Color Reflectance Research Articles

High brightness and angle insensitive full-color structural colors based on silicon mixture absorber

Here we propose a thin-film structure that produces bright and angle robust reflective structural colors based on spectral selective absorption mechanism. Through numerical analysis, the optical constants n and k of the lossy metal films were calculated under ideal absorption conditions, and the Si–Al (90%) films with a close refractive index was successfully prepared. Amorphous silicon, SiOX, and Si–Al (90%) films were constructed into appropriate thin film structures, which achieve optimal optical performance, broadband absorption, and excellent angle insensitivity. Using a series of silicon-based films, dual-frequency and broadband absorption can be achieved while minimizing the inevitable absorption losses at the peak wavelengths. These red, green, and blue reflective colors achieve a high level of reflectance with 96%, 83%, and 91% at resonance. With its excellent color properties, safety, and low fabrication costs, this scheme is suitable for wide-ranging applications, including display, color decoration, counterfeit detection, etc.

Structural Colors on Al Surface via Capped Cu-Si3N4 Bilayer Structure

Tunable structural colors have a multitude of applications in the beautification of mobile devices, in the decoration of artwork, and in the creation of color filters. In this paper, we describe a Metal-Insulator-Metal (MIM) design that can be used to systematically tune structural colors by altering the thickness of the top metal and intermediate insulator. Cu and Si3N4 were selected as the top metal and intermediate insulator layers, respectively, and various reflection colors were printed on Al. To protect the Cu surface from scratchiness and oxidation, a number of capping layers, including SiO2, LPSQ, PMMA, and the commercially available clear coat ProtectaClear, were applied. In addition to their ability to protect Cu from a humid environment without deteriorating color quality, ProtectaClear and LPSQ coatings have minimal angle dependency. Furthermore, a bilayer of PMMA/SiO2 can protect the Cu surface from the effects of humidity. In addition, the PMMA/SiO2 and ProtectaClear/SiO2 bilayers can also protect against corrosion on the Cu surface. The colors can be tuned by controlling the thickness of either the metal layer or intermediate insulator layer, and vivid structural colors including brown, dark orange, blue, violet, magenta, cyan, green-yellow, and yellow colors can be printed. The measured dielectric functions of Cu thin films do not provide any evidence of the plasmonic effect, and therefore, it is expected that the obtained colors are attributed to thin-film interference.

Reduced reflectance and altered color: The potential cost of external particulate matter accumulation on urban Rock Pigeon (Columba livia) feathers

Feather quality in birds is considered an honest signal of individual health as feather appearance and function depend on an individual’s ability to maintain them. In addition to flight and insulation, feathers are essential for social interactions and sexual selection in the form of visual signals. Airborne particulate matter (PM) can accumulate on feather surfaces and alter feather appearance. We quantified PM accumulation on Rock Pigeon (Columba livia) feathers and analyzed the spectral properties of extracted particulates. Feathers were sampled from two pigeon populations, one rural and one urban, in the Dallas-Fort Worth area, with 47 and 93% developed land cover, respectively, within 2 km of the populations. We determined accumulated PM gravimetrically after rinsing feathers and then measured the visible-near-infrared diffuse reflectance and color properties of extracted particulates. The rate of PM accumulation on rural birds was higher than on urban birds. However, feather particulates collected from urban pigeons had significantly lower total reflectance, reflectance in the visible portion of the spectrum, lightness, and hue angle compared to those of rural pigeons. The hue angle of rural feather particulates displayed a negative relationship with PM accumulation while total reflectance, reflectance in the visible range, and lightness of urban feather particulates were negatively related to PM accumulation. Our findings suggest that wild birds could incur an urban pollution penalty as PM accumulation has the potential to alter feather properties. Further research is needed to better understand the influence of external PM accumulation on the physiological and behavioral health of birds.

Customized Structural Color Filters by Pixel‐Level Electrothermal Regulation

AbstractReflective structural colors have backlight‐free supremacy and can provide energy‐efficient and environmental‐friendly color production for many applications. However, they are hindered by the complex process and incorrect colors after fabrication. Here, a pixelated electrothermal oxidation technique is proposed that can precisely tailor and produce reflective structural colors covering the entire visible range, providing the possibility for customized structural color filter arrays from a titanium canvas. Furthermore, as a demonstration, a photonic‐firework‐like color filter array is achieved in a single step via thermal engineering. Based on this one‐step‐forming color array, a computational spectrometer is also realized featuring a resolvability of 10 nm and operating bandwidth covering the whole visible range. Considering its compactness and mass production capability, this method has numerous potential applications in, e.g., imaging, anti‐counterfeiting, printing, color display, and spectroscopy.

Reversible Electrochromic Pattern in 3D Photonic Crystals Film from Thiol‐Acrylate‐Based Polymer‐Stabilized Blue Phase Liquid Crystals

AbstractEndowing advanced 3D photonic nanoarchitectures with dynamic controllability is currently in the spotlight of soft‐matter photonics research. Herein, a well‐defined photopatterned polymer‐stabilized blue phase (PSBP) film with 3D electrically tunable photonic bandgap is fabricated by a liquid crystalline thiol–acrylate monomers system. Remarkably, the addition of thiol monomers can effectively improve the electrical tuning performance of the PSBP film and maintain excellent thermal stability simultaneously. The proportions of acrylate and thiol monomers are proved to be a key factor on the morphology structure of the polymer network in the PSBP system. By masked UV photopolymerization, a patterned sample with a high‐quality image and reversible nonsynergistic electrochromism is obtained. With increasing electric field, the patterned region changes from initial green reflection color to a red one, while no obvious visual color change occurs for the shading region with initial blue reflection color. The distinct electrically responsive behaviors are mainly caused by the microstructural difference of polymer networks, which is engendered by the migration of polymerizable monomers during polymerization. The research disclosed herein may provide new insights into the development of patterned 3D self‐assembled liquid crystalline photonic crystal toward anticounterfeiting, displays, colorimetric sensors, labeling technology, and programming photonics.

Scalable Superabsorbers and Color Filters Based on Earth-Abundant Materials.

Optical materials based on unconventional plasmonic metals (e.g., magnesium) have lately driven rising research interest for the quest of possibilities in nanophotonic applications. Several favorable attributes of Mg, such as earth abundancy, lightweight, biocompatibility/biodegradability, and its active reactions with water or hydrogen, have underpinned its emergence as an alternative nanophotonic material. Here, we experimentally demonstrate a thin film-based optical device composed exclusively of earth-abundant and complementary metal-oxide semiconductor (CMOS)-compatible materials (i.e., Mg, a-Si, and SiO2). The devices can exhibit a spectrally selective and tunable near-unity resonant absorption with an ultrathin a-Si absorbing layer due to the strong interference effect in this high-index and lossy film. Alternatively, they can generate diverse reflective colors by appropriate tuning of the a-Si and SiO2 layer thicknesses, including all the primary colors for RGB (red, green, blue) and CMY (cyan, magenta, yellow) color spaces. In addition, the reflective hues of the devices can be notably altered in a zero power-consumption fashion by immersing them in water due to the resulted dissolution of the Mg back-reflection layer. These compelling features in combination with the lithography-free and scalable fabrication steps may promise their adoption in various photonic applications including solar energy harvesting, optical information security, optical modulation, and filtering as well as structure reuse and recycling.

Comparison of Electrodeposited and Sputtered Tungsten Trioxide Films for Inorganic Electrochromic Nanostructures

Electrochromic materials and their implementation with structural colors are currently being intensely researched because of their promising applications as non-emissive display devices utilizing ambient light. In particular, several fully inorganic devices that rely on electrochromic tungsten trioxide (WO3) have been presented. For preparing nanoscale films of this material, sputtering is the most established technique, but electrodeposition has recently been shown to be capable of achieving exceptionally high electro-optical modulation contrast without the need for expensive equipment. In this work, we investigate the possibilities of electrodeposited WO3 and present a systematic comparison with sputtered WO3 with respect to performance in electrochromic devices. Importantly, we show that “ultralarge” electro-optical modulation (∼95% change in transmission) is possible for both types of films. However, it is only the sputtered films that enable such high contrast in a stable electrolyte such as LiClO4 in propylene carbonate. The electrodeposited films are less uniform and difficult to make thicker than ∼500 nm. We find no evidence that the electrochromic properties of the electrodeposited WO3 are intrinsically better than those of sputtered WO3. However, the electrodeposited films are much rougher and/or porous on the nanoscale, which increases the switching speed considerably. We conclude that electrodeposited WO3 is mainly useful in applications in which high contrast is not essential while switching speed is. As an example, we present the first electrodeposited WO3 integrated with structural colors by sandwiching the material between two metal films. By electrical control, the reflective colors can then be tuned at least one order of magnitude faster (a few seconds) than previously reported while having fair color quality and without any loss of brightness.

Complex 3D‐Printed Mechanochromic Materials with Iridescent Structural Colors Based on Core–Shell Particles

AbstractA scalable protocol for design and subsequent 3D‐printing of polymeric core‐shell‐particles is reported. The particle synthesis by emulsion polymerization in starved‐feed mode is used for tailoring particle architecture and composition. Control of size, mechanical properties, and chemical functionalities allow to achieve the specific requirement profile for subsequent extrusion‐based additive manufacturing. The core‐shell particles consist of hard polystyrene cores and a comparably soft polyalkylacrylate‐based shell. Size and monodispersity, as well as core‐to‐shell ratio, are determined by means of dynamic light scattering and transmission electron microscopy. Thermal and rheological properties are investigated by means of dynamic scanning calorimetry and thermogravimetric analysis as well as oscillation and capillary rheometry. During 3D‐printing, the monodisperse particles self‐assemble into an ordered close packed lattice structure, leading to visible reflection colors according to Bragg's law of diffraction. Distinct and angle‐dependent reflection colors are recorded via UV‐vis spectroscopy. As the structural color depends, inter alia, on the underlying particle sizes, resulting colors are easily tunable by adjusting the applied synthesis parameters. Under mechanical deformation, the color changes due to controlled lattice deformation, which enables mechanochromic sensing with the printed objects. They are also promising candidates for decorative ornaments, smart optical coatings, or advanced security devices.

Organic Matter and Generation Potential of Paleozoic Deposits in the Tal’bei Block of the Chernyshev Ridge (Timan–Pechora Petroleum Province)

Abstract—The study area is located within the Chernyshev Ridge, a fold–thrust structure, which is a promising area for oil and gas exploration in the Timan–Pechora province. Petroleum source rocks of Paleozoic (from Upper Ordovician to lower Carboniferous) deposits stripped by the Vorgamusyurskaya-1 well in the Tal’bei block of the Chernyshev Ridge were studied by lithological, petrographic, organic-petrology, and organic-geochemistry methods. Most of the section is characterized by low values of Corg (<0.5%), genetic potential (S1 + S2 < 0.5 mg HC/g rock), and hydrogen index (HI < 100 mg HC/g Corg). Petroleum source rocks with elevated values of Corg (up to 0.9%), S1 + S2 (up to 2.1 mg HC/g rock), and HI (up to 277 mg HC/g Corg) were identified at different stratigraphic levels of the section: Upper Devonian (D3tm–sr), upper Silurian (S2gj), and Upper Ordovician (O3mt). The maceral composition of organic matter (OM) and the distribution of hydrocarbons in the saturated fraction of bitumens indicate that petroleum source rocks include only marine OM (type II kerogen). The degree of catagenesis of OM was estimated based on the bituminite reflectance (RVeq), Rock-Eval pyrolysis (Tmax), and conodont color alteration index (CAI). The available RVeq (0.63–0.84%), Tmax (430–443 °C), and CAI (1.5–2.0) data indicate that the entire Paleozoic well section is in the oil window (MC1–MC3). The new data refined the catagenetic zoning of the sedimentary section, according to which the OM reached the following gradations: MC1 in the lower Carboniferous and Famennian deposits, MC2 in the interval from the Frasnian to the upper part of the lower Silurian, and MC3 in the lower part of the lower Silurian and in the Upper Ordovician. The results of the study of petroleum source rocks and the type and maturity of OM will be the basis for basin modeling, which will increase the reliability of reconstructions of oil and gas formation at the Chernyshev Ridge.

Centrifugation-Assisted Growth of Single-Crystalline Grains in Microcapsules.

Colloidal crystals have been tailored in a format of microspheres to use them as a building block to construct macroscopic photonic surfaces. However, the polycrystalline grains grown from the spherical surface usually exhibit low reflectivity. Although single-crystalline microspheres have been produced, it is difficult to control the crystal orientation. Here, we design spherical microcapsules with density anisotropy that contain single-crystalline grains along the heavy side. The microcapsules spontaneously align to have a heavy side down under the action of gravity and display a bright and uniform reflection color from the entire surface of the grains. Key to the success is the use of gentle centrifugal force to initiate nucleation and grow single-crystalline grains from the heavy side through depletion attraction. The microcapsules have density anisotropy due to the heterogeneity of the shell thickness, which causes them to self-align under centrifugation. At the same time, particles are accumulated on the heavy side, which produces many tiny grains on the heavy side immediately after the centrifugation. With controlled depletion attraction among particles, only a few grains survive during postincubation through Ostwald ripening, and one or a few giant single-crystalline grains are finally produced along the heavy side of each microcapsule.

Tuning of Optical Stopband Wavelength and Effective Bandwidth of Gel-Immobilized Colloidal Photonic Crystal Films.

We show that both the optical stopband wavelength and effective bandwidth of films of gel-immobilized loosely packed colloidal photonic crystals can be controlled over a wide range. When the gelation reagent of the charge-stabilized colloidal crystals was photopolymerized under ultraviolet light using different upper- and bottom-light intensities, it resulted in a gel-immobilized colloidal crystal film with a broadened Bragg reflection peak. Moreover, the width of the Bragg peak increased from 30 to 190 nm as the difference between the light intensities increased. Films with wider Bragg peaks exhibited a brighter reflection color because of the superposition of the shifted Bragg reflections. Furthermore, the Bragg wavelength could be varied over a wide range (500-650 nm) while maintaining the same broadened effective bandwidth by varying the swelling solvent concentration. These findings will expand the applicability of colloidal crystals for use in photonic devices and color pigments.

Structural Color of Multi-Order Fabry–Perot Resonator Based on Sc0.2Sb2Te3 Enhanced Saturated Reflection Color

The structural color based on the Fabry–Perot (F–P) resonator has been extensively applied lithography-free and tunable color displays. Conventional F–P cavity-based structural color technology exhibits a wide half maximum full width (fwhm), thus causing low color saturation. In this study, a Sc0.2Sb2Te3(SST) based structure of multi-order F–P cavity resonance was proposed to obtain high-saturation colors. The surface absorber of the multi-order F–P resonator structure was coated with an SST film, such that the reflection effect at nonresonant wavelengths was reduced. Moreover, ITO layer stacking served as F–P cavity resonance for multi-level modulation, and only a resonant wavelength was allowed to reflect. On that basis, the fwhm of nearly 25 nm and a peak reflectance of 90 was achieved. With the above structure, the color saturation can be dynamically regulated by the phase state of the SST. It is noteworthy that 60% sRGB color gamut space and 50% aRGB color gamut space can be currently achieved. The proposed modulation subsurface is expected to expand the color range of high-level and micro-nano display technology.

Monolithic MOF-Based Metal-Insulator-Metal Resonator for Filtering and Sensing.

Metal-insulator-metal (MIM) configurations based on Fabry-Pérot resonators have advanced the development of color filtering through interactions between light and matter. However, dynamic color changes without breaking the structure of the MIM resonator upon environmental stimuli are still challenging. Here, we report monolithic metal-organic framework (MOF)-based MIM resonators with tunable bandwidth that can boost both dynamic optical filtering and active chemical sensing by laser-processing microwell arrays on the top metal layer. Programmable tuning of the reflection color of the MOF-based MIM resonator is achieved by controlling the MOF layer thicknesses, which is demonstrated by simulation of light-matter interactions on subwavelength scales. Laser-processed microwell arrays are used to boost sensing performance by extending the pathway for diffusion of external chemicals into nanopores of the MOFs. Both experiments and molecular dynamics simulations demonstrate that tailoring the period and height of the microwell array on the MIM resonator can advance the high detection sensitivity of chemicals.

LCP /TLC based composite multi-dimensional polarization-dependent anti-counterfeiting device

Modern anti-counterfeiting technology can effectively suppress and combat forgery and counterfeiting behaviors, which is of great significance in information security, national defense and economy. However, the realization of multi-dimensional, integrated, difficult-to-copy and easy-to-detect optical anti-counterfeiting devices is still a challenge. In this paper, a multi-dimensional and polarization-dependent anti-counterfeiting device with structure color is designed, which is composed of patterned liquid crystal polymer (LCP) nematic layer and thermotropic cholesteric liquid crystal (TLC) layer. It has the advantages of displaying and hiding polarization states, wide color tuning range, convenient operation, high integration and security. For incident light with a specific polarization state, the patterned nematic phase LCP layer can carry out regionalized phase editing and polarization state modulation, while the TLC layer can selectively reflect the incident light. Therefore, a patterned structural color security label is subtly realized. The anti-counterfeiting device can realize the display, hiding, color adjustment and image/background conversion of patterns by adjusting the polarization direction of incident light. In addition, the TLC layer in the device can meet the application requirements of the anti-counterfeit device at different environmental temperatures through the flexible design of the system weight ratio. Furthermore, the device can be easily heated by body temperature, realize dynamic real-time wide-spectrum color modulation and reversible pattern erasure, and further enhance its security dimension and security. The multi-polarization-type anti-counterfeiting device has three-dimensional anti-counterfeiting efficacy. The first dimensional anti-counterfeiting efficacy is achieved by the thermochromic liquid crystal layer. The thermochromic liquid crystal layer has no reflection color outside the operating temperature range of TLC material, and the entire device displays black background. The second and the third dimensional anti-counterfeiting efficacy are related to the polarization state of the incident light and the linear polarization direction, respectively. Only when the incident light is linearly polarized light and its polarization direction makes an angle of 45° or –45° with respect to the optical axis of the liquid crystal, will the device show the designed pattern. Consequently, our proposed anti-counterfeiting device is expected to provide a new idea for developing the anti-counterfeiting field.

Anti-counterfeiting holographic liquid crystal gels with color and pattern control

Optically tunable and switchable reflection grating LC gels can be used in anti-counterfeiting materials for optical security and encryption. Reflection colors and patterns appear by applying an AC field and a DC field controls the holographic color of the samples.

Direct detection of neuron-specific enolase using a spectrometer-free colorimetric plasmonic biosensor.

Sensitive detection of a tumor marker, neuron-specific enolase (NSE), was performed by a label-free direct immunoassay based on a colorimetric plasmonic biosensor. Reflective plasmonic colors of silver nanodome arrays provided a way for a sensitive refractive index sensor based on spectrometer-free colorimetric detection. The direct detection of NSE was demonstrated by a combination of a sensitive sensor substrate and image processing. The limit of detection (LOD) for NSE was determined to be 270 pM, which is lower than the clinical threshold value of NSE used for medical diagnostics of small-cell lung cancer. Since our substrate-based colorimetric plasmonic biosensor is compatible with smartphone detection, we believe that the presented biosensor will open up a way for biosensor technology for point-of-care testing as well as mobile health applications.

Reflective high-efficiency color filters for wide color gamut LCD

Reflective high-efficiency color filters for wide color gamut LCD

Room-Temperature Cholesteric Liquid Crystals of Cellulose Derivatives with Visible Reflection.

Hydroxypropyl cellulose (HPC) derivatives with alkanoyl side chains have attracted attention as bio-based cholesteric liquid crystal (CLC) materials with reflection colors. By taking advantage of the ability to change the reflection color in response to external stimuli, the thermotropic CLCs can be applied to a wide variety of photonic devices for a sustainable society of future generations. However, the thermotropic CLCs of HPC derivatives substituted with only one kind of alkanoyl group are not suitable for such applications because they do not exhibit visible reflection at room temperature. In this report, we describe a promising strategy to control the reflection colors of HPC derivatives at room temperature by introducing two kinds of alkanoyl groups with different lengths into the side chains of HPCs, which also enables the fine control of temperature dependence on the reflection wavelength. By chemically optimizing the side chain, we successfully prepared room-temperature thermotropic CLCs of HPC derivatives with visible reflection. This report would contribute toward the development of versatile photonic applications by CLCs produced from biomass.

A new bunting species in South China revealed by an integrative taxonomic investigation of the Emberiza godlewskii complex (Aves, Emberidae)

Species boundaries of birds across the Euro-Asian continent, especially the widely distributed passerines with multiple subspecies, are not completely resolved. The Rock Bunting complex, Emberiza cia and E. godlewskii, is subject to ongoing debate due to geographic variation in its morphology, and deep genetic splits. In this study, the phylogenetic relationships of the E. cia/godlewskii complex were evaluated based on multilocus coalescence methods combined with analyses of plumage color and morphological variation. Although the northern and southern populations of E. godlewski formed reciprocally monophyletic groups, the northern E. godlewskii clade is the sister group to E. cia, rendering E. godlewskii as currently defined paraphyletic. The significant differences in morphometry and plumage color reflectance are complementary and support the phenotypic validity of northern and southern E. godlewskii. Thus, we recommend the following taxonomic revisions: the northern and southern E. godlewskii subspecies should be recognized as different full species, such as E. godlewskii and E. yunnanensis, respectively. This resolves the issue that E. godlewskii, as currently defined, is paraphyletic. Our research provides direct evidence of cryptic species in old world buntings and highlights the underestimation of species diversity in birds in East Asia.

Application of a hyperspectral camera for colorimetric measurements on polychrome surfaces in a controlled environment and evaluation of three image processing software for displaying colorimetric data: Pros and cons of the methodology presented

AbstractThis work explores the potential of a compact hyperspectral camera, Specim IQ, for the remote colorimetric study of polychrome surfaces in controlled environments. These measurements are generally made with dedicated instruments, such as colorimeters or spectrophotometers, which require contact with the surface and coverage areas of the order of 10s of mm2. These two characteristics, contact and a very small analysis area, can severely limit the study of polychrome surfaces, since the measured areas are not necessarily representative of the entire surface. In addition, it is not always possible to touch the objects being analyzed. A possible alternative is the use of compact hyperspectral cameras, such as Specim IQ, for the in situ study of the spectral and colorimetric characteristics of these surfaces. To better address this research, which is still in the preliminary phase, a 2 × 45°/0° geometry measuringement was used. The illumination of the image plane at 45° with respect to the camera made it possible to eliminate the components reflected specularly on the camera lens. With this shooting geometry, Labsphere Color and Gray Reflectance Standards (eight color and four gray standards) were analyzed and placed on seven different color backgrounds. With the spectral data acquired, it was possible to calculate the color of the targets and display the colorimetric values by means of three commonly used image processing software packages. In this way, it was possible to define for this hyperspectral camera a measurement‐data processing procedure applicable to measurements in the laboratory aimed at studying the color of polychrome surfaces.