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Nematic liquid crystals exhibit nanosecond electro-optic response to an applied electric field which modifies the degree of orientational order without realigning the molecular orientation. However, this nanosecond electrically-modified order parameter (NEMOP) effect requires high driving fields, on the order of 108 V/m for a modest birefringence change of 0.01. In this work, we demonstrate that a nematic phase of the recently discovered ferroelectric nematic materials exhibits a robust and fast electro-optic response. Namely, a relatively weak field of 2×107 V/m changes the birefringence by ≈ 0.04 with field-on and -off times around 1 μs. This microsecond electrically modified order parameter (MEMOP) effect shows a greatly improved figure of merit when compared to other electro-optical switching modes in liquid crystals, including the conventional Frederiks effect, and has a potential for applications in fast electro-optical devices such as phase modulators, optical shutters, displays, and beam steerers. 

Ferroelectric nematic liquid crystals are formed by achiral molecules with large dipole moments. Their three-dimensional orientational order is described as unidirectionally polar. We demonstrate that the ground state of a flat slab of a ferroelectric nematic unconstrained by externally imposed alignment directions is chiral, with left- and right-handed twists of polarization. Although the helicoidal deformations and defect walls that separate domains of opposite handedness increase the elastic energy, the twists reduce the electrostatic energy and become weaker when the material is doped with ions. This work shows that the polar orientational order of molecules could trigger chirality in soft matter with no chemically induced chiral centers.

 

Elastic constants of splay K_11, twist K_22, and bend K_33 of nematic liquid crystals are often assumed to be equal to each other in order to simplify the theoretical description of complex director fields. Here we present examples of how the disparity of K_11 and K_33 produces effects that cannot be described in a one-constant approximation. In a lyotropic chromonic liquid crystal, nematic droplets coexisting with the isotropic phase change their shape from a simply-connected tactoid to a topologically distinct toroid as a result of temperature or concentration variation. The transformation is caused by the increase of the splay-to-bend ratio K_11/K_33. A phase transition from a conventional nematic to a twist-bend nematic implies that the ratio K_11/K_33 changes from very large to very small. As a result, the defects caused by an externally applied electric field change the deformation mode of optic axis from bend to splay. In the paraelectric-ferroelectric nematic transition, one finds an inverse situation: K_11/K_33 changes from small to large, which shapes the domain walls in the spontaneous electric polarization field as conic sections. The polarization field tends to be solenoidal, or divergence-free, a behavior complementary to irrotational curl-free director textures of a smectic A. 

Exotic structures with interesting physical and chemical properties can be achieved by self-organizing engineered building blocks. The central aim for self-assembly is to precisely control the position and orientation of individual building blocks. In this work, we use topological defects (disclinations) in nematic liquid crystals as templates to direct the self-assembly of colloidal particles into designable 3D structures. By photopatterning preprogrammed molecular orientations at two confining surfaces, we created pre-designable disclination networks and characterized their interactions with spherical colloidal particles. We find that colloidal particles are attracted to different disclinations depending on the orientation of the point defect (elastic dipole) around the colloids. We demonstrate that the positions, network structures, and orientation of the elastic dipoles of the colloidal chains can be pre-designed and reconfigured with remote illumination of polarized light. 

Tunable optical lenses are in great demand in modern technologies ranging from augmented and virtual reality to sensing and detection. In this work, electrically tunable microlenses based on a polymer‐stabilized chiral ferroelectric nematic liquid crystal are described. The power of the lens can be quickly (within 5 ms) varied by ≈500 diopters by ramping an in‐plane electric field from 0 to 2.5 V µm⁻¹. Importantly, within this relatively low‐amplitude field range, the lens is optically isotropic; thus, its focal length is independent of the polarization of incoming light. This remarkable performance combines the advantages of electrically tuned isotropic lenses and the field‐controlled shape of the lens, which are unique properties of chiral ferroelectric nematic liquid crystals and have no counterpart in other liquid crystals. The achieved lens performance represents a significant step forward as compared to liquid lenses controlled by electrowetting and opens new possibilities in various applications such as biomimetic optics, security printing, and solar energy concentration.

 

Azobenzene-based chiral dopants in cholesteric liquid crystals are of interest since the properties they induce in the liquid crystal could be tuned photochemically. Here, we use a substituted binaphthyl with a halogenated azobenzene as a chiral dopant to induce a photoswitchable cholesteric phase in the nematic 4-n-pentyl-4’-cyanobiphenyl. The azobenzene group chemically attached to the chiral dopant undergoes isomerization from trans to cis upon irradiation with green light (wavelength 535 nm), and from cis to trans upon irradiation with blue light (wavelength 450 nm). The transition between the two isomers causes helicity inversion of the cholesteric, with a left-handed trans isomer and a right-handed cis isomer. We report on the kinetics of photoisomerization of both processes (trans-to-cis and cis-to-trans) in the nematic host by following the pitch evolution over time. We show that the kinetic mechanism corresponds to a two-step process: a first-order isomerization followed by a second-order autocatalytic isomerization. This mechanism differs from the typical first-order kinetics for cis-to-trans or trans-to-cis isomerization in azobenzenes. The autocatalytic process is attributed to interactions between the chiral dopant and the nematic host. 

We explore the structures and confinement-induced edge dislocations in Grandjean-Cano wedge cells filled with the recently discovered chiral ferroelectric nematic (N_F^*) and chiral antiferroelectric smectic-Z 〖(SmZ〗_A^*). The chiral mixture is formed by DIO mesogen doped with a chiral additive. Wedge cells with parallel and antiparallel rubbing at the opposite plates show quantitatively different structures which is attributed to the polar in-plane anchoring of the spontaneous polarization at the rubbed substrates. The helical pitch shows a non-monotonous temperature dependence upon cooling, increasing as the temperature is lowered to the N^*-SmZ_A^* phase transition. The SmZ_A^* formed from an untwisted N^* in the thin portion of the wedge shows a bookshelf (BK) geometry, whereas the twisted N^* transforms into a twisted planar (PA) SmZ_A^* structure. In the N_F^* phase, the untwisted N^* becomes twisted in a wedge with antiparallel assembly of plates and monodomain in wedges with parallel assembly. The twisted regions of N_F^* show only one type of Grandjean zones separated by thick edge dislocations with Burgers vector b=P; the neighboring regions differ by 2π- twist. 

Lenses with tunable focal lengths play important roles in nature as well as modern technologies. In recent years, the demand for electrically tunable lenses and lens arrays has grown, driven by the increasing interest in augmented and virtual reality, as well as sensing applications. In this paper, we present a novel type of electrically tunable microlens utilizing polymer-stabilized chiral ferroelectric nematic liquid crystal. The lens offers a fast response time (5ms) and the focal length can be tuned by applying an in-plane electric field. The electrically induced change in the lens shape, facilitated by the remarkable sensitivity of the chiral ferroelectric nematic to electric fields, enables the tunable focal length capability. The achieved performance of this lens represents a significant advancement compared to electrowetting-based liquid lenses and opens exciting prospects in various fields, including biomimetic optics, security printing, solar energy concentration, and AR/VR devices. 

Morphogenesis of living systems involves topological shape transformations which are highly unusual in the inanimate world. Here, we demonstrate that a droplet of a nematic liquid crystal changes its equilibrium shape from a simply connected tactoid, which is topologically equivalent to a sphere, to a torus, which is not simply connected. The topological shape transformation is caused by the interplay of nematic elastic constants, which facilitates splay and bend of molecular orientations in tactoids but hinders splay in the toroids. The elastic anisotropy mechanism might be helpful in understanding topology transformations in morphogenesis and paves the way to control and transform shapes of droplets of liquid crystals and related soft materials.