Chiral Pitch Research Articles

Short chiral pitch and blue phase stability in cholesteric liquid crystal mixtures by adding achiral benzoic acid derivative

ABSTRACT Cholesteric liquid crystal (CLC) is anticipated to have applications such as blue light and ultraviolet (UV) light blocking for human skin owing to its selective wavelength reflection against incident sunlight through several hundred nanometres of a periodic helical twist structure. In this study, we investigated short chiral pitch control and enlargement of the blue phase (BP) temperature range by adding an achiral constituent of 4-n-octylbenzoic acid (8 BA) into CLC mixtures composed of three kinds of cholesteryl compounds: cholesteryl chloride, cholesteryl pelargonate and cholesteryl palmitate. Hydrogen bonding dependence of chiral pitch and BP temperature range were also investigated for achiral dopant (8 BA) and CLC mixture through temperature-dependent Fourier-transform infrared analysis. It was found that 40 wt% of 8 BA within the CLC mixture selectively reflected UV and blue light over 360–400 nm with a decrease of chiral pitch. It was also found that the BP temperature range increased by 15.6°C owing to hydrogen bond dimers composed of 8 BA molecules in the CLC mixture.

Full degree-of-freedom polarization hologram by freeform exposure and inkjet printing

Since the invention of holography by Dennis Gabor, the fabrication of holograms has mainly relied on direct recording of wavefront by engraving the intensity fringes of interfering electric fields into the holographic material. The degree-of-freedom (DoF) is often limited, especially for its usage as a holographic optical element in imaging or display systems, as what is recorded is what to use. In this work, based on the emerging self-assembled photo-aligned liquid crystal, a polarization hologram with full DoF for local manipulation of optical structure is demonstrated. The ability to record an arbitrary wavefront (in-plane DoF) is achieved by freeform surface exposure, while the local adjustment of deposited liquid crystal (out-of-plane DoF) is realized by inkjet printing. The methodology for designing and fabricating such a hologram is exemplified by building a full-color retinal scanning display without color crosstalk. Here, the arbitrary wavefront modulation capability helps to eliminate the aberrations caused by mismatched exposure and display wavelengths. The local liquid crystal adjustment ability enables the suppression of crosstalk by variation of chiral pitch and film thickness to tune the peak and valley of Bragg diffraction band. The demonstrated method is expected to greatly impact the fields of advanced imaging and display, such as augmented reality and virtual reality, that require optics with an ultrathin form factor and high degrees of design freedom simultaneously.

Cellulose nanocrystals-based bio-composite optical materials for reversible colorimetric responsive films and coatings

Photonic materials with a tunable chiral nematic structure that can selectively reflect light dynamically are valuable for applications in smart responsive materials. Here, we prepared potential photonic composites with a chiral nematic structure by forming cellulose nanocrystals (CNCs) and waterborne polyurethane (WPU) composites with different compositions on different substrates by evaporation-induced self-assembly. With increasing WPU content, the reflected wavelength increased from 400 to 680 nm, which was mainly caused by the increase of the chiral nematic pitch. In addition, the mechanical properties were better for higher WPU content. WPU was sensitive to small amounts of moisture in ethanol owing to the swollen WPU after absorbing water will increase the helical pitch. The reversible red shift induced by moisture was approximately 100 nm. When wood was used as the substrate, the CNCs still self-assembled to form chiral nematic structures and the adhesion forces of the composites to the wood substrate were strong. By using MgCl2 solution as an ink, invisible patterns can be written on the coating, which can be revealed temporarily by ethanol. In addition, the invisible pattern of photonic coating is rewritable. The easily prepared environmentally friendly photonic composite has great potential in sensors, anti-counterfeiting labels and smart decorative coatings.

A bistable light shutter based on polymer stabilized cholesteric liquid crystals

In this paper, a bistable light shutter that can sustain both transparent and opaque states without applied voltage was fabricated based on the polymer stabilized cholesteric liquid crystals (PSCLCs) with negative dielectric anisotropy. By applying a high-frequency or low-frequency electric pulse, the two stable states could be switched into each other. Moreover, the role of polymer concentration and chiral pitch was also investigated. It was found that they could affect the polymer micromorphology and further influence the bistable properties and transmission spectrum. Finally, an optimized precuring formula with 3 wt% of polymerizing monomer and 5 wt% of chiral dopant was applied to prepare a light shutter, which showed good bistability and optically shielding properties. Accordingly, the light shutter was highly energy efficient.

Phase Behavior, Self-Assembly, and Adhesive Potential of Cellulose Nanocrystal-Bovine Serum Albumin Amyloid Composites.

Structural organization is ubiquitous throughout nature and contributes to the outstanding mechanical/adhesive performance of organisms including geckoes, barnacles, and crustaceans. Typically, these types of structures are composed of polysaccharide and protein-based building blocks, and therefore, there is significant research interest in using similar building blocks in the fabrication of high-performance synthetic materials. Via evaporation-induced self-assembly, the organization of cellulose nanocrystals (CNCs) into a chiral nematic regime results in the formation of structured CNC films with prominent mechanical, optical, and photonic properties. However, there remains an important knowledge gap in relating equilibrium suspension behavior to dry film structuring and other functional properties of CNC-based composite materials. Herein, we systematically investigate the phase behavior of composite suspensions of rigid CNCs and flexible bovine serum albumin (BSA) amyloids in relation to their self-assembly into ordered films and structural adhesives. Increasing the concentration of BSA amyloids in the CNC suspensions results in a clear decrease in the anisotropic fraction volume percent via the preferential accumulation of BSA amyloids in the isotropic regime (as a result of depletion interactions). This translates to a blue shift or compression of the chiral nematic pitch in dried films. Finally, we also demonstrate the synergistic adhesive potential of CNC-BSA amyloid composites, with ultimate lap shear strengths in excess of 500 N/mg. We anticipate that understanding the systematic relationships between material interactions and self-assembly in suspension such as those investigated here will pave the way for a new generation of structured composite materials with a variety of enhanced functionalities.

Chiral smart bionic skin film with changeable structural colors and tunable luminescence by polymer-assisted supramolecular assembly of the photonic crystals

The biological skin system of advanced organisms can obtain various environmental stimuli to change skin colors by adjusting the micro-nano structure of the skin surface or by electroluminescence and photoluminescence. The assembly of biominetic skin materials with both changeable structural colors and tunable luminescence is a significant challenge in the fields of advanced materials and light science. This work describes a novel chiral smart bionic skin film with changeable structural colors, tunable luminescence by the polymer-assisted supramolecular assembly of the photonic crystals. A smart nanodot based on the molecule with four-armed mimosa-like structure was assembled on the cellulose nanocrystals (CNC) photonic crystal. The chiral bionic skin film obtained had multi-channel fluorescence-emission, sensitive structural color stimulus responses to the light and humidity and the tunable chiral helical nematic domains. It demonstrates a series of the supramolecular interactions that trigger multiple intelligent signals, including the adjustable chiral pitch, photo-induced electron transfer (PET), intramolecular fluorescence resonance energy transfer (FRET), π-π stacking and the changes of the hydrogen bonds. The photonic crystal skin materials with sensitive multi-signals intelligent responses as smart bionic skin have potential applications for the tissue engineering, 3D printing materials, bionic artificial skin materials, and optical devices.

Stable hazy states by electrohydrodynamic convection of ultraviolet-treated chiral nematic liquid crystal

In this paper, we show that electrohydrodynamic convection of ultraviolet (UV)-treated chiral nematic liquid crystals (CNLCs) generates stable foggy states. UV-irradiation produces relatively heavier and larger molecular ions at CNLC mixture, which activates electroconvection, and leads to easily dynamic scattering mode (DSM) with strong turbulence. Chirality of LC combined with the DSM builds two stable states (planar state and focal conic state) more strongly, unlike the conventional method generating the two states in the CNLCs. The molecular ion effect enables state conversion to a much lower switching voltage than that of the conventional cholesteric devices. The focal conic state connected to DSM2 exhibits a stable deep hazy state, the mixed state combined with DSM1 displays a stable weak hazy state, and the planar state activated in a particular frequency region creates a stable and very clear transparent state due to the chiral pitch of the infrared region. Therefore, a smart window with the tri-stable-hazy state specialized in low power consumption can be realized. The tri-stable-hazy state device also generates various hazy states according to the applied voltage; thus, it can provide a proper indoor temperature for users. Furthermore, a stable planar state with a chiral pitch designed in the infrared region may save energy through selective reflection of infrared rays from the sunlight in the summer. The proposed technology is suitable for commercial use in the next-generation smart window.

Multivalued Logic for Optical Computing with Photonically Enabled Chiral Bio-organic Structures.

Photonic bio-organic multiphase structures are suggested here for integrated thin-film electronic nets with multilevel logic elements for multilevel computing via a reconfigurable photonic bandgap of chiral biomaterials. Herein, inspired by an artificial intelligence system with efficient information integration and computing capability, the photonically active dielectric layer of chiral nematic cellulose nanocrystals is combined with printed-in p- and n-type organic semiconductors as a bifunctional logical element. These adaptive logic elements are capable of triggering tailored quantized electrical output signals under light with different photon energy and at the different photonic bandgaps of the active dielectric layer. The bifunctional structures enable complex memory behavior upon repetitive changes of photonic bandgap (controlled by expansion/contraction of chiral nematic pitch) and photon energy (controlled by light absorption wavelength of complementary organic semiconductor layers), exhibiting effectively a reconfigurable ternary logic response. This proof-of-concept bio-assisted multivalued logic structure facilitates an optical computing system for low-power optical information processing integrated with human-machine interfaces.

Characterization of orientation correlation kinetics: chiral-mesophase domains in suspensions charged DNA-rods

Bacteriophage DNA fd-rods are long and stiff rod-like particles which are known to exhibit a rich equilibrium phase behavior. Due to their helical molecular structure, they form the stable chiral nematic (N*) mesophases. Very little is known about the kinetics of forming various phases with orientations. The present study addresses the kinetics of chiral-mesophases and N*-phase, by using a novel image-time correlation technique. Instead of correlating time-lapsed real-space microscopy images, the corresponding Fourier images are shown for time-correlated averaged orientations. This allows to unambiguously distinguish to detect the temporal evolution of orientations on different length scales, such as domain sizes (depending on their relative orientations), and the chiral pitch within the domains. Kinetic features are qualitatively interpreted in terms of replica symmetry breaking of elastic deformations in the orthogonal directional axes of chiral-mesophase domains, as well by the average twist angle and the order parameter. This work can be interesting for characterizing other types of charged rods, mimicking super-cooled liquids and orientation glasses.

Stabilization of Long-Pitch Supertwisted Nematic Structures

Stabilized reverse twisted nematic liquid crystal devices (RTN-LCDs) were fabricated using formation of a polymer matrix under UV irradiation with an applied voltage (sustain voltage) in the vicinity of the alignment layers. In the absence of an applied voltage, the non-stabilized RTN structure gradually returns to a splay twist structure. The sustain voltage was decreased with an increase in temperature. A stabilized long-pitch supertwisted nematic (LPSTN) structure could also be formed during the RTN structure stabilization process with a much lower sustain voltage at a temperature near the clearing point. The chiral pitch for the LPSTN structure is longer than that for a typical STN structure. LPSTN-LCDs similar to RTN-LCDs show a large reduction in both the threshold and saturation voltage compared with those for TN-LCDs consisted of the same LC materials. Furthermore, a notable feature of LPSTN-LCDs is a change to a TN structure when a high voltage is applied. A black state can be realized due to the change from the LPSTN structure to the RTN structure unlike the typical STN mode under the crossed nicols condition. In contrast STN-LCDs retain their color due to the retardation because the RTN and LPSTN states are considered topologically equivalent.

Circularly Polarized Light Detection by Chiral Photonic Cellulose Nanocrystal with ZnO Photoconductive Layer in Ultraviolet Region.

Circularly polarized light (CPL) detection and polarization state recognition are required for a wide range of applications. Conventional polarization detection with optical components causes difficulties for miniaturization and integration. An effective design strategy is proposed for direct CPL detection with chiral material. Here, we realized direct CPL detection based on the combination of chiral photonic cellulose nanocrystal (CNC) and ultraviolet-sensitive ZnO photoconductive material. The CNC layer deposited by evaporation-induced self-assembly established the left-handed chiral nematic structure with a photonic bandgap (PBG) to recognize left-handed CPL (LCPL) and right-handed CPL (RCPL) at specific wavelengths. The PBG of CNC layer has been modulated by the adjustment of chiral nematic pitch to match the semiconductor bandgap of ZnO film in ultraviolet region. The photocurrents under RCPL and LCPL are 2.23 × 10−6 A and 1.77 × 10−6 A respectively and the anisotropy factor Δgpc of 0.23 is acquired for the CPL detection based on the chiral photonic CNC. This design provides a new approach to the detection of CPL polarization state with competitive performance.

Raman scattering of plane-wave and twisted light off chiral molecular liquids

We present an experimental study of the quasi-elastic Raman scattering (QES) of plane-wave and twisted light by liquid crystals. Depending on their temperature, these crystals can exhibit isotropic, nematic and chiral nematic phases. The question is addressed of how the phase of a crystal and the state of incident light can affect the quasi–elastic energy spectra of the scattered radiation, whose shape is usually described by the combination of Lorentzian and Gaussian components. Special attention is paid to the chiral phase, for which the Raman QES spectrum is dominated by a Lorentzian with reduced linewidth, pointing to diminished disorder and configurational entropy. Moreover, this phase is also known for a regime of iridescence (selective backscattering) which arises when the wavelength of incident light becomes comparable with the chiral pitch length. Detailed measurements, performed in this resonant regime and by employing twisted light, carrying various projections of the orbital angular momentum (OAM), have indicated a low-energy scattering surplus depending on OAM. We argue that this observation might indicate a transfer of angular momentum between light and liquid crystal.

Flexible cellulose nanocrystal-based bionanocomposite film as a smart photonic material responsive to humidity

A flexible bionanocomposite film with a visible structural color was developed by co-assembling cellulose nanocrystals with glycerol, glucose, and poly(vinyl alcohol), and its application as a photonic humidity-responsive material was investigated. This chiral nematic film exhibited an indigo, cyan, olive green, orange, pink, and transparent color when exposed to relative humidity of 33%, 75%, 85%, 89%, 96%, and 100%, respectively. The color change was caused by reversible change in chiral nematic pitch due to hydration and dehydration. The nanocomposite displayed a stable structural color after alternatively conditioned at 33% and 100% relative humidity for ten cycles. Notably, material cost for a piece of such a smart film with an area of 28 cm2 is less than $0.1 USD. This flexible, uniform bionanocomposite film has great potential to be used as a smart photonic material responsive to humidity.

Multi-Pitch Liquid Crystal Filters with Single Layer Polymer Template.

Multi-reflective peak and bandwidth scalable liquid crystal (LC) filters were investigated. By refilling a cholesteric LC (CLC) whose chiral pitch is different to the target template into a blue phase LC (BPLC) template, a multi-reflective peak single layer LC filter can be fabricated. With multiple templating and refilling processes, the number of reflective peaks can be further increased. Moreover, by refilling the CLCs of designed chiral pitch into a CLC template sequentially, a bandwidth scalable single layer CLC filter can be fabricated. The LC filters show great potential applications in optical communication, display, and LC lasing.

Template Effect of Multi-Phase Liquid Crystals

The template effects on stability of twist structure liquid crystals (LCs) were investigated. By refilling a cholesteric LC (CLC) of different pitch into a blue phase LC (BPLC) template or a sphere phase LC (SPLC) template, a multi-phase and multi-pitch twist structure LC, which includes the refilling CLC and intrinsic template BPLC or SPLC, can be fabricated. By refilling a CLC of different chiral pitch into a CLC template, a multi-pitch CLC that includes the refilling CLC and intrinsic CLC, can be fabricated. Twist structure LC devices with multi-phase and multi-pitch show great potential for applications in optical communication, displays, and LC lasing.

Heliconical cholesteric liquid crystals as electrically tunable optical filters in notch and bandpass configurations

ABSTRACT Heliconical cholesteric liquid crystals are a recently proposed novel class of liquid crystalline materials in which the chiral pitch can be tuned by an external electric field acting parallel to the helical axis. Here we show that these materials can be used as tunable optical filters of a simple architecture. The spectral properties of transmitted light can be tuned within the whole visible range. In particular, the bandpass wavelength can be shifted from 405 nm to 637 nm by changing the applied field from 2.7 to 1.7 V/µm. The electric field can also switch the filters on and off, within 1 s or less. Moreover, the filters work with both polarised and unpolarised light and can be switched from a notch to a bandpass configuration by changing the polarisation of incident light. An optical switch among three different laser lines is also reported as an additional possible application.

Light- and Humidity-Responsive Chiral Nematic Photonic Crystal Films Based on Cellulose Nanocrystals.

Light- and humidity-responsive chiral nematic photonic crystal (PC) films containing cellulose nanocrystals (CNCs) were fabricated. A photoactive polymer with hydrophilic groups, poly-(3,3'-benzophenone-4,4'-dicarboxylic acid dicarboxylate polyethylene glycol) ester, was coassembled with CNCs to form flexible iridescent films with a tunable chiral nematic order. In the coassembly process, the intermolecular hydrogen bonds of CNCs were weakened, which facilitated the fine regulation of the chiral PC nanostructure. The PC films displayed sensitive responses to both light and humidity. With increasing humidity from 30 to 100%, the chiral nematic helix pitch increased from 328 to 422 nm. The color of the PC films changed from blue to green, yellow, orange, and dark red with increasing relative humidity. Over 15 min of light irradiation, the absorption intensity of the films increased gradually. The light and humidity responses of the films were reversible. The films maintained their variable cholesteric liquid crystal texture and helical lamellar structure after light irradiation at different humidities. These PC films are expected to be useful in intelligent coatings and 3D printing.

Tilted Chiral Liquid Crystal Gratings for Efficient Large‐Angle Diffraction

AbstractDeflecting light with high efficiency over large angles with thin optical elements is challenging but offers tremendous potential for applications, such as wearable displays and optical communication systems. Compared to the complex production of metasurfaces, the self‐organization of liquid crystal (LC) superstructures provides an elegant and flexible way to produce high‐quality thin optical components. The periodically varying dielectric tensor in short‐pitch chiral LC gives rise to a photonic bandgap, which can be exploited to realize efficient diffractive mirrors in the visible wavelength range. However, large‐angle diffractive devices require a small in‐plane period, leading to complex self‐assembly behavior in the bulk. This work demonstrates that by patterning photoalignment layers at the surfaces with a period comparable to the chiral pitch, the LC self‐assembles into a tilted, defect‐free helical structure. The director configuration is calculated by finite element simulations and it is experimentally demonstrated that a single photoaligned substrate is sufficient to template the tilted chiral structure in the bulk. This structure effectively (88%) diffracts light over large angles (≈46°) and enables novel micrometer‐thin (≈3 µm) optical components that can be produced with an elegant manufacturing process. Due to flexibility of photoalignment, this process could easily be implemented in emerging photonic applications.

Tuning Chiral Nematic Pitch of Bioresourced Photonic Films via Coupling Organic Acid Hydrolysis

AbstractControlling the iridescent photonic film generated by self‐assembly of colorless cellulose nanocrystal (CNC) from the nanoscale to macroscale is challenging. This study combines experimental and computational approaches to systematically investigate the correlation between electrostatic interactions and chiral nematic structures. The chiral nematic order of the CNC colloidal is preserved in solid films after the evaporation of water. The cross‐sections of the iridescent film show a clear left‐handed helical arrangement of nanocrystals. The helical structure with the aid of acrylic acid exhibits longer organized patterns. This work reveals that compared to CNC prepared by pure sulfuric acid (zeta potential −37.1 mV), the CNC prepared using coupled mineral sulfuric and organic acrylic acid has a higher zeta potential (−67.2 mV), which induces the increase of helical pitch of the cholesteric nematic phase from 312 to 409 nm and a red shift of the iridescent film. Consequently, the tuning of reflected light wavelength lies in the variation of chiral nematic pitch inside the layered structure, which gives rise to different iridescent colors. The bioresourced photonic film is appealing to both academia and industry where optical and photonic components are essential.

Thermotropic liquid crystalline properties of (hydroxypropyl)cellulose derivatives with butyryl and heptafluorobutyryl substituents

(Hydroxypropyl)cellulose (HPC) derivatives with butyryl (Bu) and heptafluorobutyryl (7FBu) substituents were prepared in various proportions of the Bu/7FBu groups and at a fixed total DS (DSBu + DS7FBu) of 3.0. Thermotropic liquid crystallinity of the derivatives (Bu7FBu-HPC) was investigated to specify the effect of the fluoroacylation on the mesophase behavior. Thermal transition data were collected using differential scanning calorimetry and polarized light microscopy. The Bu7FBu-HPC samples formed a chiral nematic phase between their glass transition and isotropization temperatures, Tg and Ti-a, respectively; these transition temperatures rose moderately as the 7FBu proportion increased (Tg = − 44 to − 27 °C and Ti-a = 158–190 °C for DS7FBu = 0.04–1.60). The structural property of the mesophase was examined at 70 °C by circular dichroism and other optical measurements. The chiral nematic pitch (P) sensitively increased with increasing 7FBu proportion, while the supramolecular helical arrangement remained right-handed. Selective light-reflection colors were observed for the samples of DS7FBu = 0.04–0.8, covering an entire spectrum range from violet to red. Temperature dependence of P was also examined for selected samples below Ti-a, and it was found to increase with increasing temperature; however, there was no indication of inversion in the handedness of the helical structure. Wide-angle X-ray diffractometry revealed that the increases of P responding to the increases in DS7FBu and temperature were attributable to the decrease of the twist angle between adjacent thin nematic layers.