Correlated diffraction and fluorescence in the backscattering iridescence of the male butterfly Troides magellanus (Papilionidae).

The male Troides magellanus--a birdwing butterfly that lives in a restricted area of the Philippines--concentrates on its hindwings at least two distinct optical processes that contribute to its exceptional visual attraction. The first is the very bright uniform yellow coloration caused by a pigment which generates yellow-green fluorescence, and the other is a blue-green iridescence which results from light diffraction at grazing emergence under a specific illumination. Detailed optical measurements reveal that these optical effects are correlated, the fluorescence being enhanced by illuminations conditions that favor the occurrence of the iridescence. These effects are analyzed, with the conclusion that both of them depend on the same optical device: a one-dimensional microribs grating which appear on the sides of the ridges that run along the yellow scales.

Report. Dual gratings interspersed on a single butterfly scale.

Iridescent butterfly wing colours result from the interaction of light with sub-micrometre structures in the scales. Typically, one scale contains one such photonic structure that produces a single iridescent signal. Here, however, we show how the dorsal wings of male Lamprolenis nitida emit two independent signals from two separate photonic structures in the same scale. Multiple independent signals from separate photonic structures within the same sub-micrometre device are currently unknown in animals. However, they would serve to increase the complexity and specificity of the optical signature, enhancing the information conveyed. This could be important during intrasexual encounters, in which iridescent male wing colours are employed as threat displays. Blazed diffraction gratings, like those found in L. nitida, are asymmetric photonic structures and drive most of the incident light into one diffraction order. Similar gratings are used in spectrometers, limiting the spectral range over which the spectrometer functions. By incorporating two interchangeable gratings onto a single structure, as they are in L. nitida, the functional range of spectrometers could be extended.

Switchable reflector in the Panamanian tortoise beetle Charidotella egregia (Chrysomelidae: Cassidinae).

The tortoise beetle Charidotella egregia is able to modify the structural color of its cuticle reversibly, when disturbed by stressful external events. After field observations, measurements of the optical properties in the two main stable color states and scanning electron microscope and transmission electron microscope investigations, a physical mechanism is proposed to explain the color switching of this insect. It is shown that the gold coloration displayed by animals at rest arises from a chirped multilayer reflector maintained in a perfect coherent state by the presence of humidity in the porous patches within each layer, while the red color displayed by disturbed animals results from the destruction of this reflector by the expulsion of the liquid from the porous patches, turning the multilayer into a translucent slab that leaves an unobstructed view of the deeper-lying, pigmented red substrate. This mechanism not only explains the change of hue but also the change of scattering mode from specular to diffuse. Quantitative modeling is developed in support of this analysis.

Orange reflection from a three-dimensional photonic crystal in the scales of the weevil Pachyrrhynchus congestus pavonius (Curculionidae).

The three-dimensional structure that causes the coloration of the tropical weevil Pachyrrhynchus congestus pavonius was studied, using a combination of electron microscopy, optical spectroscopy, and numerical modeling. The orange scales that cover the colored rings on the animal's body were opened, to display the structure responsible for the coloration. This structure is a three-dimensional photonic polycrystal, each grain of which showing a face-centered cubic symmetry. The measured lattice parameter and the observed filling fraction of this structure explain the dominant reflected wavelength in the reddish orange. The long-range disorder introduced by the grain boundaries explains the paradoxical observation that the reflectance, although generated by a photonic crystal, is insensitive to changes in the viewing angle.

Gleaming and dull surface textures from photonic-crystal-type nanostructures in the butterfly Cyanophrys remus.

Photonic-crystal-type nanostructures occurring in the scales of the butterfly Cyanophrys remus were investigated by optical and electron microscopy (scanning and transmission electron microscopy), reflectance measurements (specular, integrated, and goniometric), by fast Fourier transform analysis of micrographs, by modeling, and by numerical simulation of the measured reflectance data. By evaluating the collected data in a cross-correlated way, we show that the metallic blue dorsal coloration originates from scales which individually are photonic single crystals of 50 x 120 microm2 , while the matt pea-green coloration of the ventral side arises from the cumulative effect of randomly arranged, bright photonic crystallites (blue, green, and yellow) with typical diameters in the 3-10-mum range. Both structures are based on a very moderate refractive index contrast between air and chitin. Using a bleached specimen in which the pigment has decayed with time, we investigated the role of pigment in photonic-crystal material in the process of color generation. The possible biologic utility of the metallic blue (single-crystal) and dull green (polycrystal) textures both achieved with photonic crystals are briefly discussed. Potential applications in the field of colorants, flat panel displays, smart textiles, and smart papers are surveyed.

Spectral filtering of visible light by the cuticle of metallic woodboring beetles and microfabrication of a matching bioinspired material.

Samples of the cuticle taken from the body of Buprestidae Chrysochroa vittata have been studied by scanning electron microscopy and optical reflectance measurements, related to numerical simulations. The cause of the metallic coloration of the body of these insects is determined to be the structure of the hard carapace constructed as a stack of thin chitin layers separated by very thin irregular air gaps. In particular the change of color as a function of the observation angle is elucidated in terms of an infinite photonic-crystal model, confirmed by finite multilayer calculations. These mechanisms are used to develop an artificial bioinspired multilayer system which reproduces the visual effects provided by the insect surface.

Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora).

Using transmission electron microscopy, analytical modeling, and detailed numerical simulations, the iridescence observed from the comb rows of the ctenophore Beroë cucumis was investigated. It is shown that the changing coloration which accompanies the beating of comb rows as the animal swims can be explained by the weakly-contrasted structure of the refractive index induced by the very coherent packing of locomotory cilia. The colors arising from the narrow band-gap reflection are shown to be highly saturated and, as a function of the incidence angle, cover a wide range of the visible and ultraviolet spectrum. The high transparency of the structure at the maximal bioluminescence wavelength is also explained.

Structural origin of the colored reflections from the black-billed magpie feathers.

The structural origin of the weak iridescence on some of the dark feathers of the black-billed magpie, Pica pica (Corvidae), is found in the structure of the ribbon-shaped barbules. The cortex of these barbules contains cylindrical holes distributed as the nodes of an hexagonal lattice in the hard layer cross section. The cortex optical properties are described starting from a photonic-crystal film theory. The yellowish-green coloration of the bird's tail can be explained by the appearance of a reflection band related to the photonic-crystal lowest-lying gap. The bluish reflections from the wings are produced by a more complicated mechanism, involving the presence of a cortex second gap."

Waveguides in inverted opal photonic crystals.

Waveguiding phenomena are investigated in an inverted opal photonic crystal made of interpenetrating air spheres, coated with amorphous Ge. Here we focus on the complete gap between the 8th and the 9th band, since a projected band analysis reveals that it is difficult to use the large lower incomplete gap for guiding purposes. Two kinds of line defects are analyzed within this photonic structure, with the plane-wave expansion method. The first one consists of an air cylinder in the Gamma-K direction. It gives rise to a large number of defect modes in the bandgap. Most of these modes have large field components at the surface. The second defect is an array of air spheres, also along the Gamma-K direction. This is shown to avoid the surface-like modes and sustain only two modes associated with different polarizations, in the frequency range of interest. The air mode waveguiding bandwidth reaches up to 113 nm centered at a wavelength of 1.5 microm.

Optical structure and function of the white filamentary hair covering the edelweiss bracts.

The optical properties of the inflorescence of the high-altitude Leontopodium nivale subsp. alpinum (edelweiss) is investigated, in relation with its submicrometer structure, as determined by scanning electron microscopy. The filaments forming the hair layer have been found to exhibit an internal structure which may be one of the few examples of a photonic structure found in a plant. Measurements of light transmission through a self-supported layer of hair pads taken from the bracts supports the idea that the wooly layer covering the plant absorbs near-ultraviolet radiation before it reaches the cellular tissue. Calculations based on a photonic-crystal model provide insight on the way radiation can be absorbed by the filamentary threads.

Natural layer-by-layer photonic structure in the squamae of Hoplia coerulea (Coleoptera).

The microscopic structure of the hard external parts of the body of the iridescent blue-violet chaffer beetle Hoplia coerulea is studied using scanning electron microscopy. The blue iridescence is shown to originate from the structure of the squamae within scales covering the dorsal side of the beetle. The internal structure of the scales shows a stack of planar sheets, separated by a well-organized network of spacers, a structure which belongs to the family of the layer-by-layer photonic crystals. The blue iridescence is easily explained by a planar multilayer approximation model, deduced from the observed three-dimensional structure.

Angular and polarization properties of a photonic crystal slab mirror.

It was recently demonstrated that a photonic crystal slab can function as a mirror for externally incident light along a normal direction with near-complete reflectivity over a broad wavelength range. We analyze the angular and polarization properties of such photonic crystal slab mirror, and show such reflectivity occurs over a sizable angular range for both polarizations. We also show that such mirror can be designed to reflect one polarization completely, while allowing 100% transmission for the other polarization, thus behaving as a polarization splitter with a complete contrast. The theoretical analysis is validated by comparing with experimental measurements.