A multifunctional blue phase liquid crystal lens based on multi-electrode structure

A blue phase liquid crystal (BPLC) lens with multifunction using multi-electrode structure and a dielectric layer with high dielectric constant is proposed. The refractive index of the BPLC can be changed flexibly in different regions. Some functional or technical requirements such as switch between positive and negative lenses can be achieved. The lens reveals a good parabolic refractive index distribution and focus adjustment capacity simultaneously. The applied voltage on the electrodes is regular and computable. To decrease the applied voltage of the proposed lens with a large diameter, a drive-type adopted Fresnel lens is introduced.     

Switchable polarisation-independent blue phase liquid crystal Fresnel lens based on phase-separated composite films

A simple method for fabricating a polarisation independent blue-phase liquid crystal Fresnel lens (BPLCFL) is demonstrated by utilising the photo-polymerisation-induced phase separation. The BPLC/polymer binary Fresnel zones is obtained well by periodic UV illumination with phase separation of the BPLC molecules and UV-curable pre-polymer mixture. The diffraction efficiency can be controlled when applying a uniform electric field which modulates the phase difference between even and odd Fresnel zones. Experimental results show that the maximum diffraction efficiency reaches 24.3%, which is close to the measured diffraction efficiency of the used Fresnel zone-plate mask of 25%. We also characterise the tunable lens performance at different applied voltages.     

Tuning of Photonic band gap via combined effect of electric and optical fields in a blue phase liquid crystal composite

ABSTRACT

Blue phase liquid crystals are soft 3D photonic crystals in which the liquid crystal molecules self-assemble to form a cubic structure with lattice spacing of a few hundred nanometers resulting in selective reflection of colours in the visible spectrum. The corresponding wavelength or the ‘photonic band gap’ can be tuned using various external stimuli such as thermal, electric, magnetic and optical fields. Here, we report efficient tuning of photonic band gap by utilising the combination of electric and optical fields in a blue phase liquid crystalline system. The studies indicate that the chirality of the medium has a direct bearing on the direction of the wavelength shift and the extent of the photonic band gap tunability. More importantly, the synergistic effect of the two fields helps in reversible tuning of the band gap.     

Analytical determination of anchoring energy coefficient in liquid crystal cells

An analytical solution for the anchoring energy coefficient of liquid crystal (LC) cells with arbitrary values of pretilt angles is derived. When phase retardation is plotted against applied voltage, one acquires the extrapolation length of the anchoring, and the anchoring energy coefficient is derived in the low-voltage regime. This solution can be applied to LC cells with various pretilt angles straightforwardly. Finally, the anchoring energy coefficient of 4.9 × 10^?5 J/m² is obtained for a homemade LC cell with a pretilt angle of 32.7°.     

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.     

Hysteresis-free and fast response polymer-stabilised blue phase liquid crystals

ABSTRACT

The electro-optical properties of γ-Fe₂O₃ nanoparticles doped polymer-stabilized blue phase liquid crystals are investigated. The experimental results show that the hysteresis were effectively suppressed by doping γ-Fe₂O₃. When doped with γ-Fe₂O₃ of 1.0 wt. %, hysteresis-free and fast response were achieved by the coupling effect of polymer and nanoparticle. Our research provides excellent guidance for the development of electro-optical devices with high grey scale accuracy and fast response.     

Tunable negative index metamaterial employing in-plane switching mode at terahertz frequencies

We analysed the response of a tunable liquid crystal metamaterial transducer in the terahertz frequency range. Tunability of scattering parameters is achieving by an in-plane switching (IPS) effect. The metamaterial structure is based on Ω-shape resonators. A full-wave analysis technique based on the finite-difference time-domain (FDTD) method was performed using the QuickWave 3D electromagnetic solver. Terahertz transmission properties of the metamaterial structure can be controlled by the director of the liquid crystal layer. The effective refractive index for operation frequency varies from negative to positive values. A novel approach to switching of metamaterial transducer by IPS mode is presented.     

The effects of azo-oxadiazole-based bent-shaped molecules on the temperature range and the light-responsive performance of blue phase liquid crystal

A series of light-responsive azo-oxadiazole-based bent-shaped molecules was synthesised and characterised. Their effects as dopants on the blue phases (BPs) range of the chiral nematic liquid crystal (N*LC) matrix and light-responsive properties including the photo-inducing change of UV absorption properties, photo-adjusting the BP structure as well as the temperature range were investigated. It was found that the azo-oxadiazole-based bent-shaped molecules with branched terminal had better miscibility in LC host than the molecules with straight terminal, and that increasing the length of rigid core of bent-shaped molecules will greatly improve the effect of widening the BP temperature range.     

Analysis of surface anchored lattice plane orientation in blue phase liquid crystal and its in-plane electric field-dependent capacitance response

Uniformly oriented macroscopic monodomain of cholesteric blue phase liquid crystal has been realised by the influence of surface anchoring. Orientation of the lattice planes in surface-treated (ST) and non-surface-treated (NST) cell were analysed and compared by Kossel diagram technique. NST cell has revealed the green and blue domains corresponding to reflection from oriented (110) and (112) planes of the body-centred cubic lattice. However, in the ST cell only the lattice plane (110) oriented uniformly and tailored the macroscopic monodomain. Electric field driven reorientation of the (110) lattice plane was noticed in NST cell whereas for ST cell such reorientation was absent. Two distinct electric field-induced capacitive responses have been observed in the two different cells. In NST cell anomalous electrostriction was observed, whereas for ST cell normal electrostriction was observed. Interestingly, the capacitance has decreased with an increasing electric field for anomalous electrostriction in NST cell, whereas for normal electrostriction in ST cell it was increased with increasing the field. Such a capacitive change behaviour is explained by dielectric anisotropic change followed by the electric field induced elongation and contraction of the cubic unit cell along and perpendicular to the electric field.     

Electro-optics of polymer-stabilised blue phase liquid crystal in in-plane switching mode

In the present work, polymer-stabilised blue phase liquid crystal ***(PS-BPLC) that exhibit the blue phase (BP) in a temperature range of 312.15 K to 298.15 K has been prepared. The textural and electro-optic studies were performed in the BP range using an in-plane switching (IPS) cell. Platelet-type textures of cubic BP having an average domain size of ~12 µm were observed. The on-state voltage increased with increasing the temperature due to reduced value of the Kerr constant. Further, the hysteresis was found to be reduced from 19.2% to 5.1% by operating the PS-BPLC sample cell at an elevated temperature.     

Study of polymer-stabilised blue phase liquid crystal on a single substrate

One outstanding feature of the polymer-stabilised blue phase (PSBP) is that it is unnecessary to form an alignment film, which requires a high-temperature baking process. Therefore, PSBPs may enable flexible liquid crystal displays (LCDs) on plastic substrates. In this study, polymer stabilisation of a blue phase (BP) on a single substrate was performed without using a conventional sandwich-type cell, and the electro-optical properties are demonstrated to be similar to those of a sandwich-type PSBP LCD cell. It was experimentally shown that the oxygen which inhibits radical polymerisation is required to be excluded in order to complete the polymer stabilisation in blue phase.     

Biaxial film design for full-colour,wide-view and high-contrast blue phase liquid crystal displays

To achieve full-colour, wide-view and high-contrast blue phase liquid crystal display (BPLCD), the compensated biaxial film is analysed in this paper. The dispersion of birefringence affects the light leakage of dark state and then affects the viewing angle and the contrast ratio at large polar viewing angle. Iso-transmission of dark state for BPLCD without compensational film, with conventional or wide-band biaxial film, is simulated. The simulated results show that the wide-band biaxial film is good for obtaining a BPLCD with excellent viewing angle and high-contrast ratio.     

Synthesis of chiral azobenzene derivatives and the performance in photochemical control of blue phase liquid crystal

A series of chiral azobenzene compounds with branched terminal were synthesised, and the photosensitive performances were investigated accordingly. It was found that the photochemical properties were mainly affected by the trans–cis configuration of azobenzene and the linked position between the azobenzene and chiral centre. The para-type azobenzenes showed general photochemical decrease in helical twisting power (HTP), but the meta-type ones appeared interestingly showed photochemical increase in HTP. This work provided an effective method for designing molecules to control blue phase (BP) including adjusting colours, inducing BP and extending BP range, which was promising in the applications of optically addressable devices.     

Toluene-induced phase transitions in blue phase liquid crystals

ABSTRACT

We report phase transitions in blue phase-forming liquid crystals (LCs) that are triggered by exposure to toluene vapours. Specifically, we reveal that room-temperature cholesteric phase mixtures of MLC-2142 and S-811 form blue phases (BP I, II and III) with increasing vapour pressure of toluene. To probe the mechanism underlying this observation, we investigated the phase behaviour of mixtures of BP-forming LCs containing a range of non-volatile aromatic compounds (e.g. pyrene). We interpret our observations to indicate that the principal effect of small aromatic compounds is to decrease the energy penalty associated with the formation of disclination lines in BPs. We also conclude that the absorption of toluene into the BP-forming LCs lowers the energy required for the formation of disclination cores in the BP phase, thus allowing the elastically favoured double-twist cylinders to form at lower temperatures. We demonstrate that BP-forming LCs containing pyrene can be used to detect toluene at concentrations below 200 ppm at room temperature. Overall, these results guide the design of LC-based materials that respond to VOCs at concentrations relevant to occupational settings.     

A polarisation-independent blue-phase liquid crystal microlens using an optically hidden dielectric structure

A polarisation-independent blue-phase liquid crystal microlens using an optically hidden dielectric structure is proposed. In this design, the non-uniform electric field across the lens aperture is obtained by the modulation of the effective dielectric constant of an optical hidden layer. As the applied voltage varies from 0 to 150V_rms, the focal length of the lens can be tuned from ∞ to 16.6 mm. Simulation results show that this device has a parabolic-like profile and exhibits polarisation-independent property.     

Low-voltage and high-transmittance blue-phase liquid crystal display with concave electrode

A low-voltage and high-transmittance blue-phase liquid crystal display (BPLCD) with concave electrodes is proposed. We use in-plane switching electrodes on the etched substrates to generate the concave electrodes. The proposed device can generate a strong in-plane field with a large horizontal component to increase the transmittance and reduce the operating voltage. As a result, a low voltage ~9 V and reasonably high transmittance ~71.7% can be achieved. Moreover, due to the generated multi-domain structures in the etched areas, this BPLCD can obtain a symmetric and wide viewing angle and the contrast ratio of 1000:1 is obtained over 60° viewing cone.     

A blue-phase liquid crystal lens array based on dual square ring-patterned electrodes

We propose a blue-phase liquid crystal (BPLC) lens array based on dual square ring-patterned electrodes. A high dielectric constant layer is used to smoothen out the horizontal electric field and reduce the operating voltage. By creating a potential difference between the dual square ring-patterned electrodes, gradient electric fields are generated and lens-like phase profile is obtained. Besides, the focal length of the BPLC lens is adjustable with voltage changes and all simulation results indicate that the BPLC lens array is polarisation-insensitive.     

Blue-phase liquid crystal display with insulating protrusion

In order to lower the saturation voltage and enhance the transmittance of in-plane switching blue-phase liquid crystal display (IPS-BPLCD), IPS-BPLCD with insulating protrusion is proposed. The single-protrusion (only set on the top of pixel electrode) and double-protrusion (set on the top of pixel and common electrodes) structures are investigated in this work. The potential distribution changes when the protrusion is used. There is a thicker transverse electric field in BPLC range, because the stronger electric field at the edges of the electrodes is decentralised into BPLC range. As a result, the saturation voltage is reduced from 36.3 V to 28.9 V when the double-protrusion structure is used, and transmittance is increased by ~20%. The contrast ratio is larger than 1000:1 in 60° viewing cone using a half-wave biaxial film. Both single-protrusion and double-protrusion structures have the uniform gamma curves at large oblique viewing angles. Moreover, the off-axis image distortion index is 0.1590 at 60º polar angle when zigzag electrodes are used.     

Electromagnetic simulations of tunable terahertz metamaterial infiltrated with highly birefringent nematic liquid crystal

In this article electromagnetic simulations of tunable terahertz metamaterial infiltrated with nematic liquid crystal are described. A full-wave analysis technique based on the finite-difference time-domain (FDTD) method was done by employing QuickWave 3D electromagnetic solver. Scattering parameter analysis shows a resonant frequency shift of 0.8 THz obtained by reorienting the layered highly birefringent nematic liquid crystal. Effective refractive index for operation frequency varies from negative to positive values.