Photonic Films Research Articles

Self‐Assembled Nanorods and Microspheres for Functional Photonics: Retroreflector Meets Microlens Array (Advanced Optical Materials 9/2021)

The back-cover image displays an artistic scheme of micropatterned photonic film that is composed of cholesteric organized cellulose nanocrystals and a monolayer of single-crystal polystyrene microspheres, as demonstrated in article number 2002258 by Guang Chu and Orlando J. Rojas. Both bottom-up self-assembly of the micro/nano building blocks and top-down transfer imprinting approach are involved in the preparation process, providing a custom-tailored route to integrate polarization-sensitive retroreflection and microlens array in one structure.

Tailoring Disorder and Quality of Photonic Glass Templates for Structural Coloration by Particle Charge Interactions

To obtain high-quality homogeneous photonic glass-based structural color films over large areas, it is essential to precisely control the degree of disorder of the spherical particles used and reduce the crack density within the films as much as possible. To tailor the disorder and quality of photonic glasses, a heteroaggregation-based process was developed by employing two oppositely charged equal-sized polystyrene (PS) particle types. The influence of the particle size ratio on the extent of heteroaggregation in the suspension mixes is investigated and correlated with both the morphology and the resultant optical properties of the films. The results show that the oppositely charged particle size ratio within the mix greatly influences the assembled structure in the films, affecting their roughness, crack density, and the coffee-ring formation. To better differentiate the morphology of the films, scanning electron microscopy images of the microstructures were classified by a supervised training of a deep convolutional neural network model to find distinctions that are inaccessible by conventional image analysis methods. Selected compositions were then infiltrated with TiO2 via atomic layer deposition, and after removal of the PS spheres, surface-templated inverse photonic glasses were obtained. Different color impressions and optical properties were obtained depending on the heteroaggregation level and thus the quality of the resultant films. The best results regarding the stability of the films and suppression of coffee-ring formation are obtained with a 35 wt % positively charged over negatively charged particle mix, which yielded enhanced structural coloration associated with improved film quality, tailored by the heteroaggregation fabrication process.

Hydrogen‐Bond‐Induced High Performance Semitransparent Ternary Organic Solar Cells with 14% Efficiency and Enhanced Stability

AbstractSemitransparent organic solar cells (ST‐OSCs) have huge potential in terms of building integrated photovoltaics (BIPVs). However, the inherent contradiction between active layer absorption and average visible‐light transmittance (AVT) hinders the follow‐up development of ST‐OSC. To solve this problem, hydrogen bond strategy has been adopted to simultaneously improve the photon trapping capability and film thickness tolerance of the devices. Here, an organic small molecule material DIBC is introduced into PM6:Y6 system to form an intramolecular hydrogen bond with Y6, through which a high power conversion efficiency (PCE) of 17.20% is obtained. It is noted that when the active layer thickness is varied from 70 to 150 nm, the PCE values distributed in the range of 16.39–17.20%, which exhibits excellent film thickness tolerance. Moreover, ST‐OSCs are achieved with a maximum PCE of 14% and a high AVT of 21.60%, which is among the best reported results of ST‐OSCs. In addition, hydrogen‐bond‐based ST‐OSCs show superior thermal and light stability in the atmospheric environment corresponding to the control devices. This work provides a feasible solution for ST‐OSC with outstanding efficiency and high AVT, which is of great significance for the industrial production of BIPVs in the future.

Co‐Assembly of Cellulose Nanocrystals and Silk Fibroin into Photonic Cholesteric Films

AbstractControlled self‐assembly of bio‐sourced nanocolloids is of high importance for the development of sustainable and low‐cost functional materials but controlling nanocomposite fabrication with both satisfactory optical properties and composition remains challenging. Silk fibroin (SF) and cellulose nanocrystals (CNCs) have independently demonstrated their ability to produce high‐quality photonic materials, in part due to their low absorbance and their transparency in the visible range. While SF is able to replicate inverse structures by high‐resolution nano‐templating, CNCs can spontaneously assemble into cholesteric liquid crystalline structures that are retained upon solvent evaporation, yielding photonic films. In this work, the conditions of successful co‐assembly of regenerated SF, extracted from silkworm silk, with CNCs extracted from cotton, are investigated. Their co‐assembly is investigated for various relative concentration ratios and pH, combining polarized optical microscopy and spectroscopy, SEM, and other characterization techniques (XRD, ATR‐FTIR, TGA). The appearance of photonic properties is observed when CNC and SF are assembled at pH ≥ 4.15, highlighting the importance of suppressing attractive electrostatic interactions between the two species for an organized structure to emerge. Beyond its fundamental motivations for colloidal co‐assembly with structural proteins, this work is relevant to design sustainable optical materials compatible with food packaging coatings and edible coloring pigments.

Fabrication of imprinted photonic films via predesigned multiple UV‐polymerizations and their ability to detect solvents and metal ions in aqueous solution

AbstractTo synthesize photonic films without a chiral dopant, a predesigned multiple photopolymerization process was carried out. The photonic films were prepared by the photopolymerization of a mixture of chiral nematic liquid crystals. After polymerization, the chiral dopant, CB15, was removed and recycled. The imprinted photonic polymer films showed Bragg reflection without the presence of the chiral dopant. Upon the sensing of solvents in aqueous solution, significant color changes and peak shifts were observed by the naked eye and ultraviolet–visible spectroscopy, respectively. A linear calibration curve between the central wavelength of the reflection band of the fabricated imprinting film and the volume ratio of 1,4‐dioxane in water was observed. Furthermore, the sensing of chloroform content in methanol, ethanol, and acetone via the imprinted film were also investigated. The results suggest that the synthesized imprinted photonic films can detect different kinds of mixed solvents. The sensing properties of the photonic films were further improved by copolymerization with a rhodamine‐derived monomer. The synthesized modified photonic films can detect heavy metal ions in aqueous solution. This study reports a novel, recyclable, and easy approach to detect organic solvents and copper ions in aqueous solution.

Self‐Assembled Nanorods and Microspheres for Functional Photonics: Retroreflector Meets Microlens Array

AbstractPatterned micro/nanomaterials display efficient light management capabilities owing to their control of light propagation within multiscale periodic structures. Here a hierarchical photonic structure composed of polystyrene microspheres and cholesteric assembly of cellulose nanocrystals is described, acting as a polarization‐sensitive retroreflective coating and microlens array. Micropatterned photonic films are prepared by casting an aqueous cellulose nanocrystal suspension onto a monolayer of polystyrene microspheres substrate through evaporation‐assisted transfer imprinting lithography, integrating a bulk cholesteric matrix and patterned surface. By directing light at the as‐assembled polystyrene surface, an enhanced structural color develops from the circularly polarized light retroreflection. Whereas when light travelling across the photonic film, the transparent layer of polystyrene microspheres forms into plano‐convex microlens to converge the transmitted light into the focus plane and reduce centimeter‐scale illuminated image into a high‐fidelity miniaturized replica. This simple method, combining self‐assembly with imprinting lithography, is expected to pave the way for designing custom‐tailored optics with novel functions.

Lateral Gradient Ambidextrous Optical Reflection in Self-Organized Left-Handed Chiral Nematic Cellulose Nanocrystals Films.

Artificial photonic materials displaying ordered reflected color patterns are desirable in the field of photonic technologies, however, it is challenging to realize. Here we present that self-assembly of cellulose nanocrystals (CNC) in a tilted cuvette leads to the formation of rainbow color CNC films. We show that the self-organized CNC films enable simultaneous reflection of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light with lateral gradient transmittance ratio (LCP/RCP: 8.7–0.9) and the maximum reflectance value up to ca. 72%. This unique ambidextrous optical reflection arises from left-handed chiral photonic architectures with lateral gradient photonic bandgaps and nematic-like defects at the film-substrate interface and between left-handed photonic bandgap layers acting as a half-wavelength retarder. We demonstrate that the tilted angle self-assembly method provides a feasible step toward color patterning of CNC-based photonic films capable of ambidextrous optical reflection.

Nano-slit assisted high-Q photonic resonant perfect absorbers.

We propose and demonstrate a new kind of resonant absorber via introducing the nano-slit into a photonic film. The combination of the nano-slit cavity and the photonic waveguide provides a powerful way to manipulate the light behaviors including the spectral Q factors and the absorption efficiency. Ultra-sharp resonant absorption with the Q factors up to 579.5 is achieved, suggesting an enhancement of ∼6100% in contrast to that of the metal-dielectric flat film structure. Moreover, in comparison with the low absorption of 5.4% for the system without nano-slit, the spectral absorption is up to ∼96.6% for the nano-slit assisted photonic absorber. The high Q resonant absorption can be further manipulated via the structural parameters and the polarization state. The operation wavelengths can be tuned by the lattice constant. As the nano-slit introduced into the dielectric film, strong optical field confinement effects can be achieved by the cavity resonance via the nano-slit itself, and the guided resonant effect in the photonic waveguide cavity formed by the adjacent nano-slits. Otherwise, the photonic-plasmonic hybridization effect is simultaneously excited between the dielectric guided cavity layer and the metal substrate. These findings can be extended to other photonic nano-cavity systems and pave new insights into the high Q nano-optics devices.

Structural colours controlled by mixing two sized silica nanoparticles

Structural colour has attracted much attention because of its features of never fading, high saturation, high brightness, green environmental protection, the application of structurally coloured photonic crystal (PC) materials as pigments and coatings is becoming more and more mature. In this work, silica nanoparticles (SNPs) with two particle diameters were prepared by using Stöber method, PC films were fabricated by mixing two sized SNPs with different mixing ratios, the result showed that the structural colours of the photonic films had a blueshift when the ratio of the smaller sized SNPs increased, i.e. the structural colours changed from red to orange, yellow, yellow-green and green for the five selected increased ratios. In addition, PC films with uniform SNPs give iridescent structural colours, while those with mixed SNPs give non-iridescent structural colours. The method of producing different structural colours from two sized SNPs is simple and fast, and it can be applied to the development of a wide range of colourful photonic pigments.

Near-Infrared Light Induced Dynamic Structural Color Change of Amorphous Photonic Hydrogel

Light-induced photonic systems are rare in nature and usually lack a full color response in artificial systems. Herein, we first report a near-infrared (NIR) light controlled optical hydrogel; its structural color covers the full visible spectrum range. The hydrogel was fabricated by embedding dry thermoresponsive microgels film into a thermoresponsive hydrogel. Under the synergistic effect of the microgels and hydrogel, the hydrogel displayed a dynamic color with temperature changing, and it was reversible. Moreover, the structural colors were angle independent for the amorphous microgel arrays. When carbon nanotubes were integrated into the matrix hydrogel, dynamic structural colors can be displayed under NIR irradiation. Such NIR light-responsive photonic hydrogel film may provide potential applications in artificial skin, the military, and other fields.

Optical anticounterfeiting photonic bilayer film based on handedness of solid-state helicoidal structure.

Anticounterfeiting photonic bilayer films were fabricated by sandwiching two solid-state cholesteric liquid crystal films having different handedness. The fabricated photonic bilayer films were successfully applied to patterning by selective photopolymerization. This photonic bilayer film as a new cryptographic technology is of interest for its anticounterfeiting application.

Graphene Oxide Films for Ultra-Thin Optics and Linear and Nonlinear Integrated Photonic Circuits

With superior optical properties, high flexibility in engineering its material properties, and strong capability for large-scale on-chip integration, graphene oxide (GO) is an attractive solution for on-chip integration of two-dimensional (2D) materials to implement functional integrated photonic devices capable of new features. Over the past decade, integrated GO photonics, representing an innovative merging of integrated photonic devices and thin GO films, has experienced significant development, leading to a surge in many applications covering almost every field of optical sciences. This paper reviews the recent advances in this emerging field, providing an overview of the optical properties of GO as well as methods for the on-chip integration of GO. The main achievements made in GO hybrid integrated photonic devices for diverse applications are summarized. The open challenges as well as the potential for future improvement are also discussed.

Graphene Quantum Dot-TiO2 Photonic Crystal Films for Photocatalytic Applications.

Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO2 photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO2 interfacial electron transfer. TiO2 inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO2 photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO2 films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films.

Flexible Photonic Cellulose Nanocrystal Films as a Platform with Multisensing Functions

Biomimetic optical cellulose nanocrystal (CNC) materials have shown great potential for application in colorimetric sensing, anticounterfeiting, and decorative coatings because of simple recognitio...

Sweat-Based Noninvasive Skin-Patchable Urea Biosensors with Photonic Interpenetrating Polymer Network Films Integrated into PDMS Chips.

A wearable noninvasive biosensor for in situ urea detection and quantification was developed using a urease-immobilized photonic interpenetrating polymer network (IPNurease) film. The photonic IPN film was intertwined with solid-state cholesteric liquid crystals (CLCsolid) and a poly(acrylic acid) (PAA) network on a flexible poly(ethylene terephthalate) substrate adhered to a poly(dimethylsiloxane) (PDMS) chip that was fabricated using an aluminum mold. The presence of urea in the chemical matrix of human sweat red-shifted the reflected color of the photonic IPNurease film, and quantification was achieved by observing the wavelength at the photonic band gap (λPBG) with a limit of detection of 0.4 mM and a linear range of 0.9-50 mM. The color changes observed in the photonic IPN film were digitalized using the CIE 1931 xy coordinates on a cell phone image, thereby enabling fast, direct diagnosis via a downloadable app. This novel PDMS chip can be expanded for use with other biosensors.

Graphene Oxide for Integrated Photonics and Flat Optics.

With superior optical properties, high flexibility in engineering its material properties, and strong capability for large-scale on-chip integration, graphene oxide (GO) is an attractive solution for on-chip integration of 2D materials to implement functional integrated photonic devices capable of new features. Over the past decade, integrated GO photonics, representing an innovative merging of integrated photonic devices and thin GO films, has experienced significant development, leading to a surge in many applications covering almost every field of optical sciences such as photovoltaics, optical imaging, sensing, nonlinear optics, and light emitting. This paper reviews the recent advances in this emerging field, providing an overview of the optical properties of GO as well as methods for the on-chip integration of GO. The main achievements made in GO hybrid integrated photonic devices for diverse applications are summarized. The open challenges as well as the potential for future improvement are also discussed.

BaTiO3: Barium Titanate Nanostructures and Thin Films for Photonics (Advanced Optical Materials 24/2020)

Artemios Karvounis, Rachel Grange and co-workers review the recent progress of photonic nanostructures and thin films based on barium titanate (see article number 2001249). Barium titanate is a noncentrosymmetric crystal utilised in a various spectrum of applications from nonlinear optics and biophotonics up to electro-optics and photonic waveguides. The significance of this material platform relies on the plethora of growth capabilities either through bottom-up or top-down methods. The strong electrooptic response, the intrinsic second-order nonlinearity, and the tunable optoelectronic properties configure a technological platform of high interest.

Functionalized Mesoporous Photonic Crystal Film for Ultrasensitive Visual Detection and Effective Removal of Mercury (II) Ions in Water

AbstractMercury ion (Hg2+) contamination is a worldwide serious environment problem; exploring smart sensing and adsorption materials are urgently demanded for Hg2+ monitoring and removal. Herein, a simple 1D photonic crystal to first ever feature capabilities of visually quantitative determination and effective adsorption toward Hg2+ is facilely constructed by integrating a specifically designed thiourea‐functionalized nanocopolymer layer with a mesoporous TiO2 layer. Based on strong chelation and porous structure, Hg2+ is easily adsorbed on the copolymer and triggers vertically volumetric shrinkage, resulting in highlighted wavelength blue‐shifts and color changes in a broad Hg2+ level scope. Utilizing the adsorption characteristic, Hg2+ existing in aqueous media can be effectively removed by the photonic film with a remarkable uptake capacity of 739.6 mg g−1. This portable nanolayered film exhibits full regeneration, facile recovery, desirable selectivity toward Hg2+, and shields interference from other metal ions, which enables future application for environmental determination and remediation. Furthermore, a novel nanopolymer‐based two‐regime Hg2+‐capturing mechanism is first revealed by quartz crystal microbalance with dissipation monitoring, providing valuable references for future relevant adsorption researches.

Barium Titanate Nanostructures and Thin Films for Photonics

AbstractBarium titanate (BaTiO3) is a synthetic crystal used in electromechanical transducers and multilayer ceramic capacitors. Since it is not available in nature, a variety of growth methods has been employed to produce in large scale, with high quality and low‐cost. BaTiO3, as a metal oxide meets practical requirements such as physical hardness, stability and tunable optoelectronic properties. The plethora of characteristics renders it functional in diverse fields of applications from energy harvesting to biophotonics. Related to optical properties, it is a dielectric material from the near ultraviolet to the near‐infrared part of the spectrum with low optical losses and relatively high refractive index. The strong second‐order nonlinear response has resulted in several breakthroughs in bioimaging, while its intrinsic electrooptic response is among the highest within the existing materials. The properties of the BaTiO3 may also be modified by doping or hybridization with other materials. This review presents the basic optoelectronic properties of BaTiO3, reports on the recent advances in BaTiO3 nanostructures and thin films related to photonic applications, and oversees photonic technologies that may benefit from this material platform in the near future.

Photonic composite materials from cellulose nanorods and clay nanolayers

Cellulose nano crystals (CNCs) are promising materials for energy efficient buildings related to the control of reflectivity and heat absorption/reflection of light. In this sense it is important to improve CNCs films fire retardant properties, which can be achieved by adding clays. Cellulose nanocrystals (CNCs) and nanolayers obtained from Sodium Fluorohectorite (NaFh) synthetic clay are both known to form liquid crystalline phases in aqueous suspensions. CNCs form cholesteric phases, which structure is preserved after water evaporation, while dry NaFh nanolayers aligned films collapse. In this initial work, it is shown that CNCs are compatible with NaFh clay. We demonstrate that the liquid crystalline phase of CNCs in water is not destroyed by the presence of NaFh nanolayers. The NaFh nanolayers act as planar anchoring surfaces to the cellulose nanorods and, after evaporation of the water coloured films are obtained. The precursor solutions and the photonic films were investigated by Describe several techniques.