Nontrivial phase matching in helielectric polarization helices: Universal phase matching theory, validation, and electric switching.

Second-order optical nonlinearity is the essential concept for realizing modern technologies of optical wavelength conversion. The emerging helical polarization fluid, dubbed helielectric nematic, now makes it possible to design and easily fabricate various polarization structures and control their optical responses. The matter family is demonstrated as an ideal liquid platform for nonlinear optical conversion and amplification with electric-reconfigurable tunability. We here develop a universal phase matching theory and reveal a nonclassic chirality-sensitive phase-matching condition in the polarization helices through both the numerical calculation and the experimental validations. The nonlinear optical amplification can be dramatically modulated with a contrast ratio of >100:1 by an in-plane electric field. Furthermore, we employ the director relaxation under electric fields coupled with nonlinear optical simulation to clarify the topology-light interactions.

Topology of ferroelectric nematic droplets: the case driven by flexoelectricity or depolarization field.

The recent discovery of ferroelectric nematics provides new opportunities for exploring polar topology in liquid matter. Here, we report numerous potential polarization topological states (e.g., polar vortex-like and line disclination mediated structures) in confined ferroelectric nematics with similar free-energy levels. In the experiment, they appear according to the confinement size and surface anchoring conditions. Based on a minimal analytical approach, we reveal that the topological transformation is balanced among the nematic elasticity, the polarization gradient, the flexoelectric and the depolarization interactions.

Extended free-energy functionals for achiral and chiral ferroelectric nematic liquid crystals: theory and simulation.

Polar nematic liquid crystals are new classes of condensed-matter states, where the inversion symmetry common to the traditional apolar nematics is broken. Establishing theoretical descriptions for the novel phase states is an urgent task. Here, we develop a Landau-type mean-field theory for both the achiral and chiral ferroelectric nematics. In the polar nematic states, the inversion symmetry breaking adds three new contributions: an additional odd elastic term (corresponding to the flexoelectricity in symmetry) to the standard Oseen-Frank free energy, electrostatic effect and an additional Landau term relating to the gradient of local polarization. The coupling between the scalar order parameter and polarization order should be considered. In the chiral and polar nematic state, we reveal that the competition between the twist elasticity and polarity dictates effective compressive energy arising from the quasi-layer structure. The polarization gradient is an essential term for describing the ferroelectric nature. Besides, we successfully simulate an experimentally reported structural transition in ferroelectric nematic droplets from a concentric-vortex-like to a line-disclination-mediated topology based on the developed theory. The approaches provide theoretical foundations for testing and predicting polar structures in emerging polar liquid crystals.

Response of helielectric nematics under an in-plane electric field.

In traditional chiral nematic liquid crystals, the apolar cholesterics, the dielectric effect is the main driving force for responding to an electric field. The emerging polar chiral nematics, dubbed helielectric nematics, are the polar counterparts of the cholesterics. The head-to-tail symmetry breaking of the new matter state enables it to respond sensitively to the polarity of an electric field. Here, we report on the observation of a sequential polar winding/unwinding process of polarization helices under an electric field applied perpendicular to the helical axes, which behaves distinctly from the unwinding of the apolar cholesteric helices. Understanding the helix-unwinding behaviors provides insights for developing switchable devices based on helielectric nematics.

Spontaneous helielectric nematic liquid crystals: Electric analog to helimagnets.

Recently, a type of ferroelectric nematic fluid has been discovered in liquid crystals in which the molecular polar nature at molecule level is amplified to macroscopic scales through a ferroelectric packing of rod-shaped molecules. Here, we report on the experimental proof of a polar chiral liquid matter state, dubbed helielectric nematic, stabilized by the local polar ordering coupled to the chiral helicity. This helielectric structure carries the polar vector rotating helically, analogous to the magnetic counterpart of helimagnet. The helielectric state can be retained down to room temperature and demonstrates gigantic dielectric and nonlinear optical responses. This matter state opens a new chapter for developing the diverse polar liquid crystal devices.

Macroscopic Nematic Orientation Dictated by an Orientationally Frustrated Random-Field Surface: Equilibrium Structure and Kinetics.

Liquid crystals subjected to frustrated surfaces with mixed anchoring conditions demonstrate a rich variety of orientational patterns. Particularly, it would trigger either continuous or discontinuous variation of the bulk orientation, i.e., a phenomenon known as the anchoring or orientational transition. Despite its prime importance in developing novel optoelectronic devices, how the surface anchoring patterns dedicate the energy landscape of a system, thus the equilibrium state, still needs to be understood. Here, we designed a simulation to model boundary substrates with two randomly mixed anchoring domains in space, which exhibit planar and homeotropic preferences. We numerically obtain general bulk orientational state diagrams under various surface and electric field conditions, which reveal the roles of each domain's size and surface fraction and anchoring strength on the bulk orientational state. Furthermore, we examine how the external electric field modifies the orientational state diagram and uncovers a field-assisted anchoring transition. We discuss the observed bistability and compare it to experimental evidence.

Collective and non-collective molecular dynamics in a ferroelectric nematic liquid crystal studied by broadband dielectric spectroscopy.

A great deal of effort has been recently devoted to the study of dielectric relaxation processes in ferroelectric nematic liquid crystals, yet their interpretation remains unclear. In this work, we present the results of broadband dielectric spectroscopy experiments of a prototypical ferroelectric nematogen in the frequency range 10 Hz-110 MHz at different electrode separations and under the application of DC bias fields. The results evidence a complex behavior in all phases due to the magnitude of polar correlations in these systems. The observed modes have been assigned to different relaxation mechanisms based on existing theoretical frameworks.

Nontrivial ultraslow dynamics under electric-field in nematics of bent-shaped molecules.

For over decades, nematic liquid crystals have been recognized as highly fluidic materials that respond to electric field on the millisecond scale. In contrast to traditional nematics with fast responsivity, we herein report nontrivial ultraslow electric-driven dynamics in bent-shaped nematic materials. Varying the alkyl chain spacers of bent-shaped cyanobiphenyl dimers (COOm and OCOm) shows a 'transition' in the dynamics behavior between the bent-dimeric and bent-core materials. Interestingly, with short alkyl chain spacers, COO2 exhibits unexpected ultra-slow dynamic pathways, i.e., "quasi-static" electrohydrodynamic convection. A significant observation is that the on/off-electro-switching time of COO2 is 10 000 times higher than that of typical nematic materials, which is the largest value reported ever in the kilo-second range. In addition, the threshold voltage for inducing the reorientation of the nematic director for COO2 is higher than 5 V, which is uncommon in traditional N materials. These properties are distinct from those of traditional nematic materials and discussed in terms of dielectric constants and electrohydrodynamic convection.

Nontrivial topological defects of micro-rods immersed in nematics and their phototuning.

Combinations of different geometries and surface anchoring conditions give rise to the diversity of topological structures in nematic colloid systems. Tuning these parameters in a single system offers possibilities for observing the evolution of the topological transformation and for manipulating colloids through topological forces. Here we investigate the nontrivial topological properties of micro-rods dispersed in nematic liquid crystals through experimental observation and computer simulation. The topological variation is driven by photodynamically changing the surface anchoring using azobenzene-based surface-commander molecules, the majority of which are localized on both the substrates and the surface of micro-rods. By comparing experimental and simulation results, we show previously unidentified topological properties of the two-body LC-rod-colloid system. Moreover, unlike the traditional photoresponsive liquid crystal systems, the localization of azobenzene molecules on the surfaces makes it possible to change only the direction of the surface orientation, not disordering of the bulk structures. The results assist in the development of photo-driven micro-robotics in fluids.

Development of emergent ferroelectric nematic liquid crystals with highly fluorinated and rigid mesogens.

The emerging ferroelectric nematic liquid crystals have been attracting broader interests in new liquid crystal physics and their unique material properties. One big challenge for the ferroelectric nematic research is to enrich the material choice, which is now limited to RM734 and DIO families as representatives, in sharp contrast to the enormously diverse variety of the traditional apolar nematic liquid crystals. Here, we report a design of novel ferroelectric nematic materials with highly fluorinated and rigid mesogens. Noteworthily, they show distinct chemical structural features compared with previous aromatic ester-based molecules. The ferroelectric nematic phase was identified and confirmed through rigorous experiments. The bulk polarization was found to become purely along the long axis director, creating giant dielectric anisotropy. This work demonstrates a great potential for expanding ferroelectric nematic material diversity and will accelerate the corresponding application research and technology innovation.

How Far Can We Push the Rigid Oligomers/Polymers toward Ferroelectric Nematic Liquid Crystals?

The emerging ferroelectric nematic (NF) liquid crystal is a novel 3D-ordered liquid exhibiting macroscopic electric polarization. The combination of the ultrahigh dielectric constant, strong nonlinear optical signal, and high sensitivity to the electric field makes NF materials promising for the development of advanced liquid crystal electroopic devices. Previously, all studies focused on the rod-shaped small molecules with limited length (l) range and dipole moment (µ) values. Here, through the precision synthesis, we extend the aromatic rod-shaped mesogen to oligomer/polymer (repeat unit up to 12 with monodisperse molecular-weight dispersion) and increase the µ value over 30 Debye (D). The NF phase has a widespread existence far beyond our expectation and could be observed in all the oligomer/polymer length range. Notably, the NF phase experiences a nontrivial evolution pathway with the traditional apolar nematic phase completely suppressed, i.e., the NF phase nucleates directly from the isotropic liquid phase. The discovery of thte ferroelectric packing of oligomer/polymer rods not only offers the concept of extending the NF state to oligomers/polymers but also provides some previously overlooked insights in oxybenzoate-based liquid crystal polymer materials.

Photo-reconfigurable twisting structure in chiral liquid crystals triggered by photoresponsive surface.

Shape-transformable molecular additives with photoresponsivity, such as azobenzene or spiropyran, in matter are known to decrease the local order parameter and lead to drastic state variations under light irradiation. For example, a liquid crystalline state can be transformed to an isotropic liquid state by photo-exciting a tiny amount of azobenzene additives from trans- to cis-conformers. On the other hand, structural or shape transformation without changing the phase state is also intriguing since it offers an opportunity for manipulating specific structures. Here, we demonstrate an active control of the topology of chiral particle-like twisting structures, dubbed toron, by light. Interestingly, the individual twisting structure is fully reconfigurable between spherical and unique branched topological states. We reveal that the shape transformation is driven by the free-energy competition between the variation of surface anchoring strength and the elastic energy stored in the twisting structure. The mean-field simulation based on the Landau-de Gennes framework shows that the elastic anisotropy plays the dominant role in modifying the toron topology upon weak anchoring. The results offer a new path for understanding the process of topology-involved shape transformation and fabrication of novel functional materials.

Viscoelastic properties of a thioether-based heliconical twist-bend nematogen.

The twist-bend nematic (NTB) phase is one of the new types of nematics found recently, which possesses local nematic order with a heliconical orientational modulation at the nanoscale. Herein, we quantitatively determined, for the first time, the temperature-dependent elastic and viscosity properties in both the nematic (N) and NTB phases using a thioether-linked cyanobiphenyl dimer CBS7SCB exhibiting a broad temperature range of the NTB phase which is stable down to room temperature. In the N phase, the fundamental elastic moduli: splay and bend elastic moduli (K11 and K33, respectively) are found to be in the order of 10-12 N, and the effective rotational viscosity (γ1) is determined to be in the range of 5-200 mPa s. Meanwhile, the NTB phase is found to exhibit a compressive elastic modulus B in the order of several tens of kilopascals, the effective K11 in the order of 10-10-10-8 N, and a considerably large γ1 value of ∼68.7 Pa s right below the N-NTB phase transition. The present study provides insights into the comprehensive viscoelastic properties based on comparison of the obtained experimental data with not only the existing theoretical prediction but also the preceding experimental works.

Supramolecular Hexagonal Platelet Assemblies with Uniform and Precisely-Controlled Dimensions.

Geometric structures are commonly encountered in natural and designed systems. However, the bottom-up fabrication of regular geometric assemblies with precise dimensional control, especially from soft materials, poses an outstanding challenge in contemporary materials science and chemistry. Herein, we present a general method for the preparation of colloidally stable, hexagonal platelets via the formation of crystalline inclusion complexes of tris-o-phenylenedioxycyclotriphosphazene and block copolymers bearing interactive blocks. Dictated by the screw dislocation growth of inclusion complexes, uniform hexagonal platelets with precisely controllable dimensions can be prepared. This supramolecular assembly approach is further utilized to produce concentric hexagonal platelets via stepwise seeded growth from various inclusion complexes.

The role of structural anisotropy in the magnetooptical response of an organoferrogel with mobile magnetic nanoparticles.

We investigate the structure and the magnetooptical response of isotropic and anisotropic fibrillous organoferrogels with mobile magnetic nanoparticles (MNPs). We demonstrate that the presence of the gel network restricts the magnetooptical response of the ferrogel. Even though the ferrogel exhibits no magnetic hysteresis, an optical hysteresis has been found. This suggests that the magnetooptical response is primarily determined by the dynamics of self-assembly of the MNPs into shape-anisotropic agglomerates. Furthermore, we show that the optical anisotropy of the system can be fine-tuned by varying the concentration of the gelator and the MNPs, respectively. The optical response in structurally anisotropic gels becomes orientation-dependent, revealing an intricate interplay between the gel mesh and the MNPs.

Reversible Switching of the Magnetic Orientation of Titanate Nanosheets by Photochemical Reduction and Autoxidation.

Optical properties of aqueous colloidal dispersions of 2D electrolytes, if their aspect ratios are extra-large, can be determined by their orientation preferences. Recently, we reported that a colloidal dispersion of diamagnetic titanate(IV) nanosheets (TiIVNSs), when placed in a magnetic field, is highly anisotropic because TiIVNS anomalously orients its 2D plane orthogonal to the magnetic flux lines due to its large anisotropic magnetic susceptibility. Herein, we report a serendipitous finding that TiIVNSs can be in situ photochemically reduced into a paramagnetic species (TiIV/IIINSs), so that their preference of magnetic orientation changes from orthogonal to parallel. This transition distinctly alters the structural anisotropy and therefore optical appearance of the colloidal dispersion in a magnetic field. We also found that TiIV/IIINSs is autoxidized back to TiIVNSs under non-deaerated conditions. By using an elaborate setup, the dispersion of TiIVNSs serves as an optical switch remotely operable by magnet and light.

Thermodynamically Anchoring-Frustrated Surface to Trigger Bulk Discontinuous Orientational Transition.

Surface-specific liquid crystal (LC) nanostructures provide a unique platform for studying surface-wetting phenomena and also for technological applications. The most important studies on LC properties are related to bulk alignment, surface anchoring, and so on. Here, we study an LC system with a nematic liquid crystal (NLC) on a perfluoropolymer-coated substrate, in which a discontinuous bulk orientational transition has recently been found. Using free-energy analysis based on experimental results of the newly-conducted grazing-incidence X-ray diffraction (GI-XRD) measurements, we have confirmed a thermodynamic growth process of smectic liquid crystalline wetting nanosheets on the surface and successfully explained that a frustrated surface of planar and vertical anchoring states accompanied by an elastic energy cost kinetically triggers the bulk reorientation in the first-order manner. This interfacial bottom-up process may offer a general insight into how interfacial hierarchical molecular architectures alter the bulk properties of matter thermodynamically.

Photoresponsive stripe pattern in achiral azobenzene liquid crystals.

A periodic stripe pattern is found in the nematic phase close to the smectic phase of photoresponsive achiral liquid-crystalline compounds. The origin of the stripe patterns can be ascribed to an extremely large bent elastic constant K33 . In addition, we succeeded in controlling the pattern by the following two methods: 1) the stripe disappears by a trans-cis photoisomerization upon UV light irradiation and reappears upon light termination, and 2) the stripe pattern is stabilized over the whole nematic phase, at approximately 10 °C, by polymerization of the compounds.

Discontinuous thermal diffusivity change due to the anchoring transition of a liquid crystal on a perfluoropolymer surface.

Thermal diffusivity of a liquid crystal, 4'-butyl-4-heptyl-bicyclohexyl-4-carbonitrile, was measured using a temperature wave method. The liquid crystal was sandwiched by two glass substrates, which were treated with three different surface agents for providing distinct molecular orientations. Here, we demonstrate that: 1) a large thermal diffusivity anisotropy arising from different orientations, that is, planar and homeotropic states, was found in the nematic and smectic A phases; 2) when substrates were coated with a perfluoropolymer, abrupt changes of the thermal diffusivity were observed in the nematic phase both on cooling and heating due to the discontinuous anchoring transition between planar and homeotropic states. The temperature dependence of the thermal diffusivity anisotropy was well described by a power law, with an exponent of 0.27 according to the mean-field theory.