth distortion index is defined to describe the gamma shift between the minimum and maximum gamma shift curves at a certain polar angle. The results show that the off-axis image distortion index can be reduced to 0.0834, the azimuth distortion index is 0.0810 and the viewing cone of contrast ratio larger than 1,000:1 is over 50ºas the zigzag proposed BPLCD is used.     

Stability of liquid crystal micro-droplets based optical microresonators

Liquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.     

An innovative technique based on voltammetric profiling and current–voltage characteristics for determination of residual direct-current voltage of liquid crystal cells

ABSTRACT

Generation of residual direct-current (DC) voltage (V_rDC) induces a serious image sticking of liquid crystal displays (LCDs). In this study, an innovative technique to determine the V_rDC of LC cells was proposed from the current-voltage (I-V) curves obtained by application of triangular voltage. The I-V curve shows the maximum and minimum current peaks due to the rotation of the LC director, and the V_rDC is determined from the average value of the two peaks. The V_rDC values determined from the proposed technique corresponded to the values determined from the flicker minimisation, which is the well-known method to determine the V_rDC. The results indicate that the proposed technique is useful for evaluation of the image sticking of the LCDs.     

Monotropic smectic A to double layer smectic C transition of biphenyl containing liquid crystal acetylene

5-[（4’-Heptoxy-4-biphenylyl）carbonyloxy]-1-pentyne（A-3,7） was synthesized and the phase structures and transitions were investigated by differential scanning calorimetry（DSC）,wide angle X-ray diffraction（WAXD）,polarized light microscopy（PLM） and the molecular packing in the crystal and liquid crystalline phases were simulated by molecular dynamic simulation.The results showed that the sample formed thermodynamically metastable SmA and SmC₂ phases before crystallized during cooling and the crystal phase directly transformed into isotropic phase during heating.     

Synthesis and mesomorphic properties of trifluorobenzoate liquid crystal

Four series of trifluorobenzoate liquid crystals have been synthesized. Their phase transition temperatures have been also measured by texture observation in a polarizing microscope and confirmed by DSC. The influence of the lateral fluoro-substitution and triple bond has been also discussed.     

Low voltage and high transmittance transflective blue-phase liquid crystal display with opposite polar electrodes

A single-cell-gap transflective polymer-stabilised blue-phase liquid crystal display with opposite polar pixel electrodes on an etched substrate is proposed. In the proposed structure, the space between common electrodes is adopted as transmissive region, and the space above the common electrode is adopted as reflective region. By optimising the electrode parameters of the transmissive and reflective regions, well-matched voltage-dependent transmittance and reflectance curves can be obtained. In addition, the device has good performances of low operating voltage (~3.2 V), high optical efficiency and a wide viewing angle.     

Synthesis of azobenzene-containing liquid crystalline gelator for use in liquid crystal gels

A liquid crystalline gelator containing the azobenzene chromophore was synthesized for the first time; it was used to form self- assembled network in nematic liquid crystals resulting in liquid crystal gels with distinct features.     

Reliability of liquid crystal cell and immiscibility between dual-curable adhesives and liquid crystal

The dual-curable adhesive used to attach thin film transistors (TFTs) to color filters in the construction process of liquid crystal display (LCD) panels requires fast curing by UV irradiation and strong bond strength after thermal-curing. In addition, it is necessary to consider the immiscibility of the dual-curable adhesives with the liquid crystal, because they come directly into contact with the liquid crystal without curing process in the large LCD panel production. In this study, dual-curable adhesives based on partially acrylated epoxy acrylate oligomers were prepared and investigated with nematic liquid crystals using gas chromatography (GC), polarized optical microscopy and the measurement of the transmittance of the liquid crystal.As the concentration of CC bonds was increased, the immiscibility was enhanced due to the fast curing rate of the partially acrylated epoxy acrylate oligomers and reduced visual contamination was observed in the polarized optical microscope images. Moreover, the transmittance of the liquid crystal cells was not changed before and after the dual-curing of the adhesives and was maintained for 100 h.     

Molecular orientation of a model liquid crystal alignment layer

An analytical methodology, involving the use of a combination of second harmonic generation (SHG) and linear dichroism, was utilized to probe the molecular orientation and angular distribution of a model liquid crystal (LC) alignment layer. In order to determine which film structure would be best suited for use as an alignment layer, the azo dye o-methyl red (MR) was covalently bound to a glass substrate using both monofunctional and trifunctional silane chemistry. The influence of solvent on the orientation and angular distribution of both thin films was also investigated. For the monofunctional silane film under water, the mean orientation angle of the MR molecular long axis was 67±4° and the width of an assumed Gaussian distribution was 32±2°. Under hexanes, the mean orientation angle was the same within error (63±1°) but the distribution width narrowed considerably to 22±1°. Molecular orientation within the trifunctional silane film exhibited little dependence on solvent. Under water, the mean orientation angle and angular distribution width were 76±3° and 30±1°, respectively. With hexanes as the solvent, the mean orientation angle and angular distribution width were 79±1° and 30±1°, respectively. Orientation insensitive SHG measurements indicated that surface coverage in the tri-functional silane film was twice that in the mono-functional silane film. The observed orientational differences were attributed to differences in the forces that dictate molecular orientation for the two systems. Based on the higher orientation angle, higher surface coverage and the lack of solvent dependence, MR-tri exhibits more desirable characteristics for use as an LC alignment layer.     

Thermophysical behaviour of monofunctional acrylate and liquid crystal systems

The phase behavior of monofunctional acrylate and low molecular weight nematic liquid crystals (LC) is considered. Systems involving the monomeric 2-ethylhexylacrylate (2-EHA), the eutectic LC mixture known as E7 and the 4-cyano-4^′-n-pentyl-biphenyl (5CB) are investigated. A similar investigation is performed on mixtures involving a polymer poly-2-EHA with molecular weight M_w=48,000 g/mol and both LCs. The experimental phase diagrams are established using polarized optical microscopy and analyzed using a theoretical formalism which combines the Flory-Huggins theory of isotropic mixing and the Maier-Saupe theory of nematic order. The results lead to characterization of the miscibility of E7 and 5CB with monomeric and analogous polymeric 2-EHA systems.     

New liquid crystal polyethers and polyesters based on diphenylbutadiene mesogens

New liquid-crystalline (LC) polymers and small molecules have been synthesized based on the 1,4-diphenylbutadiene mesogen, a chromophore with χ⁽³⁾ nonlinear optical properties. Polyethers were formed by phase-transfer reaction, and polyesters were prepared by Schotten-Bauman polymerization from these 4,4'-bisphenolic mesogenic structures. Liquid crystallinity was examined using differential scanning calorimetry (DSC) and optical microscopy. Most of the model compounds were found to be nematic. All mesomorphic polymers were nematic and displayed LC ranges of up to 50°C as well as a wide diversity of melting points. The melting points varied with the flexible (methylene) spacer length and were strongly dependent upon the presence of substituents in the mesogenic core. Crosslinking and cis-trans isomerization were observed under the influence of heat and ultraviolet radiation. © 1992 John Wiley & Sons, Inc.     

Polymorphic transitions mediated by surfactants in liquid crystal nanodroplet

Recently, various techniques have been developed using photonic crystals. Liquid crystals (LC) confined in a nanodroplet mimicked photonic crystals, such as those of opal. Therefore, investigating the phase behaviour of LC molecules in nanodroplets is very important in the next-generation optical field. In this study, the chemical interaction between surfactants and LCs in nanodroplets is reproduced using a dissipative particle dynamics method. We identify the phase behaviour of LCs and investigate how the chemical interaction affect on the orientation of LCs. In particular, by adding surfactant molecules, various morphological behaviours were observed in the LC nanodroplet. The phase transition temperature varied depending on R_ND (amount of surfactant molecules). Furthermore, difference of the self-assembly structure also appeared inside the droplet depending on R_ND. Our simulation offers a theoretical guide to control morphologies of self-assembled LCs inside a nanodroplet, a novel system that may find applications in nanofluidic devices or in photonic crystal technology.     

Polyfluorinated cycloalkoxyphenyl ether systems as dopants for liquid crystal display applications

The effects on response times of representative commercially available dielectrically negative (−LC) and positive (+LC) type liquid crystal mixtures doped with small quantities of various tetrafluorinated cycloalkoxyphenyl ether derivatives were investigated. The synthesis of various polyfluoroaryl systems by nucleophilic aromatic substitution processes is described and electro-optical measurements concerning the rise and decay times and voltage holding ratios of each doped LC formulation are reported. The response time of the dielectrically negative LC host was improved upon doping with each of the tetrafluorinated cycloalkoxyphenyl ether derivatives, although no improvement in the performance of the dielectrically positive LC host was achieved.