Liquid Crystal Superstructures Research Articles

Angle selectivity of liquid crystal superstructure and applications in bio-detection

In this paper, a temperature-controlled angle selection device based on a photonic bandgap is proposed, consisting of MLC-6608 liquid crystal (MLC) and common electrolytes stacked in layers. This device has an angular transmission stability for electromagnetic waves over a wide frequency band (600–660 THz). A high transmissivity (T>0.85) area, also called an angle window, is formed at 25° and 75°, and an area of zero transmissivity is formed beyond the angle window. The MLC is temperature-responsive, and the range of angle selection can be expanded or narrowed by temperature adjustment. When the MLC is replaced by the biological sample, it shows good sensing performance. It can be used to detect in vitro dermis, in vivo stratum corneum, and in vivo epidermis and it is of great significance in medicine, with a sensing sensitivity of 90.91°/refractive index unit.

Dual-Wavelength Lasing with Orthogonal Circular Polarizations Generated in a Single Layer of a Polymer–Cholesteric Liquid Crystal Superstructure

We investigate the laser emission from a polymer-cholesteric liquid crystal superstructure with coexisting opposite chiralities fabricated by refilling a right-handed polymeric scaffold with a left-handed cholesteric liquid crystalline material. The superstructure exhibits two photonic band gaps corresponding to the right- and left-circularly polarized light. By adding a suitable dye, dual-wavelength lasing with orthogonal circular polarizations is realized in this single-layer structure. The wavelength of the left-circularly polarized laser emission is thermally tunable, while the wavelength of the right-circularly polarized emission is relatively stable. Due to its relative simplicity and tunability characteristics, our design might have broad application prospects in various fields of photonics and display technology.

A long-term stable low-viscous self-organized blue phase liquid crystal superstructure with wide operation temperature range

ABSTRACT A long-term stable self-organised cubic blue-phase liquid crystal superstructure at a relatively low temperature down to −20°C was achieved through doping a low-molecular-weight and short-chain diluter [4-pentyl-4ʹ-propyl-1,1ʹ-bi(cyclohexyl)] (5CC3) with low viscosity into liquid crystal host. The prepared sample with the diluter-induced low-viscous liquid crystal host not only exhibited a considerably stable cubic structure, but also possessed durable outstanding electro-optical performances, e.g. fast response time, relatively low driving voltage and hysteresis at such a low temperature as compared with those of the conventional polymer stabilised blue-phase liquid crystal. Therefore, we have disclosed a feasible strategy for overcoming the formidable problem of low-temperature applicability of blue-phase liquid crystals, and the proposed diluter-doped polymer stabilised blue-phase liquid crystal paves broad ways for versatile applications of the relevant materials at low temperature.

Topological liquid crystal superstructures as structured light lasers

Liquid crystals (LCs) form an extremely rich range of self-assembled topological structures with artificially or naturally created topological defects. Some of the main applications of LCs are various optical and photonic devices, where compared to their solid-state counterparts, soft photonic systems are fundamentally different in terms of unique properties such as self-assembly, self-healing, large tunability, sensitivity to external stimuli, and biocompatibility. Here we show that complex tunable microlasers emitting structured light can be generated from self-assembled topological LC superstructures containing topological defects inserted into a thin Fabry-Pérot microcavity. The topology and geometry of the LC superstructure determine the structuring of the emitted light by providing complex three-dimensionally varying optical axis and order parameter singularities, also affecting the topology of the light polarization. The microlaser can be switched between modes by an electric field, and its wavelength can be tuned with temperature. The proposed soft matter microlaser approach opens directions in soft matter photonics research, where structured light with specifically tailored intensity and polarization fields could be designed and implemented.

Light-rewritable geometric phase and reflectance modulations enabled by pattern-aligned photoresponsive liquid crystal superstructures

ABSTRACT We propose a universal method of fabricating and operating a multifunctional diffractive optical element (DOE) with light-rewritable geometric phase and reflectance modulations enabled by pattern-aligned photoresponsive liquid crystal superstructures via manipulating the photoalignment layers as well as the phototunable helical pitch of the cholesteric liquid crystal (CLC) under different light stimuli, i.e. ultraviolet and visible (green) lights. The diffractive behaviour of the fabricated CLC DOE is simultaneously governed by the geometric phase induced by the pattern-aligned CLC and the reflectance (amplitude) modulation attributed to the helical pitch manipulation, endowing the proposed DOE with versatile practical light-controllable characteristics and outstanding performance.

Complex liquid crystal superstructures induced by periodic photo-alignment at top and bottom substrates.

The formation of nematic liquid crystal (LC) superstructures in cells with non-uniform photo-alignment at the confining substrates is studied experimentally and by simulations. An interference pattern of left- and right-handed circularly polarized light is used to define the alignment at both substrates separately, so that the alignment varies along the x-coordinate on one substrate and along the y-coordinate on the other substrate. The interplay between the complex surface alignment and the liquid crystalline soft matter leads to the formation of interesting 3D configurations. The periodic LC structures that are formed in the bulk of the cell are analyzed experimentally by polarizing optical microscopy (POM) for different applied voltages. In the region with strong photo-alignment at both substrates, a 2D LC polarization grating (PG) with a complex 3D director configuration is formed. Distinct periodic structures with different symmetry properties are observed in the regions with weak illumination at the top and/or bottom substrate. The director configuration in the different regions was successfully simulated with the help of finite element (FE) Q-tensor simulations. The agreement between the simulations and the experiments was verified by comparing the POM images with simulated results for the transmission between crossed polarizers.

Dynamic cholesteric liquid crystal superstructures photoaligned by one-step polarization holography

A convenient approach to modulate the fingerprint textures of methyl red (MR) doped cholesteric liquid crystals by asymmetric photoalignment in the green-light waveband is presented, resulting in the generation of voltage-controllable helical superstructures. The interaction between the MR molecules and the incident light polarization determines the initial twisted planar geometry, providing a multivariant control over the stripe directions of fingerprint textures by applying a proper electric field. The key factors for precise manipulation of fingerprint stripes in a predictable and rewritable manner are analyzed theoretically and investigated experimentally, which involves the alignment asymmetry, the ratio of cell gap to natural pitch length, and the chirality of chiral dopant. Dynamic periodic fingerprint textures in shapes of dashed curve and dashed line are further demonstrated by utilizing a facile one-step polarization holography process using two beams with orthogonal circular and orthogonal linear polarizations, respectively. It is believed that the practical approach described in this study would enrich the research contents of self-assembled hierarchical superstructures using soft liquid crystal building blocks.

Enhanced Low-temperature Electro-optical Kerr Effect of Stable Cubic Soft Superstructure Enabled by Fluorinated Polymer Stabilization

An enhanced electro-optical Kerr effect of the stable self-organized cubic blue phase liquid crystal superstructure at a relatively low temperature down to −50 °C was achieved through a judiciously designed fluorinated polymer stabilization. The fluorinated sample exhibited not only a rather stable cubic structure, but the promoted electro-optical performances of low driving voltage, weak hysteresis and high contrast ratio at such a low-temperature, which were much distinct from the common non-fluorinated polymer stabilized blue phase liquid crystal without conspicuous low-temperature Kerr response behaviours. Kerr constant, which reflects the obviousness of Kerr effect, of the fluorinated sample at −50 °C indicated a spectacular enhancement of two orders of magnitude in contrast to the commonly material, thereby corroborating the high efficiency of polymer fluorination in enhancing low-temperature Kerr effect. Such an enhancement of Kerr effect was probably resulted from the decreasing of interfacial anchoring between liquid crystal and fluorinated polymer network. The fluorinated polymer stabilization not only ensures the stability of self-organized cubic structure of blue phase, but overcomes the challenge and bottleneck problem of low-temperature inapplicability of common blue phase liquid crystal and paves a brilliant and broad way for relevant materials to abundant perspective applications at low temperature.

Light‐Patterned Crystallographic Direction of a Self‐Organized 3D Soft Photonic Crystal

Uniform and patterned orientation of a crystallographic direction of ordered materials is of fundamental significance and of great interest for electronic and photonic applications. However, such orientation control is generally complicated and challenging with regard to inorganic and organic crystalline materials due to the occurrence of uncontrollable dislocations or defects. Achieving uniform lattice orientation in frustrated liquid-crystalline phases, like cubic blue phases, is a formidable task. Taming and tailoring the ordering of such soft, cubic lattices along predetermined or desired directions, and even imparting a prescribed pattern on lattice orientation, are more challenging, due to the entropy-domination attribute of soft matter. Herein, we disclose a facile way to realize designed micropatterning of a crystallographic direction of a soft, cubic liquid-crystal superstructure, exhibiting an alternate uniform and random orientation of the lattice crystallographic direction enabled by a photoalignment technique. Because of the rewritable trait of the photoalignment film, the pattern can be erased and rewritten on-demand by light. Such an oriented soft lattice sensitively responds to various external stimuli such as temperature, electric field, and light irradiation. Furthermore, advanced reflective photonic applications are achieved based on the patterned crystallographic orientation of the cubic blue phase, soft lattice.

Controllable Dynamic Zigzag Pattern Formation in a Soft Helical Superstructure.

Zigzag pattern formation is a common and important phenomenon in nature serving a multitude of purposes. For example, the zigzag-shaped edge of green leaves boosts the transportation and absorption of nutrients. However, the elucidation of this complicated shape formation is challenging in fluid mechanics and soft condensed matter systems. Herein, a dynamically reconfigurable zigzag pattern deformation of a soft helical superstructure is demonstrated in a photoresponsive self-organized cholesteric liquid crystal superstructure under the simultaneous influence of an applied electric field and light irradiation. The zigzag-shaped pattern can not only be generated and terminated repeatedly on demand, but can also be easily manipulated by alternating irradiation of ultraviolet and visible light while under the influence of a sustained electric field. This unique behavior results from a delicate balance among the variable experimental parameters. The evolution of the zigzag-shaped pattern is successfully modeled by numerical simulations and has been monitored through diffraction of a probe laser. Interestingly, this fascinating zigzag-shaped pattern yields crescent-shaped diffraction pattern. The reversibly controllable dynamic zigzag pattern could enable the fabrication of novel photonic devices and architectures, besides greatly advancing the fundamental understanding of temporal behavior of ordered soft materials under combined stimuli.

Light-reconfigured waveband-selective diffraction device enabled by micro-patterning of a photoresponsive self-organized helical superstructure

A waveband-selective diffraction device was developed by micro-patterning of a photoresponsive self-organized helical liquid crystal superstructure.

Thermal and dynamic mechanical properties of hydroxypropyl cellulose films

AbstractDifferential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) were used to characterize the morphology of solvent cast hydroxypropyl cellulose (HPC) films. DSC results were indicative of a semicrystalline material with a melt at 220°C and a glass transition at 19°C (T1), although an additional event was suggested by a baseline inflection at about 80°C (T2). Corresponding relaxations were found by DMTA. A secondary relaxation at −55°C was attributed to the interaction between hydroxyl groups of the polymer and residual diluent. The tan δ peak at T2 was found to arise from an organized phase, presumably from a liquid‐crystal mesophase formed while in solution. Crosslinking with a diisocyanate increased the peak temperature of the two primary relaxations, and resulted in a more clearly defined peak for the T2 transition. From this behavior it was concluded that both T1 and T2 are similar to glass transitions (Tg's) associated with an amorphous component and a more highly ordered phase (due to a residual liquid crystal superstructure) in the HPC bulk.