Selective Reflection Research Articles

Thermoregulated CPL‐Active Flexible Polymer/Perovskite Hybrid Materials with High Luminescence Dissymmetry Factor

AbstractCPL‐active flexible materials with high luminescence dissymmetry factor (glum) and luminescent quantum yield are indispensable for 3D displays, wearable devices and so on. Perovskite has drawn much attention due to its fascinating photoelectric properties as well as its abundant applications. However, the existing strategies are inefficient in acquiring perovskite NCs with high glum. In this work, thermoregulated CPL‐active flexible polymer/perovskite hybrid materials with high glum by the creation of chiral photonic bandgap in polymer film with cholesteric domains using toluene as the dielectric media is reported. Based on selective CPL reflection, a satisfied CPL with a maximum glum of 0.6 is achieved. Importantly, the chiral photonic bandgap center of the polymer film can be easily tuned by thermal annealing at different temperatures to adapt various perovskite NCs with full‐spectral emission. The handedness of CPL of hybrid materials can be regulated with and without dielectric media. The presence of the flexible polymer makes hybrid materials a candidate to achieve patterned display applications. Additionally, the polymer/perovskite hybrid materials show excellent stability thanks to the encapsulation effect from polymers. This work provides a facile and general approach to induce adjustable CPL in the whole visible spectrum with high glum and photoluminescence quantum yield (PLQY).

Polymer dispersed cholesteric liquid crystals with combined photo- and mechanochromic response

The development of stimuli-sensitive systems is one of the newest and most promising directions in the field of modern materials science. The combination of various types of sensitivity in the same material opens the possibility of creating complex devices with a wide range of potential applications. This work deals with the design and preparation of new composite materials based on plasticized polyvinyl alcohol (PVA) films containing droplets of cholesteric liquid crystals (CLC). Mechanical stretching of elastic composite films results in a change in optical properties, namely selective light reflection (SLR), from CLC droplets. This in turn transforms the structure and optical properties of the entire polymer matrix. Such a process represents a certain mechano-optic response of the polymer matrix to the influence of mechanical deformation. The introduction of a chiral photochromic dopant into a composite film makes it possible to transform such a film into a photochromic material with the ability to control its optical properties using, for example, UV irradiation. Therefore, mechanical deformation of a polymer composite leads to a change in its optical properties (SLR) caused by a change chiral supramolecular structure of cholesteric mesophase. At the same time, UV irradiation of a polymer film induces E-Z isomerization process in the chiral photochromic dopant, altering its molecular shape and helical twisting power. This enables the using of this process for remote and contactless control of the composite optical properties. In addition, this feature allows one to implement the photopatterning and the recording of a latent image, which appears under the influence of a mechanical field.

Dual-frequency-addressable photochromic cholesteric mixture with photo- and electro-switchable optical properties

Cholesteric liquid crystalline mixture with photo-, electro-controllable optical properties was prepared and the features of its switching behavior were demonstrated. The working principle of the obtained switchable cell is based on using dual-frequency-addressable nematic mixture that enables effective control of selectively reflected planar and scattered focal conics texture by applied field frequency modulation. The nematic mixture was doped with the chiral substance in order to induce a cholesteric structure formation with selective light reflection in visible spectral range. Azobenzene-containing photochrome was also introduced into this mixture in order to stimulate photo-optical response. An application of low-frequency electric field as well as UV irradiation induces short-wavelength shift of the selective light reflection. Under action of an electric field, a transition to scattered focal conics texture takes place. Amplitude of the light-induced shift is ca. 50 nm, while the electric field leads to a shift ca. 120 nm. On the contrary, high-frequency electric field induces fast (less than 1 s) restoration of perfect highly reflected planar texture. UV irradiation leads to not only shift of the selective light reflection band, but alters the electrooptic response by increase of frequency necessary for the restoration of the planar state and considerably changes the switching kinetics. The possibility of photopatterning of the obtained electrooptic cells by UV irradiation through a mask is demonstrated. It is shown, that application of electric field of different strength and frequencies enables smart control of the photorecorded image contrast.

Robust and wear-durable coating based on halloysite nanotubes/polymer composite for passive daytime radiative cooling

Passive daytime radiative cooling (PDRC) is an efficient strategy to achieve cooling through spectrally selective emission and high solar spectrum reflection. Current preparation strategies for PDRC materials mainly involve doping inorganic micro-nanoparticles into polymers, but their solar reflectance is usually insufficient. Here, we designed a super-white inorganic coating on different substrates based on natural halloysite nanotubes (HNTs) through the doctor blade method. Polyvinyl alcohol (PVA) and water-based acrylic resin (AR) were introduced to increase the adhesion for obtaining robust inorganic coating. HNTs/PVA/AR composite coating shows excellent water resistance, wear resistance, and environmental tolerance, which is mainly due to the strong interfacial interactions between HNTs, PVA, and AR. Compared with the uncoated transparent plastic, the coated plastic exhibits a super-white appearance (whiteness of ∼95%) and maximum solar reflectance of ∼97%. The maximum temperature difference of the HNTs/PVA/AR coated and pristine copper sheet is 15 °C under simulated sunlight. The PDRC property is related to the high surface roughness of the coating composed of disordered high-aspect ratio nanotubes and the high whiteness of the raw halloysite materials. The composite coating can tolerate pH from 1–13, temperature change (−60∼250 °C), and different salt environments. The composite coating increases thermal stability, flame retardancy, and giving protective effect of dye against UV degradation. In addition, the HNTs/PVA/AR coated plastic as packaging materials can maintain good freshness of different fruits under strong solar irradiation. The robust HNTs/PVA/AR composite coating with high PDRC properties shows promising application in food packaging, building energy conservation, metal anticorrosive coating, and sunscreen products.

Chiral fluorescent liquid crystal polymers containing 4-bromo-1,8-naphthalimide – synthesis and properties

ABSTRACT By introducing two cholesteric liquid crystal monomers with different chain lengths (M1 and M2) and brominated naphthalimide fluorescent monomer (M3) into polymethyl hydrosiloxane, a series of fluorescent cholesteric liquid crystal polymers (FL-LCP) was prepared, which not only have excellent performance of liquid crystal and fluorescence but also own performance of selective reflection. Their chemical structures were confirmed to be in accord with molecular design by FT-IR and 1 H NMR spectroscopy, and found them with excellent thermal stability, mesomorphic behaviour and thermal performance by TGA, POM, DSC and XRD, and confirmed all the polymers belonged to the cholesteric phase. The FL-CLCPs emitted different intensity fluorescence under UV light, and conclusions about the relationship between fluorescence and temperature had been drawn. A comparative study was conducted on the fluorescence properties of powders and films of monomers and polymers.

Dual tunability of selective reflection by light and electric field for self-organizing materials

The oblique helicoidal structure is formed in right-angle cholesterics under the applied electric field. The electric field changes the pitch and cone angle but preserves the single-harmonic modulation of the refractive index. As a result, in such a supramolecular system, we can tune the selective reflection of light in a broad range. Here, we report that structural colors can be tuned by simultaneously illuminating the structure with UV light and the action of an electric field. The cholesterics with the oblique helicoidal structure were doped with newly designed rod-like, chiral, and bent-shaped azo-photosensitive materials characterized by a very low rate of thermal back cis (Z) – trans (E) isomerization. The E-Z isomerization of the photo-active compounds under UV light causes the red shift of the selective light reflection in the cholesteric mixtures. We found that the molecular structure of the photosensitive materials used affects the reflection coefficient, bandwidth, response time to UV irradiation, and tuning range. The effect was explained by considering the effect of molecular matching, cis–trans isomerization, and electric field action. We investigated the dynamics of molecular changes in the oblique helicoidal structure under the influence of external factors. The designed supramolecular system has the potential application in soft matter UV detectors.

Controlling the Nematic Liquid Crystallinity of Cellulose Nanocrystals with an Alcohol Ethoxy Sulfonate Surfactant.

Cellulose nanocrystals (CNCs) are biobased colloidal nanorods that have unlocked new opportunities in the area of sustainable functional nanomaterials including structural films and coatings, biomedical devices, energy, sensing, and composite materials. While selective light reflection and sensing develop from the typical chiral nematic (cholesteric, Nem*) liquid crystallinity exhibited by CNCs, a wealth of technologies would benefit from a nematic liquid crystal (LC) with CNC uniaxial alignment. Therefore, this study answers the central question of whether surfactant complexation suppresses CNC chirality in favor of nematic lyotropic and thermotropic liquid crystallinity. Therein, we use a common surfactant having both nonionic and anionic blocks, namely, oligo(ethylene glycol) alkyl-3-sulfopropyl diether potassium salt (an alcohol ethoxy sulfonate (AES)). AES forms complexes with CNCs in toluene (a representative for nonpolar organic solvent) via hydrogen bonding with an AES' oligo(ethylene glycol) block. A sufficiently high AES weight fraction endows the dispersibility of CNC in toluene. Lyotropic liquid crystallinity with Schlieren textures containing two- and four-point brush defects is observed in polarized optical microscopy (POM), along with the suppression of the cholesteric fingerprint textures. The results suggest a nematic (Nem) phase in toluene. Moreover, thermotropic liquid crystallinity is observed by incorporating an excess of AES, in the absence of an additional solvent and upon mild heating. The Schlieren textures suggest a nematic system that undergoes uniaxial alignment under mild shear. Importantly, replacing AES with a corresponding nonionic surfactant does not lead to liquid crystalline properties, suggesting electrostatic structural control of the charged end group of AES. Overall, we introduce a new avenue to suppress CNC chirality to achieve nematic structures, which resolves the long-sought uniaxial alignment of CNCs in filaments, composite materials, and optical devices.

Period induced reflectance tuning in transparent gold metasurfaces

Gold (Au) nanodisk arrays are modeled and fabricated for tunable near infrared reflectance with high visible transmittance. Simulations are performed in the visible and near infrared regimes between 400 and 2000 nm with varying period of nanostructures. A progressive increase in period of up to three times the diameter of the nanodisks on polymer and quartz substrates is presented. As a result, an increase in visible transmittance with a decrease in near infrared reflectance peak intensity is observed. Modeling results exhibit a high average visible transmittance of 91.5% (400–700 nm) and average reflectance of 62.9% (750–2000 nm) with diameter (D) variations from D=120 nm to D=500 nm with a period (P) equal to twice the diameter, P=2D for a polymer substrate. Electric field intensities exhibit an increase with a decrease in the period between individual nanostructures for constant unit cell dimensions. Alongside simulations, electron beam lithography and evaporation processes are utilized for the patterning and fabrication. Optical characterizations including scanning electron microscopy (SEM) and atomic force microscopy (AFM) of selected metasurfaces on quartz are performed. For a specific period and diameter, the fabricated metasurfaces exhibit near infrared reflectance having a good agreement with the simulated results about plasmonic wavelengths. Such metasurfaces offer potential to be deployed for applications that require selective reflectance intensities in the near infrared range with high visible transmittance, such as controlled environment agriculture (CEA) or multi spectral near infrared sensing. With tunability over a broadband (750–2500 nm), this simple and fabrication-friendly model may be used as a theoretical guide in quantifying specific design parameters with varying light intensity configurations.

Dynamically reconfigurable complex liquid crystal emulsions

ABSTRACT Complex emulsions, containing liquid crystals (LCs) and immiscible oils such as fluorocarbon or silicone oils, present innovative functionalities in soft materials. These emulsions exhibit dynamic reconfiguration in droplet morphology and internal LC organisation, expanding applications in optics, sensing, and active matter. This review highlights the evolution and potential of LC-containing complex emulsions. Emphasis is placed on emulsions featuring dynamic and reversible morphological transformations and LC reorganisation, underpinning their suitability for tunable micro-lenses, chemical sensing, and biosensing. Nematic LCs within complex emulsions allow for topological defect-driven functionalization, attaching antibodies to induce substantial changes in the LC director field upon interfacial recognition events. This feature may facilitate the development of ultrasensitive chemical and biological sensors. Moreover, cholesteric LCs, through their selective reflection, enable the rapid and sensitive detection of foodborne pathogens. The controlled assembly of magnetic nanoparticles at interfaces illustrates advancements towards magnetic field-responsive and active colloids. Future perspectives include exploiting these properties for intelligent colloidal systems capable of autonomous behaviour, and furthering applications in dynamic photonic devices, high-security identification tags, and responsive lens systems.

Liquid crystal polymers with dual properties of fluorescence and selective reflection-design, synthesis and characterisation

ABSTRACT In this paper, two series of side chain fluorescent cholesteric liquid crystal polymers (FL-ChLCPs) were prepared by one-step method with polymethyl-hydrosiloxane (PMHS) as the main chain. Two series of polymers differ in the structure of vinyl flexible chains of fluorescent monomer, the comparative study was conducted on their performance of liquid crystal and fluorescence. The compounds and polymers were characterised using conventional methods. The results indicated all polymers possessed dual optical properties of fluorescence and selective reflection, they also had excellent thermal stability. The FL-ChLCPs emitted different intensity fluorescence under UV light, the fluorescence intensity has a certain relationship with temperature and aggregation-caused quenching effect only occurs when the content of fluorescent monomer in the polymer exceeds 20%.

Studies on Dual Helmholtz Resonators and Asymmetric Waveguides for Ventilated Soundproofing.

Achieving the simultaneity of ventilation and soundproofing is a significant challenge in applied acoustics. Ventilated soundproofing relies on the interplay between local resonance and nonlocal coupling of acoustic waves within a sub-wavelength structure. However, previously studied structures possess limited types of fundamental resonators and lack modifications from the basic arrangement. These constraints often force the specified position of each attenuation peak and low absorption performance. Here, we suggest the in-duct-type sound barrier with dual Helmholtz resonators, which are positioned around the symmetry-breaking waveguides. The numerical simulations for curated dimensions and scattered fields show the aperiodic migrations and effective amplifications of the two absorptive domains. Collaborating with the subsequent reflective domains, the designed structure holds two effective attenuation bands under the first Fabry-Pérot resonance frequency. This study would serve as a valuable example for understanding the local and non-local behaviors of sub-wavelength resonating structures. Additionally, it could be applied in selective noise absorption and reflection more flexibly.

Competing risk analysis of cardiovascular disease risk in breast cancer patients receiving a radiation boost

BackgroundThoracic radiotherapy may damage the myocardium and arteries, increasing cardiovascular disease (CVD) risk. Women with a high local breast cancer (BC) recurrence risk may receive an additional radiation boost to the tumor bed.ObjectiveWe aimed to evaluate the CVD risk and specifically ischemic heart disease (IHD) in BC patients treated with a radiation boost, and investigated whether this was modified by age.MethodsWe identified 5260 BC patients receiving radiotherapy between 2005 and 2016 without a history of CVD. Boost data were derived from hospital records and the national cancer registry. Follow-up data on CVD events were obtained from Statistics Netherlands until December 31, 2018. The relation between CVD and boost was evaluated with competing risk survival analysis.Results1917 (36.4%) received a boost. Mean follow-up was 80.3 months (SD37.1) and the mean age 57.8 years (SD10.7). Interaction between boost and age was observed for IHD: a boost was significantly associated with IHD incidence in patients younger than 40 years but not in patients over 40 years. The subdistribution hazard ratio (sHR) was calculated for ages from 25 to 75 years, showing a sHR range from 5.1 (95%CI 1.2–22.6) for 25-year old patients to sHR 0.5 (95%CI 0.2–1.02) for 75-year old patients.ConclusionIn patients younger than 40, a radiation boost is significantly associated with an increased risk of CVD. In absolute terms, the increased risk was low. In older patients, there was no association between boost and CVD risk, which is likely a reflection of appropriate patient selection.

Preparation and properties of highly flexible cholesteric side-chain liquid crystalline elastomers with selective reflective properties in the visible region

ABSTRACT In this paper, we have synthesised cholesteryl 4-allyloxybenzoate (MB), crosslinking agent dodecane-1,12-diyl bis(undec-10-enoate) (MG), and A’BGF series liquid crystalline elastomer (LCE) films which take polymethylhydrosiloxane (A’) as the backbone, MB as the cholesteric liquid crystal (LC) monomer, and MG and bis-vinyl silicone oil (MF) with different reactivities as cross-linking agent for exploring the effect of the change of MF content on the properties of the elastomeric films. The experimental results show that the synthesised MB, MG, and the LCE films are all consistent with the molecular design; MB is an enantiotropic cholesteric LC. The synthesised LCE films are cholesteric LCs, have certain selective reflection characteristic, higher thermal stability, and their Tg decreased down to 3.9°C as MF increasing. During the heating and cooling, the liquid crystalline properties of the LCE films have a good reversibility. Among the synthesised LCE films, A’BGF-2 has stretching radio of 264.8% at room temperature, with the best elastic properties. By adjusting the ratio of MF, the content of small-molecule MB remained in the films can be changed, which will affect the optical properties and elasticity of the LCE films. The experiments provide a method for the preparation of highly elastic side chain LCE film.

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.

Transmission-mode geometric-phase signatures of circular Bragg phenomenon

A dielectric structurally chiral medium (DSCM) exhibits the circular Bragg phenomenon, whereby circular-polarization-state selective reflection occurs in a specific spectral regime that depends on the direction of propagation of the incident plane wave. Theoretical analysis shows that the geometric-phase spectrum of the plane wave transmitted through a DSCM slab contains a signature of the circular Bragg phenomenon, provided that the incident plane wave is not right-circularly polarized, regardless of the structural handedness of the DSCM. As the number of structural periods in the DSCM slab increases with the structural period fixed, the spectrum of the transmission-mode geometric phase evolves but without an easily discernible pattern. A reversal of structural handedness affects but does not lead to a simple change in the transmission-mode geometric phase.

Controllable Circularly Polarized Luminescence with High Dissymmetry Factor via Co-Assembly of Achiral Dyes in Liquid Crystal Polymer Films.

Circularly polarized luminescence (CPL) materials are highly demanded due to their great potential in optoelectronic and chiroptical elements. However, the preparation of CPL films with high luminescence dissymmetry factors (glum ) remains a formidable task, which impedes their practical application in film-based devices. Herein, a facile strategy to prepare solid CPL film with a high glum through exogenous chiral induction and amplification of liquid crystal polymers is proposed. Amplification and reversion of the CPL appear when the films are annealed at the chiral nematic liquid crystalline temperature and the maximal glum up to 0.30 due to the enhancement of selective reflection. Thermal annealing treatment at different liquid crystalline states facilitates the formation of the chiral liquid phase and adjusts the circularly polarized emission. This work not only provides a straightforward and versatile platform to construct organic films capable of exhibiting strong circularly polarized emission but also is helpful in understanding the exact mechanism for the liquid crystal enhancement of CPL performance.

Amplification of Light‐Induced Helix Inversion of Intrinsically Chiral Diarylethene Molecular Switches

AbstractPhotonics and tunable optics are rapidly developing fields that require materials with programmable properties and advanced functionalities. Cholesteric liquid crystals (CLCs) are unique materials that exhibit selective light reflection and can be tuned using stimuli‐responsive small organic molecules. The challenge lies in designing molecules that can convert external signals, such as light, into dynamic and invertible chiral states, which can be transduced to the CLC supramolecular structures inducing large differences in helicity and eventually to macroscopic properties. Here, novel intrinsically chiral phenanthrene‐based diarylethenes as light‐responsive chiral dopants for controlling the supramolecular helical architectures of CLCs are introduced. The substitution pattern and light‐invertible axial chirality of these diarylethenes make them highly compatible with liquid crystals and provide high twisting power. The light‐induced cyclization and molecular chirality transformation result in a wide tunability of the cholesteric helix pitch (reflection colors) and reversible inversion of helical handedness. These findings provide a powerful tool for controlling and manipulating the macroscopic properties of CLCs, opening new avenues for a range of applications, including diffractive optics and photonics, anticounterfeiting tags, and displays.

Significant anisotropic deformation and optical shifts in stretched cholesteric liquid crystal elastomers.

This study explores the opto-mechanical response of cholesteric liquid crystal elastomers (CLCEs) subjected to uniaxial stretching along the x-axis, perpendicular to their helical z-axis. A definitive crossover is observed in the strain (εx) dependencies of various optical and mechanical properties, such as the transmission spectra, degree of mesogen orientation, Poisson's ratios, and tensile stress. At low strains, CLCEs exhibit a blue shift in the selective reflection band due to a reduction in the helical pitch, accompanied by a decrease in reflection selectivity for circularly polarized light. Beyond a certain critical strain further pitch alterations halt. This strain regime is marked by substantial anisotropic lateral contractions without any z-axis contraction, as indicated by a Poisson's ratio (μxz) of zero. Within this intermediate strain regime, local directors predominantly reorient towards the x-direction within the xy-plane, resulting in a quasi-plateau of tensile stress. Approaching a higher critical strain a complete loss of reflective selectivity occurs. Past this threshold, while the mechanical responses resemble those of isotropic conventional rubber, they retain a periodic structure albeit without phase chirality. These observed features are accounted for by the Mao-Terentjev-Warner model, especially when the network anisotropy parameter is adjusted to match the critical strain magnitude associated with the cessation of selective reflection.

Selective reflection study of excitation-dependent dipole-dipole interaction in dense rubidium vapour

We conducted a comprehensive study of excitation dependent dipole-dipole interaction in dense rubidium gas using the resonant selective reflection of weak probe radiation from the dielectric-rubidium vapor interface. The excitation was created by the pump laser beam. The study involved exploring a diverse range of pumping radiation intensity and detuning for the number density of the rubidium vapour 2.8 × 1016 cm-3. We have measured experimental selective reflection spectra in both linear and nonlinear pumping modes.

Waterborne Polymer Coatings with Bright Noniridescent Structural Colors.

Structural color pigments offer an efficient, sustainable, and environmentally friendly approach to obtain waterborne polymer coatings. We developed polymer-based spherical photonic pigments to incorporate in aqueous dispersions of polymer nanoparticles used to obtain waterborne polymer films. Our spherical photonic pigments are assembled from polymer nanoparticles and are highly stable in water dispersion, maintaining their optical properties in the final polymer films. Unlike conventional dyes and pigments, which are prone to photobleaching because they are based on the absorption of light, photonic pigments rely on the selective reflection of light by their nanostructure and therefore are not photodegraded. Furthermore, different colors can be obtained from the same materials, changing only their nanostructure, in this case, the size of the polymer nanoparticles. Our novel spherical photonic pigments are noniridescent and can be incorporated in aqueous polymer nanoparticle dispersions without deteriorating their structure to produce waterborne polymer coatings with structural color. This approach for structural colored waterborne polymer coatings is efficient, simple, and environmentally friendly, offering excellent prospects for application in paints and coatings.