Grant Tyler England

Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna

Significance In the course of evolution, many organisms have developed unique light manipulation strategies that rely on intriguing combinations of a broad range of optical effects generated by materials with sophisticated multiscale hierarchical structural arrangements. By exploiting the optical principles underlying natural structural color, we can generate new photonic materials. Researchers have only just begun to match nature’s morphological and compositional complexity in man-made materials using nanofabrication. We present a bioinspired photonic material that mimics the reverse color-order diffraction found in the butterfly Pierella luna. Exploiting and improving the butterfly’s strategy, we create photonic materials that increase our basic understanding of the optical interplay of hierarchical structures and provide a platform for the development of novel photonic devices. Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.

Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies

Significance Controlling the internal structure over multiple length scales can produce materials with superior properties. This hierarchical design is ubiquitous in nature where materials have evolved to show maximum functionality from a limited choice of available building blocks. Mimicking the emergence of functionality from simple building blocks is a key challenge for man-made materials. Here, we show how a simple confined self-assembly of colloidal particles leads to a complex geometry that displays a surprising variety of optical effects. These effects are a result of the intricate interaction of light with the structural features at different length scales, and the geometry of the self-assembled structure. The results underline the importance of controlling assembly processes over multiple length scales to tailor properties and maximize performance. Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal’s curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies—potentially as more efficient mimics of structural color as it occurs in nature.

Combining Bottom-Up Self-Assembly with Top-Down Microfabrication to Create Hierarchical Inverse Opals with High Structural Order

Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, a bottom-up assembly of polystyrene particles in the presence of a silica sol-gel precursor material (tetraethylorthosilicate, TEOS), which creates crack-free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, is combined with top-down microfabrication techniques. Micrometer scale hierarchical superstructures having a highly regular internal nanostructure with precisely controlled crystal orientation and wall profiles are produced. The ability to combine structural order at the nano- and microscale enables the fabrication of materials with complex optical properties resulting from light-matter interactions at different length scales. As an example, a hierarchical diffraction grating, which combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micrometer scale periodicity, is demonstrated.

Multifunctionality of chiton biomineralized armor with an integrated visual system

A set of strong eyes Although many biological tissues serve more than one purpose, rarely are they optimized to do multiple tasks well. When you try to optimize for one functionality, it comes at the expense of another. Li et al. investigated the biomineralized armor of the small mollusc chiton Acanthopleura granulata. The armor appears to be optimized for both mechanical strength and for image capture by hundreds of integral aragonite-based lenses. Science, this issue p. 952 Chiton armor shows both mechanical strength and optical functionality. Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. One such example is the biomineralized armor of the chiton Acanthopleura granulata, which incorporates an integrated sensory system that includes hundreds of eyes with aragonite-based lenses. We use optical experiments to demonstrate that these microscopic lenses are able to form images. Light scattering by the polycrystalline lenses is minimized by the use of relatively large, crystallographically aligned grains. Multiscale mechanical testing reveals that as the size, complexity, and functionality of the integrated sensory elements increase, the local mechanical performance of the armor decreases. However, A. granulata has evolved several strategies to compensate for its mechanical vulnerabilities to form a multipurpose system with co-optimized optical and structural functions.

Photothermally triggered actuation of hybrid materials as a new platform for in vitro cell manipulation

Mechanical forces in the cell’s natural environment have a crucial impact on growth, differentiation and behaviour. Few areas of biology can be understood without taking into account how both individual cells and cell networks sense and transduce physical stresses. However, the field is currently held back by the limitations of the available methods to apply physiologically relevant stress profiles on cells, particularly with sub-cellular resolution, in controlled in vitro experiments. Here we report a new type of active cell culture material that allows highly localized, directional and reversible deformation of the cell growth substrate, with control at scales ranging from the entire surface to the subcellular, and response times on the order of seconds. These capabilities are not matched by any other method, and this versatile material has the potential to bridge the performance gap between the existing single cell micro-manipulation and 2D cell sheet mechanical stimulation techniques.

Hierarchical structural control of visual properties in self-assembled photonic-plasmonic pigments

We present a simple one-pot co-assembly method for the synthesis of hierarchically structured pigment particles consisting of silica inverse-opal bricks that are doped with plasmonic absorbers. We study the interplay between the plasmonic and photonic resonances and their effect on the visual appearance of macroscopic collections of photonic bricks that are distributed in randomized orientations. Manipulating the pore geometry tunes the wavelength- and angle-dependence of the scattering profile, which can be engineered to produce angle-dependent Bragg resonances that can either enhance or contrast with the color produced by the plasmonic absorber. By controlling the overall dimensions of the photonic bricks and their aspect ratios, their preferential alignment can either be encouraged or suppressed. This causes the Bragg resonance to appear either as uniform color travel in the former case or as sparse iridescent sparkle in the latter case. By manipulating the surface chemistry of these photonic bricks, which introduces a fourth length-scale (molecular) of independent tuning into our design, we can further engineer interactions between liquids and the pores. This allows the structural color to be maintained in oil-based formulations, and enables the creation of dynamic liquid-responsive images from the pigment.

The Optical Janus Effect: Asymmetric Structural Color Reflection Materials

Structurally colored materials are often used for their resistance to photobleaching and their complex viewing-direction-dependent optical properties. Frequently, absorption has been added to these types of materials in order to improve the color saturation by mitigating the effects of nonspecific scattering that is present in most samples due to imperfect manufacturing procedures. The combination of absorbing elements and structural coloration often yields emergent optical properties. Here, a new hybrid architecture is introduced that leads to an interesting, highly directional optical effect. By localizing absorption in a thin layer within a transparent, structurally colored multilayer material, an optical Janus effect is created, wherein the observed reflected color is different on one side of the sample than on the other. A systematic characterization of the optical properties of these structures as a function of their geometry and composition is performed. The experimental studies are coupled with a theoretical analysis that enables a precise, rational design of various optical Janus structures with highly controlled color, pattern, and fabrication approaches. These asymmetrically colored materials will open applications in art, architecture, semitransparent solar cells, and security features in anticounterfeiting materials.

Art on the Nanoscale and Beyond

Methods of forming and patterning materials at the nano- and microscales are finding increased use as a medium of artistic expression, and as a vehicle for communicating scientific advances to a broader audience. While sharing many attributes of other art forms, miniaturized art enables the direct engagement of sensory aspects such as sight and touch for materials and structures that are otherwise invisible to the eye. The historical uses of nano-/microscale materials and imaging techniques in arts and sciences are presented. The motivations to create artwork at small scales are discussed, and representations in scientific literature and exhibitions are explored. Examples are presented using semiconductors, microfluidics, and nanomaterials as the artistic media; these utilized techniques including micromachining, focused ion beam milling, two-photon polymerization, and bottom-up nanostructure growth. Finally, the technological factors that limit the implementation of artwork at miniature scales are identified, and potential future directions are discussed. As research marches toward even smaller length scales, innovative and engaging visualizations and artistic endeavors will have growing implications on education, communication, policy making, media activism, and public perception of science and technology.

Emerging optical properties from the combination of simple optical effects

Structural color arises from the patterning of geometric features or refractive indices of the constituent materials on the length-scale of visible light. Many different organisms have developed structurally colored materials as a means of creating multifunctional structures or displaying colors for which pigments are unavailable. By studying such organisms, scientists have developed artificial structurally colored materials that take advantage of the hierarchical geometries, frequently employed for structural coloration in nature. These geometries can be combined with absorbers-a strategy also found in many natural organisms-to reduce the effects of fabrication imperfections. Furthermore, artificial structures can incorporate materials that are not available to nature-in the form of plasmonic nanoparticles or metal layers-leading to a host of novel color effects. Here, we explore recent research involving the combination of different geometries and materials to enhance the structural color effect or to create entirely new effects, which cannot be observed otherwise.

Corrigendum: Photothermally triggered actuation of hybrid materials as a new platform for in vitro cell manipulation

This corrects the article DOI: 10.1038/ncomms14700.