Self-assembled liquid crystal architectures for soft matter photonics

Self-assembled architectures of soft matter have fascinated scientists for centuries due to their unique physical properties originated from controllable orientational and/or positional orders, and diverse optic and photonic applications. If one could know how to design, fabricate, and manipulate these optical microstructures in soft matter systems, such as liquid crystals (LCs), that would open new opportunities in both scientific research and practical applications, such as the interaction between light and soft matter, the intrinsic assembly of the topological patterns, and the multidimensional control of the light (polarization, phase, spatial distribution, propagation direction). Here, we summarize recent progresses in self-assembled optical architectures in typical thermotropic LCs and bio-based lyotropic LCs. After briefly introducing the basic definitions and properties of the materials, we present the manipulation schemes of various LC microstructures, especially the topological and topographic configurations. This work further illustrates external-stimuli-enabled dynamic controllability of self-assembled optical structures of these soft materials, and demonstrates several emerging applications. Lastly, we discuss the challenges and opportunities of these materials towards soft matter photonics, and envision future perspectives in this field.

The fate of liquid crystal topological defects on chemically patterned surfaces during phase transitions

Controlling topological defects in liquid crystals (LCs) is an essential element in the development of areas such as directed self-assembly and micropatterning materials. However, during the phase transition on confined patterned surfaces, how the morphologies in one liquid crystalline phase change from deformations or defects into another phase is much less known. Here, we examine the fate of defects in a LC confined on a patterned surface during smectic-A-nematic and nematic-isotropic phase transitions, using experiments and simulation analyses. Upon heating from smectic-A to nematic, a Toric focal conical domain (TFCD) melts into a +1 converging boojum defect, which then transitioned into a concentric configuration as temperature increases, attributed to a steeper decrease of the bend and twist modulus compared to splay modulus. During cooling, TFCDs are developed from two distinct pathways depending on the cooling rates. Our continuum simulation recapitulates these transformations and provides elastic constant-based explanations for the two pathways. Although the phase transition pathways of defects are independent of the geometry of the confined patterns, the arrangement of FCDs is highly dependent on the size and shape of the patterns. Taken together, this simple approach offers promising opportunities for tuning the micro- or nano-patterning of topological defects in liquid crystals.

Focal conic flowers, dislocation rings, and undulation textures in smectic liquid crystal Janus droplets

Liquid crystalline phases of matter often exhibit visually stunning patterns or textures. Mostly, these liquid crystal (LC) configurations are uniquely determined by bulk LC elasticity, surface anchoring conditions, and confinement geometry. Here, we experimentally explore defect textures of the smectic LC phase in unique confining geometries with variable curvature. We show that a complex range of director configurations can arise from a single system, depending on sample processing procedures. Specifically, we report on LC textures in Janus drops comprised of silicone oil and 8CB in its smectic-A LC phase. The Janus droplets were made in aqueous suspension using solvent-induced phase separation. After drop creation, smectic layers form in the LC compartment, but their self-assembly is frustrated by the need to accommodate both the bowl-shaped cavity geometry and homeotropic (perpendicular) anchoring conditions at boundaries. A variety of stable and metastable smectic textures arise, including focal conic domains, dislocation rings, and undulations. We experimentally characterize their stabilities and follow their spatiotemporal evolution. Overall, a range of fabrication kinetics produce very different intermediate and final states. The observations elucidate assembly mechanisms and suggest new routes for fabrication of complex soft material structures in Janus drops and other confinement geometries.

Hierarchical self-assembling and helical structure in focal conic domains in meniscus of ferroelectric liquid crystal

We investigate experimentally the formation of focal conic domains of the ferroelectric phase of a liquid crystal, chiral smectic C (SmC^{*}), in the meniscus geometry. The meniscus geometry is formed in the gap between two glass plates which are placed on a common substrate. This gap is called here a physical cavity. Focal conic domains (FCDs) in the physical cavity with dimensions of micrometer scale are investigated under an optical polarizing microscope which enables us to extract the information on the helical structure formation in the constraint and gradient topological meniscus interface. The helical pitch in the FCD is observed to be shorter than in planar confined geometry. A crucial phenomenon of unwrapping and wrapping of helical structure from one FCD to another is also observed. In-plane application of an electric field on a FCD revealed the asymmetric helical unwinding process whereas an increase in temperature has shown symmetrical unwinding. The helical structure based observation is significant for understanding the ferroelectric phase in focal conic domains and their application in microlenses and optical components.

Making Smectic Defect Patterns Electrically Reversible and Dynamically Tunable Using In Situ Polymer-Templated Nematic Liquid Crystals

Shaping liquid crystals (LCs) into arrays of defect patterns enables the design of composite materials with new stimuli-responsive properties. Self-assembled defect assemblies that may arise in layered smectic A (SmA) LCs such as focal conic domains (FCDs), exhibit remarkable optical features and abilities for ordering nanoparticles. However, such SmA defect patterns are essentially electrically irreversible, which currently limits their adjustability in a dynamic way. Here, in situ polymerization of the texture of SmA FCDs allows transferring them into more electrically responsive LC phases, such as nematic, making possible a dynamic switch between different textural and optical states of FCDs in a reversible manner with voltage. Moreover, the method readily enables to program the operating temperature range of the polymer/LC composite from its chemical composition, adapting the system to various potential uses. This approach may increment new applications of SmA defect patterns such as voltage-tunable privacy layers and may further inspire the design of LC-based nanostructured composite and hybrid materials with new functions that can be dynamically tuned with voltage.

Converging Microlens Array Using Nematic Liquid Crystals Doped with Chiral Nanoparticles

Nematic liquid crystals of achiral molecules or racemic mixtures of chiral ones form flat films when suspended in submillimeter size grids and submerged under water. Recently, it has been shown (Popov et al., 2017) that films of nematic liquid crystals doped with chiral molecules adopt biconvex lens shapes underwater. The curved shape together with degenerate planar anchoring leads to a radial variation of the optical axis along the plane of the film, providing a Pancharatnam-Berry-type phase lens that modifies geometric optical imaging. Here, we describe nematic liquid crystal microlenses formed by the addition of chiral nanoparticles. It is found that the helical twisting power of the nanoparticles, the key factor to form the lens, is about 400 μm^-1, greater than that of the strongest molecular chiral dopants. We demonstrate imaging capabilities and measure the shape as well as the focal length of the chiral nanoparticle-doped liquid crystal lens. We show that measuring the shape of the lens allows one to calculate the helical pitch of the chiral nematic liquid crystal and thus determine the helical twisting power of the chiral ligand-capped nanoparticles. Such measurements require the use of only nanograms of chiral nanoparticles, which is 3 orders of magnitude less than that required by conventional techniques. Since NPs are sensitive to external stimuli such as light and electric and magnetic fields, the use of chiral NPs may allow the achievement of tunable optical properties for such microlens arrays.

Hierarchical assembly of smectic liquid crystal defects at undulated interfaces

The assembly of topological defects in liquid crystals has drawn significant interest in the last decade due to their ability to trap colloidal objects and direct their arrangements. They have also brought about a high impact in modern technologies, in particular in optics, e.g., microlens arrays, soft lithography templates, and optically selective masks. Here we study the formation of defects in smectic A liquid crystal with hybrid texture at undulated surfaces. We investigate the role of surface topography on the organization of focal conic domains (FCDs) in smectic films. We demonstrate new methods for assembling FCDs and disclinations into hierarchical structures. When the liquid crystal is heated to the nematic phase, we observe stable defect lines forming at specific locations. These defects are created to satisfy anchoring conditions and the geometry of confinement imposed by the boundaries. Once the liquid crystal is cooled to the smectic A phase, the disclinations maintain their positions, but periodic structures of reversible FCDs facing opposite directions arise between them. We report the correlation between the size of these FCDs and their eccentricities with the morphology of the interface. This work paves the way for creating new procedures to control the assembly of functional nanomaterials into tunable assemblies that may find relevance in the field of energy technology including in optoelectronic and photonic applications.

Self-Assembled Asymmetric Microlenses for Four-Dimensional Visual Imaging

Visual imaging that can extract three-dimensional (3D) space or polarization information on the target is essential in broad sciences and technologies. The simultaneous acquisition of them usually demands expensive equipment and sophisticated operations. Therefore, it is of great significance to exploit convenient approaches for four-dimensional (3D and polarization) visual imaging. Here, we present an efficient solution based on self-assembled asymmetric liquid crystal microlenses, with freely manipulated phase profiles and symmetry-breaking properties. Accordingly, characteristics of multifocal functionality and polarization selectivity are exhibited, along with the underlying mechanisms. Moreover, with a specific sample featured by radially increased unit sizes and azimuthally varied domain orientations, the discriminability of four-dimensional information is extracted in a single snapshot, via referring to the coordinates of the clearest images. Demultiplexing of depth/polarization information is also demonstrated. This work will unlock a variety of revolutionary apparatuses and lighten extensive applications.

Instabilities in the electric Freedericksz state of the twist-bend nematic liquid crystal CB7CB

We report on the instabilities in the Freedericksz state of the twist-bend nematic (NTB) liquid crystal 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB). The quasi homeotropic NTB state, into which a planar (untwisted or 90°-twisted) nematic CB7CB layer transits under a strong electric field, is found to be unstable despite the material being dielectrically positive. Close to the NTB melting point, destabilization occurs through the formation of metastable toric focal conic domains (TFCDs) that, in time, transform into parabolic focal conic domains (PFCDs) with the confocal parabolae in vertical planes through the layer normal. This transformation occurs by a novel process of continued dissociation of TFCDs. We outline how the extended Volterra process could help in a general appreciation of focal conic defects in the NTB phase. At relatively lower temperatures, stripes develop competingly with TFCDs. Identifiable as oily streaks, they are both localized and polarity sensitive; they form close to the substrates; and in low frequency square wave fields, they get suppressed at the cathode and augmented at the anode at each polarity switch. The study also dwells on the N-NTB-N sandwich region, found between the N and NTB states under a small temperature gradient.

Directed Self-Assembly of Topological Defects of Liquid Crystals

One of the alluring aspects of liquid crystals (LCs) is their readily controllable self-assembly behavior, leading to comprehension of complex topological structures and practical patterning applications. Here, we report on manipulating various kinds of topological defects by adopting an imprinted polymer-based soft microchannel that simultaneously imposes adjustable surface anchoring, confinement, and uniaxial alignment. Distinctive molecular orientation could be achieved by varying the surface anchoring conditions at the sidewall polymer and the rubbing directions on the bottom layer. On this pioneering platform, a common LC material, 8CB (4'-n-octyl-4-cyano-biphenyl), was placed where various topological defect domains were generated in a periodic arrangement. The experimental results showed that our platform can change the packing behavior and even the shape of topological defects by varying the rubbing condition. We believe that this facile tool to modulate surface boundary conditions combined with topographic confinement can open a way to use LC materials in potential optical and patterning applications.

Polymer-Stabilized Micropixelated Liquid Crystals with Tunable Optical Properties Fabricated by Double Templating

Self-organized nano- and microstructures of soft materials are attracting considerable attention because most of them are stimuli-responsive due to their soft nature. In this regard, topological defects in liquid crystals (LCs) are promising not only for self-assembling colloids and molecules but also for electro-optical applications such as optical vortex generation. However, there are currently few bottom-up methods for patterning a large number of defects periodically over a large area. It would be highly desirable to develop more effective techniques for high-throughput and low-cost fabrication. Here, a micropixelated LC structure consisting of a square array of topological defects is stabilized by photopolymerization. A polymer network is formed on the structure of a self-organized template of a nematic liquid crystal (NLC), and this in turn imprints other nonpolymerizable NLC molecules, which maintains their responses to electric field and temperature. Photocuring of specific local regions is used to create a designable template for the reproducible self-organization of defects. Moreover, a highly diluted polymer network (≈0.1 wt% monomer) exhibits instant on-off switching of the patterns. Beyond the mere stabilization of patterns, these results demonstrate that the incorporation of self-organized NLC patterns offers some unique and unconventional applications for anisotropic polymer networks.

Morphogenesis of liquid crystal topological defects during the nematic-smectic A phase transition

The liquid crystalline phases of matter each possess distinct types of defects that have drawn great interest in areas such as topology, self-assembly and material micropatterning. However, relatively little is known about how defects in one liquid crystalline phase arise from defects or deformations in another phase upon crossing a phase transition. Here, we directly examine defects in the in situ thermal phase transition from nematic to smectic A in hybrid-aligned liquid crystal droplets on water substrates, using experimental, theoretical and numerical analyses. The hybrid-aligned nematic droplet spontaneously generates boojum defects. During cooling, toric focal conic domains arise through a sequence of morphological transformations involving nematic stripes and locally aligned focal conic domains. This simple experiment reveals a surprisingly complex pathway by which very different types of defects may be related across the nematic–smectic A phase transition, and presents new possibilities for controlled deformation and patterning of liquid crystals.

Defects in liquid crystals play a central role in determining their structural and dynamic properties, whilst it is challenging to characterize the defects at a molecule level. Here, Gim et al. trace the evolution pathway of defects during a phase transition from a nematic to a smectic state.

Chiral nematic liquid crystal microlenses

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water. On addition of chiral dopants to the liquid crystal, the films exhibit optical textures with concentric ring patterns and radial variation of the birefringence color. Both are related to a biconvex shape of the chiral liquid crystal film; the rings are due to interference. The curvature radii of the biconvex lens array are in the range of a few millimeters. This curvature leads to a radial variation of the optical axis along the plane of the film. Such a Pancharatnam-type phase lens dominates the imaging and explains the measured focal length of about one millimeter. To our knowledge, these are the first spontaneously formed Pancharatnam devices. The unwinding of the helical structure at the grid walls drives the lens shape. The relation between the lens curvature and material properties such as helical pitch, the twist elastic constant, and the interfacial tensions, is derived. This simple, novel method for spontaneously forming microlens arrays can also be used for various sensors.

Smectic Layer Origami via Preprogrammed Photoalignment

Hierarchical architecture is of vital importance in soft materials. Focal conic domains (FCDs) of smectic liquid crystals, characterized by an ordered lamellar structure, attract intensive attention. Simultaneously tailoring the geometry and clustering characteristics of FCDs remains a challenge. Here, the 3D smectic layer origami via a 2D preprogrammed photoalignment film is accomplished. Full control of hierarchical superstructures is demonstrated, including the domain size, shape, and orientation, and the lattice symmetry of fragmented toric FCDs. The unique symmetry breaking of resultant superstructures combined with the optical anisotropy of the liquid crystals induces an intriguing polarization-dependent diffraction. This work broadens the scientific understanding of self-assembled soft materials and may inspire new opportunities for advanced functional materials and devices.

Orientation Control of Smectic Liquid Crystals via a Combination Method of Topographic Patterning and In-Plane Electric Field Application for a Linearly Polarized Illuminator

We dynamically controlled the configuration of layering structures built by smectic A liquid crystal molecules using the combination method of the microchannel confinement and the in-plane electric field to realize the linearly polarized illuminator and bistable structures. Once a mild in-plane electric field (∼30 V) is applied between polymeric walls, the layer configuration was changed from the toric focal conic domains to periodic zigzag patterns of alternatively packed focal conic domains. The transformed zigzag patterns maintained their structures even after turning off the applied electric fields, revealing the ability for use in a bistable memory device. Indeed, a strong electric field (∼100 V) can make unidirectionally aligned LC molecules along with the applied electric field via zigzag patterns, and electro-optical performance of resultant textures when the sample is mixed with fluorescent dyes was characterized to show a linearly polarized light illuminator. Our electric field in and on the confined geometries will be used in the fabrication of functional structures built by polar soft materials which can broaden applications in patterning platforms and efficient electro-optical devices in the near future.

Creation of liquid-crystal periodic zigzags by surface treatment and thermal annealing

The orientation control of soft matter to create a large area single domain is one of the most exciting research topics in materials science. Recently, this effort has been extended to fabricate two- or three-dimensional structures for electro-optical applications. Here, we create periodic zigzag structures in liquid crystals (LCs) using a combination of surface treatment and thermal annealing. The LC molecules in the nematic (N) phase were initially guided by the alignment layer of rubbed polymers, which were quenched and subsequently annealed in the smectic A (SmA) phase to create periodic zigzag structures that represent modulated layer structures. Direct investigation of the zigzags was performed using microscopy and diffraction techniques, showing the alternately arranged focal conic domains (FCDs) formed. The resulting macroscopic periodic structures will be of interest in further studies of the physical properties of soft matters.

Smectic Gardening on Curved Landscapes

Focal conic domains (FCDs) form in smectic-A liquid crystal films with hybrid anchoring conditions with eccentricity and size distribution that depend strongly on interface curvature. Assemblies of FCDs can be exploited in settings ranging from optics to material assembly. Here, using micropost arrays with different shapes and arrangement, we assemble arrays of smectic flower patterns, revealing their internal structure as well as defect size, location, and distribution as a function of interface curvature, by imposing positive, negative, or zero Gaussian curvature at the free surface. We characterize these structures, relating free surface topography, substrate anchoring strength, and FCD distribution. Whereas the largest FCDs are located in the thickest regions of the films, the distribution of sizes is not trivially related to height, due to Apollonian tiling. Finally, we mold FCDs around microposts of complex shape and find that FCD arrangements are perturbed near the posts, but are qualitatively similar far from the posts where the details of the confining walls and associated curvature fields decay. This ability to mold FCD defects into a variety of hierarchical assemblies by manipulating the interface curvature paves the way to create new optical devices, such as compound eyes, via a directed assembly scheme.

Periodic arrays of liquid crystalline torons in microchannels

We have fabricated very well ordered periodic arrays of torons using cholesteric liquid crystal phase (CLC) in microchannels.

Three-dimensional textures and defects of soft material layering revealed by thermal sublimation

Layering is found and exploited in a variety of soft material systems, ranging from complex macromolecular self-assemblies to block copolymer and small-molecule liquid crystals. Because the control of layer structure is required for applications and characterization, and because defects reveal key features of the symmetries of layered phases, a variety of techniques have been developed for the study of soft-layer structure and defects, including X-ray diffraction and visualization using optical transmission and fluorescence confocal polarizing microscopy, atomic force microscopy, and SEM and transmission electron microscopy, including freeze-fracture transmission electron microscopy. Here, it is shown that thermal sublimation can be usefully combined with such techniques to enable visualization of the 3D structure of soft materials. Sequential sublimation removes material in a stepwise fashion, leaving a remnant layer structure largely unchanged and viewable using SEM, as demonstrated here using a lamellar smectic liquid crystal.

Topographically induced hierarchical assembly and geometrical transformation of focal conic domain arrays in smectic liquid crystals

Controlling topological defects in 3D liquid crystal phases is a crucial element in the development of novel devices, from blue-phase displays to passive biochemical sensors. However, it remains challenging to realize the 3D topological conditions necessary to robustly and arbitrarily direct the formation of defects. Here, using a series of short pillar arrays as topological templates, we demonstrate the hierarchical assembly of focal conic domains (FCDs) in smectic-A liquid crystals that break the underlying symmetry of the pillar lattice, exhibit tunable eccentricity, and together develop a nontrivial yet organized array of defects. The key to our approach lies in the selection of the appropriate ratio of the size of focal domain to the dimension of pillars such that the system favors the "pinning" of FCD centers near pillar edges while avoiding the opposing effect of confinement. Our study unequivocally shows that the arrangement of FCDs is strongly influenced by the height and shape of the pillars, a feature that promotes both a variety of nontrivial self-assembled lattice types and the attraction of FCD centers to pillar edges, especially at regions of high curvature. Finally, we propose a geometric model to reconstruct the smectic layer structure in the gaps between neighboring FCDs to estimate the energetic effects of nonzero eccentricity and assess their thermodynamic stability.

Wavelength-scale lens microscopy via thermal reshaping of colloidal particles

Lenses are by far the most simple tools for visualization. Although they are intrinsically limited in resolution, recent efforts have aimed at focusing visible light in micro-scale lenses with subwavelength resolution, triggering an intense interest in further improving and understanding their performances. Herein, we report on a distinctive library of wavelength-scale solid immersion lenses facilitated the self-assembly of polystyrene colloidal particles. The thermally activated structural changes in polystyrene colloidal spheres directly impact the optical performance of the obtained lenses. Similar melting dynamics is observed for spheres of various size spheres at different temperatures. This allows precise control of the contact angle spanning a broad range from 180° to <20°. The fabricated lenses display deviations from the ray optics, allowing us to resolve features as small as 180 nm using a simple microscopy setup. We succeed in proper self-assembly of the colloidal lenses that enables large-area optical nanoscopy through simple and reliable experimental protocols. The limitations and the artifacts of the present technique are discussed.

Pillar-assisted epitaxial assembly of toric focal conic domains of smectic-a liquid crystals

SU-8 pillar-assisted epitaxial assembly of toric focal conic domains (TFCDs) arrays of smectic-A liquid crystals is studied. The 3D nature of the pillar array is crucial to confine and direct the formation of TFCDs on the top of each pillar and between neighboring pillars, leading to highly ordered square and hexagonal array TFCDs. Excellent agreement between the experimentally obtained critical pillar diameter and elasticity calculation is found.