Self-assembled liquid crystal architectures for soft matter photonics

Responsive Liquid Crystal Network Microstructures with Customized Shapes and Predetermined Morphing for Adaptive Soft Micro-Optics

Stimuli-responsive materials have garnered substantial interest in recent years, particularly liquid crystal networks (LCNs) with sophisticatedly designed structures and morphing capabilities. Extensive efforts have been devoted to LCN structural designs spanning from two-dimensional (2D) to three-dimensional (3D) configurations and their intricate morphing behaviors through designed alignment. However, achieving microscale structures and large-area preparation necessitates the development of novel techniques capable of facilely fabricating LCN microstructures with precise control over both overall shape and alignment, enabling a 3D-to-3D shape change. Herein, a simple and cost-effective in-cell soft lithography (ICSL) technique is proposed to create LCN microstructures with customized shapes and predesigned morphing. The ICSL technique involves two sequential steps: fabricating the desired microstructure as the template by using the photopolymerization-induced phase separation (PIPS) method and reproducing the LCN microstructures through templating. Meanwhile, surface anchoring is employed to design and achieve molecular alignment, accommodating different deformation modes. With the proposed ICSL technique, cylindrical and spherical microlens arrays (CMLAs and SMLAs) have been successfully fabricated with stimulus-driven polarization-dependent focusing effects. This technique offers distinct advantages including high customizability, large-area production, and cost-effectiveness, which pave a new avenue for extensive applications in different fields, exemplified by adaptive soft micro-optics and photonics.

3D Directional Assembly of Liquid Crystal Molecules

The precise construction of hierarchically long-range ordered structures using molecules as fundamental building blocks can fully harness their anisotropy and potential. However, the 3D, high-precision, and single-step directional assembly of molecules is a long-pending challenge. Here, a 3D directional molecular assembly strategy via femtosecond laser direct writing (FsLDW) is proposed and the feasibility of this approach using liquid crystal (LC) molecules as an illustrative example is demonstrated. The physical mechanism for femtosecond (fs) laser-induced assembly of LC molecules is investigated, and precise 3D arbitrary assembly of LC molecules is achieved by defining the discretized laser scanning pathway. Additionally, an LC-based Fresnel zone plate array with polarization selection and colorization imaging functions is fabricated to further illustrate the potential of this method. This study not only introduces a 3D high-resolution alignment method for LC-based functional devices but also establishes a universal protocol for the precise 3D directional assembly of anisotropic molecules.

Omnidirectional color wavelength tuning of stretchable chiral liquid crystal elastomers

Wavelength-tunable structural colors using stimuli-responsive materials, such as chiral liquid crystals (CLCs), have attracted increasing attention owing to their high functionality in various tunable photonic applications. Ideally, on-demand omnidirectional wavelength control is highly desirable from the perspective of wavelength-tuning freedom. However, despite numerous previous research efforts on tunable CLC structural colors, only mono-directional wavelength tuning toward shorter wavelengths has been employed in most studies to date. In this study, we report the ideally desired omnidirectional wavelength control toward longer and shorter wavelengths with significantly improved tunability over a broadband wavelength range. By using areal expanding and contractive strain control of dielectric elastomer actuators (DEAs) with chiral liquid crystal elastomers (CLCEs), simultaneous and omnidirectional structural color-tuning control was achieved. This breakthrough in omnidirectional wavelength control enhances the achievable tuning freedom and versatility, making it applicable to a broad range of high-functional photonic applications.

Two-dimensional beam scanning by tunable photonic spin Hall effect

To the best of our knowledge, a novel tunable photonic spin Hall effect is proposed based on a pair of liquid crystal Pancharatnam-Berry (PB) lenses. Owing to the spin-dependent geometric phases, a PB lens focus or defocus the incident light field according to its spin angular momentum. By cascading two PB lenses with a small gap, the focus and defocus effects can be suppressed, and the transmitted light fields with opposite spin will be deflected toward opposite directions when the two PB lenses have a relative lateral displacement. The deflection angles vary linearly with the displacements, thus double-lines two-dimensional continuous beam scanning is achieved with a scanning angle of 39o × 39° and a beam diverging angle of 0.028o × 0.028°. The scanning beam is used to write different patterns on a 200 nm thick gold film. We believe this beam scanning system can find wide applications ranging from laser processing, Lidar, particle manipulation, to free space optical communications.

The association of structural chirality and liquid crystal anchoring in polymer stabilized cholesteric liquid crystals

Polymer stabilized cholesteric liquid crystals (PSCLCs) are electrically reconfigurable reflective elements. Prior studies have hypothesized and indirectly confirmed that the electro-optic response of these composites is associated with the electrically mediated distortion of the stabilizing polymer network. The proposed mechanism is based on the retention of structural chirality in the polymer stabilizing network, which upon deformation is spatially distorted, which accordingly affects the pitch of the surrounding low molar-mass liquid crystal host. Here, we utilize fluorescent confocal polarized microscopy to directly assess the electro-optic response of PSCLCs. By utilizing dual fluorescent probes, sequential imaging experiments confirm that the periodicity of the polymer stabilizing network matches that of the low molar-mass liquid crystal host. Further, we isolate distinct ion-polymer interactions that manifest in certain photopolymerization conditions.

Machine learning methods for liquid crystal research: phases, textures, defects and physical properties

Liquid crystal materials, with their unique properties and diverse applications, have long captured the attention of researchers and industries alike. From liquid crystal displays and electro-optical devices to advanced sensors and emerging technologies, the study and application of liquid crystals continue to be of paramount importance in the fields of materials science, chemistry and physics. With the ever-increasing complexity and diversity of liquid crystal materials, researchers face new challenges in understanding their behaviors, properties, and potential applications. On the other hand, machine learning, a rapidly evolving interdisciplinary field at the intersection of computer science and data analysis, has already become a powerful tool for unraveling implicit correlations and predicting new properties of a wide variety of physical and chemical systems and structures. Here we aim to consider how machine learning methods are suitable for solving fundamental problems in the field of liquid crystals and what are the advantages of this artificial intelligence based approach.

Solid-State Cholesteric Liquid Crystals as an Emerging Platform for the Development of Optical Photonic Sensors

Over the past decade, solid-state cholesteric liquid crystals (CLCsolid ) have emerged as a promising photonic material, heralding new opportunities for the advancement of optical photonic biosensors and actuators. The periodic helical structure of CLCsolid s gives rise to their distinctive capability of selectively reflecting incident radiation, rendering them highly promising contenders for a wide spectrum of photonic applications. Extensive research is conducted on utilizing CLCsolid 's optical characteristics to create optical sensors for bioassays, diagnostics, and environmental monitoring. This review provides an overview of emerging technologies in the field of interpenetrating polymeric network-CLCsolid (IPN) and CLCsolid -based optical sensors, including their structural designs, processing, essential materials, working principles, and fabrication methodologies. The review concludes with a forward-looking perspective, addressing current challenges and potential trajectories for future research.

The Influence of the Molecular Structure of Compounds on Their Properties and the Occurrence of Chiral Smectic Phases

We have designed new chiral smectic mesogens with the -CH2O group near the chiral center. We synthesized two unique rod-like compounds. We determined the mesomorphic properties of these mesogens and confirmed the phase identification using dielectric spectroscopy. Depending on the length of the oligomethylene spacer (i.e., the number of methylene groups) in the achiral part of the molecules, the studied materials show different phase sequences. Moreover, the temperature ranges of the observed smectic phases are different. It can be seen that as the length of the alkyl chain increases, the liquid crystalline material shows more mesophases. Additionally, its clearing (isotropization) temperature increases. The studied compounds are compared with the structurally similar smectogens previously synthesized. The helical pitch measurements were performed using the selective reflection method. These materials can be useful and effective as chiral components and dopants in smectic mixtures targeted for optoelectronics and photonics.

Color liquid crystal grating based color holographic 3D display system with large viewing angle

Holographic 3D display is highly desirable for numerous applications ranging from medical treatments to military affairs. However, it is challenging to simultaneously achieve large viewing angle and high-fidelity color reconstruction due to the intractable constraints of existing technology. Here, we conceptually propose and experimentally demonstrate a simple and feasible pathway of using a well-designed color liquid crystal grating to overcome the inevitable chromatic aberration and enlarge the holographic viewing angle, thus enabling large-viewing-angle and color holographic 3D display. The use of color liquid crystal grating allows performing secondary diffraction modulation on red, green and blue reproduced images simultaneously and extending the viewing angle in the holographic 3D display system. In principle, a chromatic aberration-free hologram generation mechanism in combination with the color liquid crystal grating is proposed to pave the way for on such a superior holographic 3D display. The proposed system shows a color viewing angle of ~50.12°, which is about 7 times that of the traditional system with a single spatial light modulator. This work presents a paradigm for achieving desirable holographic 3D display, and is expected to provide a new way for the wide application of holographic display.

Fabrication of Oriented Polycrystalline MOF Superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.

Optical Manipulation of Soft Matter

Optical manipulation has emerged as a pivotal tool in soft matter research, offering superior applicability, spatiotemporal precision, and manipulation capabilities compared to conventional methods. Here, an overview of the optical mechanisms governing the interaction between light and soft matter materials during manipulation is provided. The distinct characteristics exhibited by various soft matter materials, including liquid crystals, polymers, colloids, amphiphiles, thin liquid films, and biological soft materials are highlighted, and elucidate their fundamental response characteristics to optical manipulation techniques. This knowledge serves as a foundation for designing effective strategies for soft matter manipulation. Moreover, the diverse range of applications and future prospects that arise from the synergistic collaboration between optical manipulation and soft matter materials in emerging fields are explored.

Simultaneous chiral polarization and edge enhancement imaging enabled by a single geometric-phase-based element

In this Letter, we propose a multifunctional imaging system enabled by a single geometric-phase-based liquid crystal (LC) element, which integrates chiral polarization and edge enhancement imaging. The element is located at the frequency domain plane in a 4F imaging system, and the phase profile of the element consists of a fork grating in the x direction and a grating in the y direction, which provide edge enhancement and chiral polarization imaging capabilities. Benefiting from the tunable property of the LC, the system can be switched from a polarization and edge imaging mode to the normal conventional imaging mode which is capable of conveniently acquiring the needed image information. Experiments demonstrate that the system can easily achieve multifunctional and switchable imaging, which agrees well with our design, and our LC element can work in the broadband spectrum because of the geometric phase modulation. The multifunctional strategy used here can effectively avoid the need to increase the size of the original microscopic system and the need for additional mechanical rotation of components. We believe that the proposed system with the additional advantages of electric control and tunability can find applications in biological imaging, medical detection, and optical computing.

Laser-Written Tunable Liquid Crystal Aberration Correctors

In this Article, we present a series of novel laser-written liquid crystal (LC) devices for aberration control for applications in beam shaping or aberration correction through adaptive optics. Each transparent LC device can correct for a chosen aberration mode with continuous greyscale tuning up to a total magnitude of more than 2π radians phase difference peak to peak at a wavelength of λ = 660 nm. For the purpose of demonstration, we present five different devices for the correction of five independent Zernike polynomial modes (although the technique could readily be used to manufacture devices based on other modes). Each device is operated by a single electrode pair tuned between 0 and 10 V. These devices have potential as a low-cost alternative to spatial light modulators for applications where a low-order aberration correction is sufficient and transmissive geometries are required.

Saddles as rotational locks within shape-assisted self-assembled nanosheets

Two-dimensional (2D) materials are a key target for many applications in the modern day. Self-assembly is one approach that can bring us closer to this goal, which usually relies upon strong, directional interactions instead of covalent bonds. Control over less directional forces is more challenging and usually does not result in as well-defined materials. Explicitly incorporating topography into the design as a guiding effect to enhance the interacting forces can help to form highly ordered structures. Herein, we show the process of shape-assisted self-assembly to be consistent across a range of derivatives that highlights the restriction of rotational motion and is verified using a diverse combination of solid state analyses. A molecular curvature governed angle distribution nurtures monomers into loose columns that then arrange to form 2D structures with long-range order observed in both crystalline and soft materials. These features strengthen the idea that shape becomes an important design principle leading towards precise molecular self-assembly and the inception of new materials.

Cholesteric Liquid Crystal Polymeric Coatings for Colorful Artificial Muscles and Motile Humidity Sensor Skin Integrated with Magnetic Composites

Structural colorful cholesterics show impressive susceptibility to external stimulation, leading to applications in electro/mechano-chromic devices. However, out-of-plane actuation of structural colorful actuators based on cholesterics and the integration with other stimulation remains underdeveloped. Herein, colorful actuators and motile humidity sensors are developed using humidity-responsive cholesteric liquid crystal networks (CLCNs) and magnetic composites. The developed colorful actuator can exhibit synergistic out-of-plane shape morphing and color change in response to humidity, with CLCNs as colorful artificial muscles. Through the integration with magnetic control, the motile sensor can be navigated to open and confined spaces with the aid of friction to detect local relative humidity. The integration of multi-stimulation actuation of cholesteric magnetic actuators will expand the research frontier of structural colorful actuators and motile sensors for confined spaces.

Phase-Transition Microcavity Laser

Liquid-crystal microcavity lasers have attracted considerable attention because of their extraordinary tunability and sensitive response to external stimuli, and because they operate generally within a specific phase. Here, we demonstrate a liquid-crystal microcavity laser operated in the phase transition in which the reorientation of liquid-crystal molecules occurs from aligned to disordered states. A significant wavelength shift of the microlaser is observed, resulting from the dramatic changes in the refractive index of liquid-crystal microdroplets during the phase transition. This phase-transition microcavity laser is then exploited for sensitive thermal sensing, enabling a two-order-of-magnitude enhancement in sensitivity compared with the nematic-phase microlaser operated far from the transition point. Experimentally, we demonstrate an exceptional sensitivity of -40 nm/K and an ultrahigh resolution of 320 μK. The phase-transition microcavity laser features compactness, softness, and tunability, showing great potential for high-performance sensors, optical modulators, and soft matter photonics.

Dual-Function Smart Windows Using Polymer Stabilized Cholesteric Liquid Crystal Driven with Interdigitated Electrodes

In this study, we present an electrically switchable window that can dynamically transmit both visible light and infrared (IR) light. The window is based on polymer stabilized cholesteric liquid crystals (PSCLCs), which are placed between a top plate electrode substrate and a bottom interdigitated electrode substrate. By applying a vertical alternating current electric field between the top and bottom substrates, the transmittance of the entire visible light can be adjusted. The cholesteric liquid crystals (CLC) texture will switch to a scattering focal conic state. The corresponding transmittance decreases from 90% to less than 15% in the whole visible region. The reflection bandwidth in the IR region can be tuned by applying an in-plane interdigital direct current (DC) electric field. The non-uniform distribution of the in-plane electric field will lead to helix pitch distortion of the CLC, resulting in a broadband reflection. The IR reflection bandwidth can be dynamically adjusted from 158 to 478 nm. The electric field strength can be varied to regulate both the transmittance in the visible range and the IR reflection bandwidth. After removing the electric field, both features can be restored to their initial states. This appealing feature of the window enables on-demand indoor light and heat management, making it a promising addition to the current smart windows available. This technology has considerable potential for practical applications in green buildings and automobiles.

Laser-reduced graphene oxide for a flexible liquid sliding sensing surface

Flexible electronic skin is a flexible sensor system that imitates human skin. Recently, flexible sensors have been successfully developed. However, the droplet sliding sensing technology on a flexible electronic skin surface is still challenging. In this Letter, a flexible droplet sliding sensing surface is proposed and fabricated by laser-reduced graphene oxide (LRGO). The LRGO shows porous structures and low surface energy, which are beneficial for infusing lubricants and fabricating stable slippery surfaces. The slippery surface guarantees free sliding of droplets. The droplet sliding sensing mechanism is a combination of triboelectricity and electrostatic induction. After a NaCl droplet slides from lubricant-infused LRGO, a potential difference (∼0.2 mV) can be measured between two Ag electrodes. This study reveals considerable potential applications in intelligent robots and the medical field.

Tunable Circularly Polarized Luminescence via Chirality Induction and Energy Transfer from Organic Films to Semiconductor Nanocrystals

Circularly polarized luminescent (CPL) films with high dissymmetry factors hold great potential for optoelectronic applications. Herein, we propose a strategy for achieving strongly dissymetric CPL in nanocomposite films based on chirality induction and energy transfer to semiconductor nanocrystals. First, focusing on a purely organic system, aggregation-induced emission (AIE) and CPL activity of organic liquid crystals (LCs) forming helical nanofilaments was detected, featuring green emission with high dissymmetry factors g_lum ∼ 10^-2. The handedness of helical filaments, and thus the sign of CPL, was controlled via minute amounts of a small chiral organic dopant. Second, nanocomposite films were fabricated by incorporating InP/ZnS semiconductor quantum dots (QDs) into the LC matrix, which induced the chiral assembly of QDs and endowed them with chiroptical properties. Due to the spectral matching of the components, energy transfer (ET) from LC to QDs was possible enabling a convenient way of tuning CPL wavelengths by varying the LC/QD ratio. As obtained, composite films exhibited absolute g_lum values up to ∼10^-2 and thermally on/off switchable luminescence. Overall, we demonstrate the induction of chiroptical properties by the assembly of nonchiral building QDs on the chiral organic template and energy transfer from organic films to QDs, representing a simple and versatile approach to tune the CPL activity of organic materials.