Short-range order and near-field effects on optical scattering and structural coloration

An Angle-Independent Multi-Color Display Electro-Responsive Hydrogel Film

In nature, some organisms have the ability to camouflage to adapt to environmental changes; they blend with the environment by changing their skin colors. Such a phenomenon is of great significance for the research of adaptive camouflage materials. In this study, we propose a novel design scheme for the study of angle-independent photonic materials and successfully prepare an electrically tunable multi-color display angle-independent inverse opal photonic gel (IOPG). After photopolymerization of hydroxyethyl methacrylate with ionizable monomer acrylic acid (AA) in a long-range disordered opal template and etching, the angle-independent inverse opal photonic gel is obtained, presenting a single structural color. The electrically responsive color changes can be achieved at different angles. The color of the disordered AA-IOPG changes from green to blue-green when applying +4 V bias voltage and from green to orange when applying -4 V bias voltage. The electrochromism of the disordered AA-IOPG is mainly due to the local pH change caused by water electrolysis under bias voltage, which leads to a change of the swelling ratio. The disordered AA-IOPG shows high color tunability and durability through repeated opposite bias voltage tests, indicating that it is a promising conductive photonic material.

Mechanism of structural colors in binary mixtures of nanoparticle-based supraballs

Inspired by structural colors in avian species, various synthetic strategies have been developed to produce noniridescent, saturated colors using nanoparticle assemblies. Nanoparticle mixtures varying in particle chemistry and size have additional emergent properties that affect the color produced. For complex multicomponent systems, understanding the assembled structure and a robust optical modeling tool can empower scientists to identify structure-color relationships and fabricate designer materials with tailored color. Here, we demonstrate how we can reconstruct the assembled structure from small-angle scattering measurements using the computational reverse-engineering analysis for scattering experiments method and use the reconstructed structure in finite-difference time-domain calculations to predict color. We successfully, quantitatively predict experimentally observed color in mixtures containing strongly absorbing nanoparticles and demonstrate the influence of a single layer of segregated nanoparticles on color produced. The versatile computational approach that we present is useful for engineering synthetic materials with desired colors without laborious trial-and-error experiments.

A Continuous Gradient Colloidal Glass

Colloidal crystals and glasses manipulate light propagation depending on their chemical composition, particle morphology, and mesoscopic structure. This light-matter interaction has been intensely investigated, but a knowledge gap remains for mesostructures comprising a continuous property gradient of the constituting particles. Here, a general synthetic approach to bottom-up fabrication of continuous size gradient colloidal ensembles is introduced. First, the technique synthesizes a dispersion with a specifically designed gradual particle size distribution. Second, self-assembly of this dispersion yields a photonic colloidal glass with a continuous size gradient from top to bottom. Local and bulk characterization methods are used to highlight the significant potential of this mesostructure, resulting in vivid structural colors along, and in superior light scattering across the gradient. The process describes a general pathway to mesoscopic gradients. It can expectedly be transferred to a variety of other particle-based systems where continuous gradients will provide novel physical insights and functionalities.

3D Tomographic Analysis of the Order-Disorder Interplay in the Pachyrhynchus congestus mirabilis Weevil

The bright colors of Pachyrhynchus weevils originate from complex dielectric nanostructures within their elytral scales. In contrast to previous work exhibiting highly ordered single-network diamond-type photonic crystals, here, it is shown by combining optical microscopy and spectroscopy measurements with 3D focused ion beam (FIB) tomography that the blue scales of P. congestus mirabilis differ from that of an ordered diamond structure. Through the use of FIB tomography on elytral scales filled with platinum (Pt) by electron beam-assisted deposition, it is revealed that the red scales of this weevil possess a periodic diamond structure, while the network morphology of the blue scales exhibit diamond morphology only on the single scattering unit level with disorder on longer length scales. Full wave simulations performed on the reconstructed volumes indicate that this local order is sufficient to open a partial photonic bandgap even at low dielectric constant contrast between chitin and air in the absence of long-range or translational order. The observation of disordered and ordered photonic crystals within a single organism opens up interesting questions on the cellular origin of coloration and studies on bio-inspired replication of angle-independent colors.

Polymer-based self-assembled photonic crystals to tune light transport and emission

The advent of photonic crystals has made possible the idea of controlling flows of light, which has revolutionized photonics-based technology. Photonic crystals are constructed based on periodic refractive index variations in one-, two-, or three-spatial dimensions on the optical-wavelength scale. Photonic crystals show inherent photonic stop gaps or band gaps depending upon the crystal symmetry and refractive index contrast. Showing ease of fabrication, polymer-based self-assembled photonic crystals with stop gaps have been widely explored. We discuss angle- and polarization-dependent stop gap creation and splitting at higher angles of incidence. The observed stop gaps in self-assembled photonic crystals often deviate from theoretical predictions due to experimental constraints, such as finite size and fabrication disorders associated with samples. We perform micro-reflectivity experiments on a single domain, showing minimal disorder, with nearly 100% reflectivity, which is in agreement with theory. We obtain more than 75% emission intensity suppression and a 30% increase in the emission lifetime at the stop gap using micro-emission experiments in a single domain. This enables us to study the role of finite-size effects in photonic crystals in modifying the emission properties. We observe the linear scaling of the emission intensity suppression and the emission rate with the finite size of the crystal. Our single-domain experimental studies reveal that the use of low index-contrast self-assembled photonic crystals is a potential platform for strategically modifying light transport and emission properties.

Structural color from solid-state polymerization-induced phase separation

Structural colors are produced by wavelength-dependent scattering of light from nanostructures. While living organisms often exploit phase separation to directly assemble structurally colored materials from macromolecules, synthetic structural colors are typically produced in a two-step process involving the sequential synthesis and assembly of building blocks. Phase separation is attractive for its simplicity, but applications are limited due to a lack of robust methods for its control. A central challenge is to arrest phase separation at the desired length scale. Here, we show that solid-state polymerization-induced phase separation can produce stable structures at optical length scales. In this process, a polymeric solid is swollen and softened with a second monomer. During its polymerization, the two polymers become immiscible and phase separate. As free monomer is depleted, the host matrix resolidifies and arrests coarsening. The resulting polymeric composites have a blue or white appearance. We compare these biomimetic nanostructures to those in structurally-colored feather barbs, and demonstrate the flexibility of this approach by producing structural color in filaments and large sheets.

Light scattering from colloidal aggregates on a hierarchy of length scales

Disordered dielectrics with structural correlations on length scales comparable to visible light wavelengths exhibit interesting optical properties. Such materials exist in nature, leading to beautiful structural non-iridescent color, and they are also increasingly used as building blocks for optical materials and coatings. In this article, we explore the angular resolved single-scattering properties of micron-sized, disordered colloidal assemblies. The aggregates act as structurally colored supraparticles or as building blocks for macroscopic photonic glasses. We obtain first experimental data for the differential scattering and transport cross-section. Based on existing macroscopic models, we develop a theoretical framework to describe the scattering from densely packed colloidal assemblies on a hierarchy of length scales.

Self-assembling structural colour in nature

The diversity and vividness of structural colour in the natural world have been recognised as far back as William Hooke in the 17th century. Whilst it is only recently that advances in the field have revealed the elegance and finesse of the physics used to create these effects. In this topical review we will highlight some of the structures and effects responsible for colour in butterfly scales, bird feathers, plants, insects and beetle elytra that have been studied to date. We will discuss the structures responsible and look at similarities and differences in these structures between species. This will be alongside our current understanding of how these are created biologically, how they develop structurally and what control mechanisms nature has at its disposal to control structure formation.

Vivid structural colors from long-range ordered and carbon-integrated colloidal photonic crystals

A facile strategy to prepare high-quality colloidal photonic crystals (PCs) with good visibility is proposed. Based on a high refractive-index material (zinc sulfide), highly monodispersed colloidal particles are successfully produced and assembled into long-range ordered crystalline colloidal arrays. The carbon-based materials are in situ incorporated with the long-range ordered colloidal PCs, which endows PCs with the combined characteristics to simultaneously achieve an intense photonic stop band and excellent control of incoherent light scattering. Owing to these merits, the obtained ZnS colloidal PCs have demonstrated strong brightness with the maximum reflectivity of 98%. Moreover, the coloration, saturation, and viewing angle are all improved. This study provides a straightforward and cost-effective strategy to create structural colors with high-quality visibility, which is expected to facilitate future applications of colloidal PCs.

Environment and human friendly colored materials prepared using black and white components

A review describing how to prepare structural colored materials with less angle dependency using white and black substances.

Bioinspired bright noniridescent photonic melanin supraballs

Structural colors enable the creation of a spectrum of nonfading colors without pigments, potentially replacing toxic metal oxides and conjugated organic pigments. However, significant challenges remain to achieve the contrast needed for a complete gamut of colors and a scalable process for industrial application. We demonstrate a feasible solution for producing structural colors inspired by bird feathers. We have designed core-shell nanoparticles using high-refractive index (RI) (~1.74) melanin cores and low-RI (~1.45) silica shells. The design of these nanoparticles was guided by finite-difference time-domain simulations. These nanoparticles were self-assembled using a one-pot reverse emulsion process, which resulted in bright and noniridescent supraballs. With the combination of only two ingredients, synthetic melanin and silica, we can generate a full spectrum of colors. These supraballs could be directly added to paints, plastics, and coatings and also used as ultraviolet-resistant inks or cosmetics.

Non-invasive continuous monitoring of pro-oxidant effects of engineered nanoparticles on aquatic microorganisms

Engineered nanomaterials (ENMs) are key drivers for the development of highly sophisticated new technologies. As all new attainments, the rapidly increasing used of ENMs raise concerns about their safety for the environment and humans. There is growing evidence showing that if engineered nanomaterials are released into the environment, there is a possibility that they could cause harm to aquatic microorganisms. Among the divers effects triggering their toxicity the ability of ENMs to generate reactive oxygen species (ROS) capable of oxidizing biomolecules is currently considered a central mechanism of toxicity. Therefore, development of sensitive tools for quantification of the ROS generation and oxidative stress are highly sought. After briefly introducing ENMs-induced ROS generation and oxidative stress in the aquatic microorganisms (AMOs), this overview paper focuses on a new optical biosensor allowing sensitive and dynamic measurements of H₂O₂ in real-time using multiscattering enhanced absorption spectroscopy. Its principle is based on sensitive absorption measurements of the heme protein cytochrome c whose absorption spectrum alters with the oxidation state of constituent ferrous Fe^II and ferric Fe^III. For biological applications cytochrome c was embedded in porous random media resulting in an extended optical path length through multiple scattering of light, which lowers the limit of detection to a few nM of H₂O₂. The sensor was also integrated in a microfluidic system containing micro-valves and sieves enabling more complex experimental conditions. To demonstrate its performance, abiotic absorption measurements of low concentrations of dye molecules and 10 nm gold particles were carried out achieving limits of detection in the low nM range. Other biologically relevant reactive oxygen species can be measured at sub-μM concentrations, which was shown for glucose and lactate through enzymatic reactions producing H₂O₂. In ecotoxicological investigations H₂O₂ excreted by aquatic microorganisms exposed to various stressors were measured. Pro-oxidant effects of nano-TiO₂ and nano-CuO towards green alga Chlamydomonas reinhardtii were explored in various exposure media and under different light illuminations. Dynamics of Cd²⁺ induced effects on photosynthetic activity, sensitisation and recovery of cells of C. reinhardtii was also studied.

Maximal absorption regime in random media

Efficient optical energy transfer is key to many technologies, ranging from biosensing to photovoltaics. Here, for the first time we show that by introducing a random medium with appropriate filling factor, absorption in a specific volume can be maximized. Using both numerical simulations and an analytical diffusion model, we identify design rules to maximize absorption in the system with different geometrical and scattering properties. By combining a random medium with an open photonic cavity, we numerically demonstrate a 23-fold enhancement of the absorbed energy. We also show how absorption as high as 99% can be reached in a device as thin as 500 μm for normal incidence illumination. Finally, our data indicate that introducing a non-absorbing random medium into a light trapping system for thin solar cells can enhance absorption of energy by a factor of 2.2. This absorption enhancement, caused by the random medium, is broadband and wide-angle and can help design efficient solar cells, light trapping devices, biosensors and random lasers.

Production of Structural Colors with High Contrast and Wide Viewing Angles from Assemblies of Polypyrrole Black Coated Polystyrene Nanoparticles

Structural color with wide viewing angles has enormous potential applications in pigment, ink formulation, displays, and sensors. However, colors obtained from colloidal assemblies with low refractive index contrast or without black additives typically appear pale. Here, we prepare polypyrrole (PPy) black coated polystyrene (PS) nanoparticles and demonstrate well-defined colors with high color contrast and wide viewing angles under ambient light. Depending on the loading of pyrrole during polymerization, PPy nanogranules of different sizes and coverages are grafted to the surface of PS nanoparticles. The bumpy particles can self-assemble into quasi-amorphous arrays, resulting in low angle dependent structure colors under ambient light. The color can be tuned by the size of the PS nanoparticles, and the presence of the PPy black on PS nanoparticles enhances the color contrast by suppressing incoherent and multiple scattering.

Thermally Tunable Hydrogels Displaying Angle-Independent Structural Colors

We report the preparation of thermally tunable hydrogels displaying angle-independent structural colors. The porous structures were formed with short-range order using colloidal amorphous array templates and a small amount of carbon black (CB). The resultant porous hydrogels prepared using colloidal amorphous arrays without CB appeared white, whereas the hydrogels with CB revealed bright structural colors. The brightly colored hydrogels rapidly changed hues in a reversible manner, and the hues varied widely depending on the water temperature. Moreover, the structural colors were angle-independent under diffusive lighting because of the isotropic nanostructure generated from the colloidal amorphous arrays.

Near-Field Effects in Mesoscopic Light Transport

In dense multiple scattering media, optical fields evolve through both homogeneous and evanescent waves. New regimes of light transport emerge because of the near-field coupling between individual scattering centers at mesoscopic scales. We present a novel propagation model that is developed in terms of measurable far- and near-field scattering cross sections. Our quantitative description explains the increase of total transmission in dense scattering media and its accuracy is established through both full-scale numerical calculations and enhanced backscattering experiments.

Cryptic iridescence in a fossil weevil generated by single diamond photonic crystals

Nature's most spectacular colours originate in integumentary tissue architectures that scatter light via nanoscale modulations of the refractive index. The most intricate biophotonic nanostructures are three-dimensional crystals with opal, single diamond or single gyroid lattices. Despite intense interest in their optical and structural properties, the evolution of such nanostructures is poorly understood, due in part to a lack of data from the fossil record. Here, we report preservation of single diamond (Fd-3m) three-dimensional photonic crystals in scales of a 735,000 year old specimen of the brown Nearctic weevil Hypera diversipunctata from Gold Run, Canada, and in extant conspecifics. The preserved red to green structural colours exhibit near-field brilliancy yet are inconspicuous from afar; they most likely had cryptic functions in substrate matching. The discovery of pristine fossil examples indicates that the fossil record is likely to yield further data on the evolution of three-dimensional photonic nanostructures and their biological functions.

Multiscattering-enhanced absorption spectroscopy

An original scheme for sensitive absorption measurements, particularly well-suited for low analyte concentrations, is presented. The technique is based on multiscattering-enhanced absorption spectroscopy (MEAS) and benefits from the advantages of conventional absorption spectroscopy: simplicity, rapidity, and low costs. The technique relies on extending the optical path through the sensing volume by suspending dielectric beads in the solution containing the analytes of interest, resulting in multiple scattering of light, which increases the optical path length through the sample. This way, a higher sensitivity and lower limit of detection, compared to those of conventional absorption spectroscopy, can be achieved. The approach is versatile and can be used for a broad variety of analytes. Here, it is applied to the detection of phenol red, 10 nm gold nanoparticles, and envy green fluorescence dye; the limit of detection is decreased by a factor of 7.2 for phenol red and a factor of 3.3 for nanoparticles and dye. The versatility of this approach is illustrated by its application in increasing the sensitivity of colorimetric detection with gold nanoparticle probes and a commercially available hydrogen peroxide bioassay. The influence of different parameters describing the scattering medium is investigated in detail experimentally and numerically, with very good agreement between the two. Those parameters can be effectively used to tailor the enhancement for specific applications and analytes.

Structurally coloured secondary particles composed of black and white colloidal particles

This study investigated the colourful secondary particles formed by controlling the aggregation states of colloidal silica particles and the enhancement of the structural colouration of the secondary particles caused by adding black particles. We obtained glossy, partially structurally coloured secondary particles in the absence of NaCl, but matte, whitish secondary particles were obtained in the presence of NaCl. When a small amount of carbon black was incorporated into both types of secondary particles, the incoherent multiple scattering of light from the amorphous region was considerably reduced. However, the peak intensities in the reflection spectra, caused by Bragg reflection and by coherent single wavelength scattering, were only slightly decreased. Consequently, a brighter structural colour of these secondary particles was observed with the naked eye. Furthermore, when magnetite was added as a black particle, the coloured secondary particles could be moved and collected by applying an external magnetic field.

Diffraction from arrays of plasmonic nanoparticles with short-range lateral order

We have measured the angular distribution of light scattered off 2D plasmonic Al nanoparticle ensembles. We created these samples with disk-like nanoparticles, 175 and 500 nm in diameter, respectively, using hole-mask colloidal lithography and electron beam lithography. The nanoparticle arrangements in the samples display the short-range order (but no long-range order) characteristic for an ensemble formed by random sequential adsorption. As a consequence of this, the ensemble scattering patterns can be quantitatively well described by combining the single-particle scattering pattern with a static structure factor that carries information about the diffraction effects caused by the short-range order of the ensemble. We also performed sensing experiments in which we monitored changes in the angle-resolved scattering intensity for a fixed wavelength as a function of the thickness of an ultrathin SiO(2) coating covering the Al nanoparticles. The data show that the angle and strength of the main diffraction peak vary linearly with SiO(2) coating thickness in the range 1.5-4.5 nm and suggest that measurements of the scattering profile could be a competitive alternative to traditional transmission measurements in terms of sensitivity.