Photonic Crystal Films Research Articles

Vapor Absorption and Liquefication Triggered Dynamic Color Changes and Pattern Conversions on Photonic Crystal Films for Anticounterfeiting.

The widespread use of counterfeit goods caused significant damage to economy, personal data security, and public health. There is an urgent need to develop innovative anticounterfeiting materials to enhance their performance. Anticounterfeiting labels based on responsive photonic crystals have been widely researched for the inherent difficulty in replicating the delicate structures and structure colors. By integrating various distinct nanoparticles (NPs), it is expected to produce photonic crystal patterns that offer more intricate color effects and improved anticounterfeiting capabilities. In this work, we reported a photonic crystal anticounterfeiting label with dynamic color variations in solvent vapors and pattern conversions upon vapor liquefication by utilizing three different nanoparticles including d-SiO2, h-SiO2, and m-SiO2 NPs as building blocks. This anticounterfeiting label exhibits a wealth of dynamic color changes associated with the absorption time, absorption rate, absorption medium, and microstructure of the nanoparticles, offering diverse visual effects and showcasing interesting anticounterfeiting performances. The dynamic color change or pattern conversion effect of this photonic crystal (PC) pattern is realized through refractive index changes induced by vapor absorption or solvent filling, which involves no volume changes and shows exceptional durability for repeated applications. This label shows promising broad applications in anticounterfeiting areas.

Dynamic exciton-to-dopant energy transfer tuning of Mn2+-Doped perovskite nanocrystals by PBG effect

Cesium lead halide perovskite nanocrystals (CsPbX3 NCs) doped with Mn2+ brings attractive long-wavelength emission and flexible color tunability. However, modulating the exciton-to-Mn2+ energy transfer efficiency poses a challenge, particularly when relying on traditional methods such as changing halogen composition or incorporating external cations, which often complicate the process. In this work, a simple photonic bandgap (PBG)-modulation strategy is applied to modulate the exciton-to-Mn2+ energy transfer by constructing a sandwich “PC-Perovskite-PC” structure with two photonic crystal (PC) films and Mn2+ doped CsPb(ClBr)3 NPs. When the PBG and the host emission band change from deviation to coincidence, the lifetime of Mn2+ decreases from 0.54 ms to 0.36 ms owing to the enhancing energy transfer efficiency by PBG effect. Meanwhile, the ratios of orange and blue emissions (O/B) increase from 1.36 to 1.6. In addition, the back energy transfer also can be modulated. When the PBG matches well with the Mn2+ emission band, the lifetime of the host emission exhibits the largest value (1.25 ns) owing to back energy transfer from Mn2+ the host. Meanwhile, the O/B value shows the smallest value (0.52) attributed to the decreased spontaneous radiation of Mn2+ and the enhanced host emission. When PBG deviates from the Mn2+ emission band, the lifetime of the host emission decreases but O/B value increases gradually. This work provides new strategy for the modulation of energy transfer in Mn2+ doped CsPb(ClBr)3 NPs.

All‐In‐One Photonic Crystals With Multi‐Stimuli‐Chromic, Color‐Recordable, Self‐Healable, and Adhesive Functions

AbstractFabricating photonic crystals (PCs) with dynamic multi‐stimuli‐responsive structural colors, recordable colors, and self‐healable properties is significant for their emerging applications, yet remains a big challenge. Here, an all‐in‐one multifunctional PC (MFPC) film with outstanding mechanochromic, thermochromic, solvatochromic, color/shape‐recordable, self‐healable, and adhesive functions is designed and prepared by simply non‐close‐assembling silica particles into the unique 2‐[[(Butylamino)carbonyl]oxy]ethyl acrylate (BCOEA) followed by photopolymerization. The success is mainly due to the rational combination of non‐close‐packing structures and BCOEA's characteristics, including its urethane groups with numerous sacrificial hydrogen bonds, temperature‐dependent refractive index, swellable and deformable polymer network, as well as low glass‐transition temperature. The MFPC's hue and color saturation can be reversibly and dynamically modulated by strain/pressure/solvents and temperature, respectively, thereby realizing visually and spectrally sensing these stimuli. More interestingly, the color‐responsiveness combined with other functions endows MFPCs with fascinating emerging applications, including multilayer optical filters, inkless printing, reconfigurable multicolor patterns, stretching‐based anti‐counterfeiting, etc. This work offers a new perspective for designing next‐generation smart photonic materials and will facilitate their all‐round applications.

Mechanochromic 3D Soft Photonic Crystals Enabled Anticounterfeiting and Encryption Information Storage

AbstractInspired by the color‐changing abilities of natural organisms, this research focuses on the design and fabrication of mechanochromic liquid crystal photonic crystal thin films with periodic self‐assembly structures. Benefiting from its unique 3D self‐assembled structure with a body‐centered cubic lattice, BP I film exhibits excellent mechanical properties, characterized by superior elasticity and toughness due to the presence of rigid and flexible microdomains and suitable dislocation line volumes. As the strain increases, the structural color of the BP I film demonstrates a repeatable and rapid shift from red to blue, covering the entire visible light spectrum with high reflectivity. The incorporation of fluorescent molecules further enhances the mechanical properties and endowing the film with circularly polarized luminescence under UV light irradiation. Utilizing different fluorescent dyes as inks, arbitrary patterned writing can be performed on the BPI film, enabling specific information hiding and encryption. The prepared label exhibits excellent environmental stability, presenting promising applications in information storage, anti‐counterfeiting, and flexible 3D displays. The study may contribute to a deeper understanding of the relationship between microstructure, mechanical deformation, and optical properties, potentially advancing the development of innovative responsive materials and multifunctional devices.

The Correlations between Microstructures and Color Properties of Nanocrystalline Cellulose: A Concise Review.

Cellulose nanocrystals (CNCs) are a green resource which can produce photonic crystal films with structural colors in evaporation-induced self-assembly; CNC photonic crystal films present unique structural colors that cannot be matched by other colored materials. Recently, the mechanisms of CNC photonic crystal films with a unique liquid crystal structure were investigated to obtain homogenous, stable, and even flexible films at a large scale. To clarify the mechanism of colorful CNC photonic crystal films, we briefly summarize the recent advances from the correlations among the preparation methods, microstructures, and color properties. We first discuss the preparation process of CNCs, aiming to realize the green application of resources. Then, the behavior of CNCs in the formation of liquid crystal phases is studied, considering the influence of the CNCs' size and shape, surface properties, and the types and concentrations of solvents. Finally, the film formation process of CNCs and the control of structural colors during the film formation are summarized, as well as the mechanisms of CNC photonic crystal films with full color. In summary, considering the above factors, obtaining reliable commercial CNC photonic crystal films requires a comprehensive consideration of the subsequent preparation processes starting from the preparation of CNCs.

Facile Synthesis of Soft Core-Shell Nanospheres for Constructing Multiresponsive Photonic Crystal Security Patterns.

Responsive photonic crystals (RPCs) presenting tunable structural colors under external stimuli are widely applied in the fields of dynamic displays, sensors, and anticounterfeiting. However, the development of multiresponsive photonic crystal (MRPC) films possessing versatile variable optical properties remains a significant challenge due to the tedious synthetic procedure of multifunctional building blocks and complex assembly processes, thereby constraining their extensive applications. In the present work, a series of soft nanospheres with a hydrophobic cores and responsive hydrophilic shells have been synthesized by a facile one-step surfactant-free emulsion polymerization method. The MRPC patterns were then prepared by depositing soft nanosphere emulsions onto the 3D-printed substrates with a topological structure followed by drying at room temperature. The shells of soft nanospheres deformed and fused with each other, resulting in the formation of transparent MRPC film patterns. The MRPC patterns exhibited brilliant structural color in a wet state but lost the color again after complete drying. Such a reversible structural color was ascribed to the change of the refractive index (RI) of the hydrophilic shell layers of nanospheres according to wetting/drying state shifting. Moreover, the on-demand designed MRPC patterns could rapidly respond to external stimuli of temperature, pH, and organic solvents in a reversible manner, and multichannel encrypted security labels were also demonstrated. We postulate that our facile and feasible approach can be applied to the systematic design and large-scale production of MRPC patterns for a variety of applications.

Optically Ambidextrous Reflection Circularly Polarized Luminescence Film with High Dissymmetry Factor and On‐Demand Handedness

AbstractThe self‐assembled cellulose nanocrystal (CNC) film has a left‐handed layered structure, which makes the reflection of right‐handed circularly polarized (CP) light a challenge. Herein, a nematic phase layer is designed and inserted into the chiral organization, to work as a half‐wave retarder and make the ambidextrous CP light reflection. The maximum reflectivity exceeds 80%, which breaks the 50% limitations of single‐direction light reflection and is the current maximum in all the reported CNC‐based films. This “sandwich‐like” structure displays different optical properties on both sides, displayed as the chromatism and the inversion of the circular dichroism signals. The dual CP light reflection and direction‐dependent optical phenomena are reserved in the synthesized circularly polarized luminescence (CPL) film, with the dissymmetry factor (glum) of −0.4245. However, this luminescent intensity and single‐direction emission are not enough in advanced optical systems. This work designs a triple CPL amplification path and develops the handedness inversion strategy, with the glum of −1.0551 and 0.4082. Then, the dual‐directional CPL emission films are designed, where the chiral optics can be switched on‐demand. Finally, the photonic crystal films are applied in the anti‐counterfeit and chiral superstructure induction.

Cellulose-Based Photonic Crystal Film with Multiple Stimulus-Responsive Performances

Cellulose-Based Photonic Crystal Film with Multiple Stimulus-Responsive Performances

Highly Flexible and Reversible Stimuli-Responsive Photonic Crystal Film in Anti-Counterfeit, Detector, and Coating.

Various fascinating optical characteristics in organisms encourage scientists to develop biomimetic synthesis strategies and mimic their unique microstructure. Inspired by the Chameleon's skin with tunable color and superior flexibility, this work designs the evaporated-induced self-assembly technique to synthesize the chiral photonic crystal film. Ultrasonic-intensified and additive-assisted techniques synergistically optimize the film properties, on the aspects of optic and mechanic. The film shows considerable rigidity and superior flexibility, which can undergo multiple mechanical deformations. Without destroying the chiral nematic structure, the ultimate strain approaches 50%, which exceeds most cellulose-derived film materials. It also integrates excellent optical performance. The film color can cover the total visible region by tuning the photonic bandgap and has angle-dependent properties. It can make the response to humidity and solvents, and chromaticity variation reflects the degree of stimulation. Importantly, this structural-dependent color change is reversible. Lastly, the photonic crystal materials with excellent mechanics and unique optics have been applied in the security. The anti-counterfeiting material design contains photonic crystal ink, repeatable writing paper, information-hiding film, and color-changing labels, with the features of environmentally friendly, economical, non-destructive, and convenient for authentication.

Preparation and Adsorption Photocatalytic Properties of PVA/TiO2 Colloidal Photonic Crystal Films.

Polyvinyl alcohol (PVA)/TiO2/colloidal photonic crystal (CPC) films with photocatalytic properties are presented, where TiO2 nanoparticles were introduced into the PVA gel network. Such PVA/TiO2/CPC films possess three-dimensional periodic structures that are supported with a PVA/TiO2 composite gel. The unique structural color of CPCs can indicate the process of material preparation, adsorption, and desorption. The shift of diffraction peaks of CPCs can be more accurately determined using fiber-optic spectroscopy. The effect of the PVA/TiO2/CPC catalyst films showed better properties as the degradation of methylene blue (MB) by the PVA/TiO2/CPC film catalyst in 4 h was 77~90%, while the degradation of MB by the PVA/TiO2 film was 33% in 4 h, indicating that the photonic crystal structure was 2.3~2.7 times more effective than that of the bulk structure.

Miniaturized spectrometers based on graded photonic crystal films.

Miniaturized spectrometers have become increasingly important in modern analytical and diagnostic applications due to their compact size, portability, and versatility. Despite the surge in innovative designs for miniaturized spectrometers, significant challenges persist, particularly concerning manufacturing cost and efficiency when devices become smaller. Here we introduce an ultracompact spectrometer design that is both cost-effective and highly efficient. The core dispersion element of this new design is a graded photonic crystal film, which is engineered by applying gradient stress during its fabrication. The film shows bandstop transmission spectral profiles, akin to a notch filter, enhancing light throughput compared to conventional narrowband filters. The spectral analysis, with a resolution of 5 nm and operating within the wavelength range of 450-650 nm, is conducted by reconstructing the spectrum from a series of such notch transmission profiles along the graded photonic crystal film, utilizing a sophisticated algorithm. This approach not only reduces manufacturing costs but also significantly improves the sensitivity (with a light throughput efficiency of 71.05%) and overall performance of the limitations of current technology, opening up new avenues for applications in diverse fields.

Robust and Durable Photonic Crystal with Liquid-Repellent Property for Self-Cleaning Coatings and Structural Colored Textiles.

Photonic crystal coatings with unique structural colors and self-cleaning properties have been providing an efficient way for substrate coloration. However, the enhancement of the robustness and durability of structural colored coatings to meet the requirements in diverse environments remains a challenging task. Here, to realize the application of photonic crystal films under various kinds of conditions, we present a poly(fluoroalkyl acrylate)-based colloidal photonic crystal (fCPC) coating. Fluorinated core-interlayer-shell (FCIS) colloidal particles of polystyrene (PS) core, poly(methyl methacrylate) (PMMA) interlayer, and poly(fluoroalkyl acrylate-ethyl acrylate-butyl acrylate) (P(FA-EA-BA)) shell copolymers have been first prepared by a stepwise emulsion polymerization. fCPCs with self-supporting property, reprocessing ability, friction resistance, as well as excellent wettability and liquid-repellent properties are successfully obtained via the bending-induced ordering technique (BIOT). When applied in antifouling applications, the fCPC film exhibits resistance against various oil and inorganic liquids. Furthermore, the fCPC coatings demonstrate their durability under outdoor conditions by maintaining stable color appearances during rainy and sunny conditions. Additionally, an electronic product adhered with the fCPC coatings is presented, which exhibits a surface that remains clean even after prolonged usage in comparison to the conventional CPC coating. Structural colored textiles with enhanced stability and functionalized liquid-repellent properties are achieved through a one-step process using FCIS particles. Therefore, the developed self-cleaning and comprehensive fCPC coatings capable of withstanding diverse conditions may open up new avenues for the advancement of structural coloration in decoration, vehicle, textile, and building.

Tamm phonon-polaritons triggered in hyperbolic material hexagonal boron nitride

Tamm phonon polaritons (TPhPs) have been recently predicted and experimentally observed for the first time in silicon carbide (SiC) film, allowing for enhanced light–matter interactions and new opportunities for manipulating light at the micro- and nano-scale. Hyperbolic material hexagonal boron nitride (hBN), a two-dimensional Van der Waals crystal, also supports phonon polaritons. However, TPhPs in hBN have not been systematically studied yet. In this paper, we theoretically investigate TPhPs triggered in the structure based on one-dimensional photonic crystal (PC) and hyperbolic material hBN film. It is found that the structure PC/spacer/semi-infinite hBN film can form TPhPs in the Type-II hyperbolic band, but not in the Type-I hyperbolic band. This phenomenon of selective excitation is attributed to the negative permittivity of hBN in the plane, rather than its out-of-plane permittivity. Importantly, TPhPs are sensitive to the thickness of the spacer, which can be regulated flexibly by changing the thickness. In addition, the selective excitation of TPhPs for hyperbolic bands in the configuration of finite hBN/spacer/PC is demonstrated using the same approach. It can be found that the absorption can reach 0.9 at the wavelength of 7.23 μm regardless of transverse electric or transverse magnetic waves, and the observed resonance has high quality factor of 181. This work provides a theoretical basis for TPhPs based on hyperbolic materials and has potential applications in highly sensitive sensors and selective absorbers.

Enhancement of Light Efficiency of Deep-Ultraviolet Light-Emitting Diodes by Encapsulation with a 3D Photonic Crystal Reflecting Layer.

Recently, UVC LEDs, which emit deep ultraviolet light, have found extensive applications across various fields. This study demonstrates the design and implementation of thin films of three-dimensional photonic crystals (3D PhCs) as reflectors to enhance the light output power (LOP) of UVC LEDs. The 3D PhC reflectors were prepared using the self-assembly of silica nanospheres on a UVC LED lead frame substrate via the evaporation-induced method (side) and the gravitational sedimentation method (bottom), respectively. These PhCs with the (111) crystallographic plane were deposited on the side wall and bottom of the UVC LED lead frame, acting as functional materials to reflect UVC light. The LOP of UVC LEDs with 3D PhC reflectors at a driving current of 100 mA reached 19.6 mW. This represented a 30% enhancement compared to commercial UVC LEDs with Au-plated reflectors, due to the UVC light reflection by the photonic band gaps of 3D PhCs in the (111) crystallographic plane. Furthermore, after aging tests at 60 °C and 60% relative humidity for 1000 h, the relative LOP of UVC LEDs with 3D PhC reflectors decreased by 7%, which is better than that of commercial UVC LEDs. Thus, this study offers potential methods for enhancing the light output efficiency of commercial UVC light-emitting devices.

Circularly Polarized Perovskite Luminescence in Composite Films with High Flexibility and Stability

AbstractFlexible circularly polarized luminescence (CPL) exhibits significant potential for applications in smart wearable devices and high‐end anti‐counterfeiting. However, existing technologies suffer from low dissymmetry factor (glum) and poor mechanical reliability. Here, a metal halide perovskite is employed as the emitter to construct efficient and robust flexible CPL films through a facile strategy of bilayer stacking, which comprises a layer of perovskite nanocrystalline embedded polymer film and a layer of photonic crystal film based on polymerized chiral liquid crystals (LCs). The photoluminescence efficiency of perovskite nanocrystals in polyacrylonitrile (PAN) matrix is improved by incorporating an appropriate amount of phenylethylammonium bromide (PEABr) as a passivation agent, and the helical arrangement and optical activity of free‐standing flexible photonic crystal films are found to be highly related to the supporting substrates. Upon stacking, the resulting flexible composite film exhibits extremely high |glum| values of up to 1.81, representing the highest value among reported flexible CPL‐active films. Moreover, the composite films demonstrate remarkable mechanical robustness, which shows negligible degradation in terms of glum value even after 1000 bending cycles at a bending radius of 2 mm. Based on the flexible CPL‐active films, advanced anti‐counterfeit labels with switchable patterns under circular polarizers are demonstrated.

A novel photonic crystal hydrogel inverse opal fluorescence enhancement platform for highly sensitive immunoassay of tumor markers

Cancer is a significant global health issue, necessitating early detection for effective diagnosis and treatment. We presented a novel method for detecting tumor markers using fluorescence-enhanced photonic crystal hydrogel inverse opal (PC-GELIO) membranes. We efficiently fabricated large-area, low-crack photonic crystal membranes using a homemade microfluidic chip. Composed of PEG-DA and PEG, the hydrogel replicated the photonic crystal structure, yielding photonic crystal hydrogel inverse opal membranes with a unique photonic band gap and inverse opal porous structure. These properties afford the PC-GELIO platform several advantages: Firstly, the high stability of the internal nanostructure in the large-area, low-crack photonic crystals film, prepared using the homemade microfluidic chip, is crucial for preserving the platform's exceptional fluorescence-enhanced sensing capabilities. Secondly, the peak wavelength emitted by fluorescent molecules aligns with the photonic band gap (PBG), significantly enhancing the fluorescence signal through the low-photon effect. Lastly, the high specific surface area provides numerous binding targets for biomolecule modification, and the hydrogel offers a uniform liquid environment conducive to biomolecule binding. We evaluated the fluorescence-enhanced sensing performance of the platform using alpha-fetoprotein (AFP). The limit of quantification (LOQ) for AFP was determined to be as low as 23 pg/mL, representing an order of magnitude improvement compared to non-enhanced platforms (0.29 ng/mL and 0.25 ng/mL). Consequently, this novel fluorescence-enhanced photonic crystal hydrogel inverse opal platform holds significant potential as a straightforward and efficient method for biomolecular detection.

Rapid Fabrication of Large-Grain Opal Films and Photonic Crystal Hydrogel Sensors by a Filter Paper-Enhanced Evaporation Chip.

Developing a rapid fabrication method for crack-free opal films is a significant challenge with broad applications. We developed a microfluidic platform known as the "filter paper-enhanced evaporation microfluidic chip" (FPEE-chip) for the fabrication of photonic crystal and inverse opal hydrogel (IOPH) films. The chip featured a thin channel formed by bonding double-sided adhesive poly(ethylene terephthalate) with a polymethyl methacrylate cover and a glass substrate. This channel was then filled with nanosphere colloids. The water was guided to evaporate rapidly at the surface of the filter paper, allowing the nanospheres to self-assemble and accumulate within the channel under capillary forces. Experimental results confirmed that the self-assembly method based on the FPEE-chip was a rapid platform for producing high-quality opal, with centimeter-sized opal films achievable in less than an hour. Furthermore, the filter paper altered the stress release mechanism of the opal films during drying, resulting in fewer cracks. This platform was proven capable of producing large-grain, crack-free opal films of up to 30 mm2 in size. We also fabricated crack-free IOPH pH sensors that exhibited color and size responsiveness to pH changes. The coefficient of variation of the gray color distribution for crack-free IOPH ranged from 0.03 to 0.07, which was lower than that of cracked IOPH (ranging from 0.07 to 0.14). Additionally, the grayscale peak value in 1 mm2 of the crack-free IOPH was more than twice that of the cracked IOPH at the same pH. The FPEE-chip demonstrated potential as a candidate for developing vision sensors.

Smartphone‐Based Molecularly Imprinted Photonic Crystal Hydrogel Sensor for the Label‐Free Detection of Bisphenol A

AbstractHere, the implementation of a smartphone‐based portable molecularly imprinted photonic crystal hydrogel (MIPCH) colorimetric sensor for the visual and label‐free detection of bisphenol A (BPA) in water samples is reported. The sensor is prepared by photopolymerizing BPA‐added hydrogel precursor solution within the voids of a photonic crystal opal film, followed by the extraction of BPA molecules. Rebinding of the BPA analyte to the MIPCH causes the hydrogel to swell, leading to a significant redshift in the diffracting spectrum. The MIPCH sensor exhibits a low limit of detection of 0.96 × 10−15 m. The sensor also shows linear response in the femtomolar range, rapid responsiveness (≤6 min), selective detection of BPA in complex matrices, long‐term stability, and reproducibility. Images of the sensor responses are used to train a deep‐learning‐based regression model on a smartphone to predict the BPA concentration quantitatively. Integration of the regression model with the developed sensor provides an accurate and portable smart sensor platform well‐suited for real‐time and on‐field monitoring of BPA.

Firefly-inspired bipolar information indication system actuated by white light

The indication of information in materials is widely used in our daily life, and optical encoding materials are ideal for information loading due to their easily readable nature and adjustable optical properties. However, most of them could only indicate one type of information, either changing or unchanging due to the mutual interference. Inspired by firefly, we present a non-interfering bipolar information indication system capable of indicating both changing and unchanging information. A photochemical afterglow material is incorporated into the photonic crystal matrix through a high-throughput technique called shear-induced ordering technique, which can efficiently produce large-area photonic crystal films. The indication of changing and unchanging information is enabled by two different utilizations of white light by the afterglow material and photonic crystals, respectively, which overcome the limitations of mutual interference. As a proof of concept, this system is used to indicate the changing photodegradation level of mecobalamin (a photosensitive medicine) and unchanging intrinsic drug information with anti-counterfeiting functionality, which is a promising alternative to instantly ascertain the efficacy of medicine at home where conventional assays are impractical.

Photonic Crystal-Integrated Semitransparent Solar Cell for Solar Greenhouse Application

Solar greenhouse technology emerges as a solution that intensifies crop yields and allows utilities to generate energy from photovoltaic cells. Here, we demonstrate a photonic crystal-integrated solar cell that selectively transmits photosynthetic light for plant cultivation and reflects the remaining light to the adjacent solar cells to generate electricity. The photonic crystal is fabricated using an e-beam evaporation method by nonperiodic stacking of TiO2 and SiO2 layers to transmit light at 400–500 nm (blue) and 600–700 nm (red) and reflect green (500–600 nm), ultraviolet, and infrared light. The photonic crystal-integrated solar cell is constructed with the vertically arranged copper indium gallium diselenide (CIGS) solar cell and the tilted photonic crystal with respect to the CIGS cell. By controlling the tilting angle of the photonic crystal film, it achieves the generated electric power of 40.2 W m-2 and the irradiance of 124.6 W m-2 for transmitted photosynthetic lights (400–500 and 600–700 nm). In contrast, a conventional horizontal CIGS cell shows a power generation of 55.6 W m-2 without any light transmission. This work provides a new optical strategy and design principle for the development of a wavelength-selective semitransparent solar cell.