Harmonic distortions enhance circular dichroism of dielectric single gyroids.

The departure from strict periodic order in two-phase dielectric materials can offer properties that are otherwise inaccessible to perfectly ordered photonic crystals. Herewith, we investigate the circular dichroism of the single gyroid photonic crystal in the presence of spatial distortions. FDTD simulations and microwave transmission measurements on 3D-printed replicas show that certain harmonic long-wavelength spatial distortions ("sinusoidal chirp") nearly doubles the imbalance of the circular polarisation reflectances, as well as significantly strengthens polarisation-incoherent reflectance. The observed changes are partially rationalised by comparison with simpler distortion models (linear chirp and tetragonal deformation) of the Gyroid.

Controlled fluorescence in a beetle's photonic structure and its sensitivity to environmentally induced changes.

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

A unique self-organization of bacterial sub-communities creates iridescence in Cellulophaga lytica colony biofilms.

Iridescent color appearances are widespread in nature. They arise from the interaction of light with micron- and submicron-sized physical structures spatially arranged with periodic geometry and are usually associated with bright angle-dependent hues. Iridescence has been reported for many animals and marine organisms. However, iridescence has not been well studied in bacteria. Recently, we reported a brilliant "pointillistic" iridescence in colony biofilms of marine Flavobacteria that exhibit gliding motility. The mechanism of their iridescence is unknown. Here, using a multi-disciplinary approach, we show that the cause of iridescence is a unique periodicity of the cell population in the colony biofilm. Cells are arranged together to form hexagonal photonic crystals. Our model highlights a novel pattern of self-organization in a bacterial biofilm. "Pointillistic" bacterial iridescence can be considered a new light-dependent phenomenon for the field of microbiology.

Structural colour from helicoidal cell-wall architecture in fruits of Margaritaria nobilis.

The bright and intense blue-green coloration of the fruits of Margaritaria nobilis (Phyllanthaceae) was investigated using polarization-resolved spectroscopy and transmission electron microscopy. Optical measurements of freshly collected fruits revealed a strong circularly polarized reflection of the fruit that originates from a cellulose helicoidal cell wall structure in the pericarp cells. Hyperspectral microscopy was used to capture the iridescent effect at the single-cell level.

Towards outperforming conventional sensor arrays with fabricated individual photonic vapour sensors inspired by Morpho butterflies.

Combining vapour sensors into arrays is an accepted compromise to mitigate poor selectivity of conventional sensors. Here we show individual nanofabricated sensors that not only selectively detect separate vapours in pristine conditions but also quantify these vapours in mixtures, and when blended with a variable moisture background. Our sensor design is inspired by the iridescent nanostructure and gradient surface chemistry of Morpho butterflies and involves physical and chemical design criteria. The physical design involves optical interference and diffraction on the fabricated periodic nanostructures and uses optical loss in the nanostructure to enhance the spectral diversity of reflectance. The chemical design uses spatially controlled nanostructure functionalization. Thus, while quantitation of analytes in the presence of variable backgrounds is challenging for most sensor arrays, we achieve this goal using individual multivariable sensors. These colorimetric sensors can be tuned for numerous vapour sensing scenarios in confined areas or as individual nodes for distributed monitoring.

Bright-white beetle scales optimise multiple scattering of light.

Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres. Here, we show that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimised anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, we show that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. Our light transport investigation unveil high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nanostructure.

Discovery of the surface polarity gradient on iridescent Morpho butterfly scales reveals a mechanism of their selective vapor response.

For almost a century, the iridescence of tropical Morpho butterfly scales has been known to originate from 3D vertical ridge structures of stacked periodic layers of cuticle separated by air gaps. Here we describe a biological pattern of surface functionality that we have found in these photonic structures. This pattern is a gradient of surface polarity of the ridge structures that runs from their polar tops to their less-polar bottoms. This finding shows a biological pattern design that could stimulate numerous technological applications ranging from photonic security tags to self-cleaning surfaces, gas separators, protective clothing, sensors, and many others. As an important first step, this biomaterial property and our knowledge of its basis has allowed us to unveil a general mechanism of selective vapor response observed in the photonic Morpho nanostructures. This mechanism of selective vapor response brings a multivariable perspective for sensing, where selectivity is achieved within a single chemically graded nanostructured sensing unit, rather than from an array of separate sensors.

Bio-inspired band-gap tunable elastic optical multilayer fibers.

The concentrically-layered photonic structure found in the tropical fruit Margaritaria nobilis serves as inspiration for photonic fibers with mechanically tunable band-gap. The fibers show the spectral filtering capabilities of a planar Bragg stack while the microscopic curvature decreases the strong directional chromaticity associated with flat multilayers. Elongation of the elastic fibers results in a shift of the reflection of over 200 nm.

Variable multilayer reflection together with long-pass filtering pigment determines the wing coloration of papilionid butterflies of the nireus group.

The dorsal wing surfaces of papilionid butterflies of the nireus group are marked by bands of brilliant blue-green-colored cover scales. The thin, cuticular lower lamina of the scales acts as a blue reflector. The thick upper lamina forms a dense two-dimensional cuticular lattice of air cavities with a pigment acting as a long-pass optical filter. Reflectance spectra of small scale areas oscillate, but for large scale areas and the intact wing they are smooth. Theoretical modeling shows that the oscillations vanish for a scale ensemble with varying layer thicknesses and cavity dimensions. The scales combine in a subtle way structural and pigmentary coloration for an optical effect.

Papiliochrome II pigment reduces the angle dependency of structural wing colouration in nireus group papilionids.

The wings of four papilionid butterfly species of the nireus group, Papilio bromius, P. epiphorbas, P. nireus and P. oribazus, are marked by blue-green coloured bands surrounded by black margins. The cover scales in the coloured bands contain a violet-absorbing, blue-fluorescing pigment. The fluorescence and absorbance spectra of the nireus group wings are very similar to those of the wings of the Japanese yellow swallowtail, Papilio xuthus, and thus the pigment is presumably papiliochrome II. The scale structures of P. xuthus are arranged irregularly, and both the fluorescence and light reflection are diffuse. In the nireus papilionids, the spatial fluorescence distribution of the scales is also diffuse, but the reflection is specular. The scales have a multilayered structure, consisting of two main laminae. We show that the papiliochrome II pigment in the upper lamina of the scales functions as a violet-blocking long-pass filter in front of the lower lamina, thus limiting the reflectance spectrum to the blue-green wavelength range. Optical modelling showed that the papiliochrome II filter effectively removes the angle dependency of the reflectance spectra - that is, it reduces the wing iridescence. The contribution of the fluorescence signal to the visual appearance is minor.

Iridescence of a marine bacterium and classification of prokaryotic structural colors.

Iridescence is a property of structural color that is occasionally encountered in higher eukaryotes but that has been poorly documented in the prokaryotic kingdom. In the present work, we describe a marine bacterium, identified as Cellulophaga lytica, isolated from the surface of an anemone, that exhibits bright green iridescent colonies under direct epi-illumination. This phenomenon has not previously been investigated in detail. In this study, color changes of C. lytica colonies were observed at various angles of direct illumination or observation. Its iridescent green appearance was dominant on various growth media. Red and violet colors were also discerned on colony edges. Remarkable C. lytica bacterial iridescence was revealed and characterized using high-resolution optical spectrometry. In addition to this, by culturing other bacterial strains to which various forms of faintly iridescent traits have previously been attributed, we identify four principal appearance characteristics of structural color in prokaryotes. A new general classification of bacterial iridescence is therefore proposed in this study. Furthermore, a specific separate class is described for iridescent C. lytica strains because they exhibit what is so far a unique intense glitter-like iridescence in reflection. C. lytica is the first prokaryote discovered to produce the same sort of intense iridescence under direct illumination as that associated with higher eukaryotes, like some insects and birds. Due to the nature of bacterial biology, cultivation, and ubiquity, this discovery may be of significant interest for both ecological and nanoscience endeavors.

Glitter-like iridescence within the bacteroidetes especially Cellulophaga spp.: optical properties and correlation with gliding motility.

Iridescence results from structures that generate color. Iridescence of bacterial colonies has recently been described and illustrated. The glitter-like iridescence class, created especially for a few strains of Cellulophaga lytica, exhibits an intense iridescence under direct illumination. Such color appearance effects were previously associated with other bacteria from the Bacteroidetes phylum, but without clear elucidation and illustration. To this end, we compared various bacterial strains to which the iridescent trait was attributed. All Cellulophaga species and additional Bacteroidetes strains from marine and terrestrial environments were investigated. A selection of bacteria, mostly marine in origin, were found to be iridescent. Although a common pattern of reflected wavelengths was recorded for the species investigated, optical spectroscopy and physical measurements revealed a range of different glitter-like iridescence intensity and color profiles. Importantly, gliding motility was found to be a common feature of all iridescent colonies. Dynamic analyses of "glitter" formation at the edges of C. lytica colonies showed that iridescence was correlated with layer superposition. Both gliding motility, and unknown cell-to-cell communication processes, may be required for the establishment, in time and space, of the necessary periodic structures responsible for the iridescent appearance of Bacteroidetes.

Structural optimization for broadband scattering in several ultra-thin white beetle scales.

Recent work discovered the remarkable optical scattering properties of the scales of the white beetle Cyphochilus. It was suggested that its brilliant whiteness and brightness were due to optimization of the microstructure within its scales. Here we compare the microstructure of Cyphochilus scales to those of two other white beetles, Lepidiota stigma and Calothyrza margaritifera. Extensive optical modeling and experimental data suggest that each species displays structural optimization designed to maximize optical scatter. Optimization of the scale filling fraction is observed, as well as optimization of scattering center spacing and diameter. Cyphochilus, in particular, displays a high degree of structural optimization, resulting in its bright white appearance.

Developing optical efficiency through optimized coating structure: biomimetic inspiration from white beetles.

The recent discovery of brilliant whiteness in ultrathin beetle scales indicated the availability of significant whiteness, brightness, and opacity from limited sample thickness. This is achieved in the beetle through optimization of the packing density of scattering centers in its elytral scales. Here, we directly test and apply this idea to whiteness and brightness in the production and appearance of mineral coatings on paper by varying the scattering center parameters that underpin its optical properties. Through biomimetic design principles, we find that desirably high optical scattering from mineral coatings can be achieved. Commercially, by using appropriately designed coating formulations, this leads to the prospect of equal optical performance using less scattering material.

Spectroscopy on the wing: naturally inspired SERS substrates for biochemical analysis.

We show that naturally occurring chitinous nanostructures found on the wings of the Graphium butterfly can be used as substrates for surface-enhanced Raman scattering when coated with a thin film of gold or silver. The substrates were found to exhibit excellent biocompatibility and sensitivity, making them ideal for protein assaying. An assay using avidin/biotin binding showed that the substrates could be used to quantify protein binding directly from changes in the surface-enhanced Raman scattering (SERS) spectra and were sensitive over a concentration range comparable with a typical enzyme-linked immunosorbent assays (ELISA) assay. A biomimetic version of the wing nanostructures produced using a highly reproducible, large-scale fabrication process, yielded comparable enhancement factors and biocompatibility. The excellent biocompatibility of the wings and biomimetic substrates is unparalleled by other lithographically produced substrates, and this could pave the way for widespread application of ultrasensitive SERS-based bioassays.

A protean palette: colour materials and mixing in birds and butterflies.

While typically classified as either 'structural' or 'pigmentary', bio-optical tissues of terrestrial animals are rarely homogeneous and typically contain both a structural material such as keratin or chitin and one or more pigments. These base materials interact physically and chemically to create colours. Combinations of structured base materials and embedded pigment molecules often interact optically to produce unique colours and optical properties. Therefore, to understand the mechanics and evolution of bio-optical tissues it is critical to understand their material properties, both in isolation and in combination. Here, we review the optics and evolution of coloured tissues with a focus on their base materials, using birds and butterflies as exemplar taxa owing to the strength of our current knowledge of colour production in these animals. We first review what is known of their base materials, and then discuss the consequences of these interactions from an optical perspective. Finally, we suggest directions for future research on colour optics and evolution that will be invaluable as we move towards a fuller understanding of colour in the natural world.

Photonic crystal fiber in the polychaete worm Pherusa sp.

Setae of the polychaete worm Pherusa exhibit remarkably strong photonic effects, which arise from their two-dimensional-periodic internal structure of hexagonally packed cylindrical channels. The hexagonal order is limited to monocrystalline domains of different orientation, which results in an overall polycrystalline effect. A detailed experimental and theoretical investigation of this structure reveals that the internal photonic structure is carefully tuned with respect to its lattice constant in order to provide an optical response coinciding with the visible wavelength rage. A further optimization is observed for the packing fraction of cylindrical channels in order to maximize the width of photonic band gaps, and hence the reflectance of incident visible light.

Pterin pigment granules are responsible for both broadband light scattering and wavelength selective absorption in the wing scales of pierid butterflies.

A small but growing literature indicates that many animal colours are produced by combinations of structural and pigmentary mechanisms. We investigated one such complex colour phenotype: the highly chromatic wing colours of pierid butterflies including oranges, yellows and patterns which appear white to the human eye, but strongly absorb the ultraviolet (UV) wavelengths visible to butterflies. Pierids produce these bright colours using wing scales that contain collections of minute granules. However, to date, no work has directly characterized the molecular composition or optical properties of these granules. We present results that indicate these granules contain pterin pigments. We also find that pterin granules increase light reflection from single wing scales, such that wing scales containing denser granule arrays reflect more light than those with less dense granule collections. As male wing scales contain more pterin granules than those of females, the sexual dichromatism found in many pierid species can be explained by differences in wing scale pterin deposition. Additionally, the colour pattern elements produced by these pterins are known to be important during mating interactions in a number of pierid species. Therefore, we discuss the potential relevance of our results within the framework of sexual selection and colour signal evolution.

Grazing-incidence iridescence from a butterfly wing.

The Troides magellanus butterfly exhibits a specialized iridescence that is visible only when its hind wings are both illuminated and viewed at near-grazing incidence. The effect is due to the presence of a constrained bigrating structure in its wing scales that has been previously observed in only one other species of butterfly (Ancyluris meliboeus). However, whereas the Ancyluris presents wide-angle flickering iridescence, the Troides butterfly uses pigmentary coloration at all but a narrow tailored range of angles, producing a characteristic effect.