**6. Three-dimensional photonic crystals**

Three-dimensional photonic crystals are also encountered in nature, especially in butterflies, weevils and longhorns. In view the very high diversity of living organisms and the frequency of structural colors, it can be speculated that other families of insects will soon reveal a similar evolution. Three-dimensional photonic crystals are periodic in all three dimensions of space. Illuminated by white light in a well-defined direction, an ideal structure produces several colored beams, each of them corresponding to a stack of reticular planes with its appropriate spacing. A good example of the visual effect produced by an ideal photonic crystal is provided by the so-called Brazilian "diamond" weevil, which displays a green color when viewed from a distance, but under an optical microscope, shows individual scales with a variety of very saturated (pure) colors. Most other weevils and longhorns show less iridescence, and this is usually explained in term of orientation disorder: natural three-dimensional photonic crystals most often appear under the form of photonic polycrystals, with well-defined domains bringing short-range order and long-range orientation disorder.

Fig. 10. The internal structure of a scale on a blue area from the cuticle of the weevil *Eupholus schoenherri*. The structure, with a face-centered cubic symmetry, can be described as an "opal" structure.

How Nature Produces Blue Color 21

coloration is reached by increase of the structure compactness, with the production of

spheres with an average diameter of 212 nm, arranged in a compact cubic structure.

Fig. 12. The array of spherical centers found in scales of the Malaysian longhorn *Pseudomyagrus waterhousei*. The compactness of the structure leads to the production of

short-wavelength scattering, providing a slightly desaturated blue-violet color.

This brief survey of the production of blue color on living organisms shows that all broad categories of structural mechanisms can be put to use to produce short-wavelengths scattering. We have seen that all structures known to be at the root of a structural coloration in nature (Vigneron & Simonis, 2010) can actually provide a blue coloration. Each device has its own rules for providing scattering solely on the short-wavelength end of the visible spectrum.

This particular objective is not always easy and often requires a multiscale solution. For living organisms that has undergone evolution over many million years, this is not a problem: the "modification-selection" algorithm, which is the engine of the past and present biodiversity, has no reluctance for complexity. Even if the range of refractive indexes in biologically prepared materials is rather narrow (typically 1.3 to 1.8), complexity in geometrical structure can provide a very wide range of functions that turn out to be an

We do not always know what can be the biological advantage of producing blue. We understand that a male metallic blue *Morpho* can be seen from far away, which is an advantage for accelerating productive mates encounters. But the answer is less obvious for the formation of iridescent blue plants, as blue is one of the spectral components of the light captured by chlorophyll molecules to achieve photosynthesis. While an answer to the physical "how" question - referring to a description of the production mechanisms - is

Priscilla Simonis, on leave from the University of Namur, Belgium, acknowledges the hospitality of the "Institut des Nanosciences de Paris", Université Pierre et Marie Curie –

relatively easy, an answer to the biological "why" question is far less obvious.

**7. Conclusion** 

advantage for species population increase.

Paris 6, where this work was carried out.

**8. Acknowledgment** 

Blue weevils are frequent, in particular in the *Eupholus* genus, as *E. loriai* (completely blue) or *E. bennetti*, *E. magnificus* or *E. schoenherri* (partially blue). At the moment, the structures that produce this blue colors can be described as a photonic polycrystal with grains locally organized in a face-centered cubic symmetry. A typical "blue" structure from a weevil is shown in Fig. 10, which shows a scale from a blue area on an elytron of *Eupholus schoenherri*. The same kind of structure has been encountered in a previous work (Parker et al., 2003) for a different weevil displaying green spots. This structure is generally referred to as an "opal" structure, making a parallel with the assembly of monodispersed spheres constituting the iridescent stone. The present photonic structure is also an arrangement of non-absorbing spheres but the constituting material is a chitinous compound, with a refractive index of the order of 1.6. In order to produce short-wavelength photonic gaps, the size of the spheres is kept small and the compactness is maximized. In such a structure, the light scatterers are effectively the tiny air-filled interstices left between the spheres, not easily seen in electron microscopy images. In weevils however, the "inverse opal" structure like the one shown in Fig. 11 is the most common case. This structure corresponds to an arrangement of spherical hollows in a chitinous matrix.

Fig. 11. Optical microscope view of the blue scales of the weevil *Cyphus hancoki*. The different colors correspond to different crystal grains (left). On the right, electron microscope image of one grain. The 3D array of spherical hollows is described as an "inverse opal" structure (Berthier, 2006).

A blue longhorn, with a three-dimensional photonic-crystal structure has also been described (Simonis et al., 2011). *Pseudomyagrus waterhousei* shows a slightly desaturated purplish blue color. These colorations arise from a dense layer of droplet-shaped scales covering the dorsal parts of the cuticle. These colors are caused by structural interferences and produced by an aggregate of internally ordered photonic-crystal grains. As in the weevils' case, the structure is built with spherical diffusion centers arranged according to a face-centered cubic symmetry. Domains are also present, with long-range orientation disorder, a complex structure which partly explains the lack of iridescence in the visual effect, in spite of a structural coloration. Theoretical considerations suggests that the contents of the observed reflectance dominantly arise from photonic crystallites with (111) reticular plane parallel to the cuticle surface. Another source of disorder lies in the observation that, in this structure, internally ordered photonic crystal grains can be separated by regions of amorphous arrangement, with spheres diameters varying over a rather wide range, from 170 to 300 nm in diameter. Here also, the short-wavelength blue

Blue weevils are frequent, in particular in the *Eupholus* genus, as *E. loriai* (completely blue) or *E. bennetti*, *E. magnificus* or *E. schoenherri* (partially blue). At the moment, the structures that produce this blue colors can be described as a photonic polycrystal with grains locally organized in a face-centered cubic symmetry. A typical "blue" structure from a weevil is shown in Fig. 10, which shows a scale from a blue area on an elytron of *Eupholus schoenherri*. The same kind of structure has been encountered in a previous work (Parker et al., 2003) for a different weevil displaying green spots. This structure is generally referred to as an "opal" structure, making a parallel with the assembly of monodispersed spheres constituting the iridescent stone. The present photonic structure is also an arrangement of non-absorbing spheres but the constituting material is a chitinous compound, with a refractive index of the order of 1.6. In order to produce short-wavelength photonic gaps, the size of the spheres is kept small and the compactness is maximized. In such a structure, the light scatterers are effectively the tiny air-filled interstices left between the spheres, not easily seen in electron microscopy images. In weevils however, the "inverse opal" structure like the one shown in Fig. 11 is the most common case. This structure corresponds to an arrangement of spherical

A blue longhorn, with a three-dimensional photonic-crystal structure has also been described (Simonis et al., 2011). *Pseudomyagrus waterhousei* shows a slightly desaturated purplish blue color. These colorations arise from a dense layer of droplet-shaped scales covering the dorsal parts of the cuticle. These colors are caused by structural interferences and produced by an aggregate of internally ordered photonic-crystal grains. As in the weevils' case, the structure is built with spherical diffusion centers arranged according to a face-centered cubic symmetry. Domains are also present, with long-range orientation disorder, a complex structure which partly explains the lack of iridescence in the visual effect, in spite of a structural coloration. Theoretical considerations suggests that the contents of the observed reflectance dominantly arise from photonic crystallites with (111) reticular plane parallel to the cuticle surface. Another source of disorder lies in the observation that, in this structure, internally ordered photonic crystal grains can be separated by regions of amorphous arrangement, with spheres diameters varying over a rather wide range, from 170 to 300 nm in diameter. Here also, the short-wavelength blue

Fig. 11. Optical microscope view of the blue scales of the weevil *Cyphus hancoki*. The different colors correspond to different crystal grains (left). On the right, electron microscope image of one grain. The 3D array of spherical hollows is described as an

hollows in a chitinous matrix.

"inverse opal" structure (Berthier, 2006).

coloration is reached by increase of the structure compactness, with the production of spheres with an average diameter of 212 nm, arranged in a compact cubic structure.

Fig. 12. The array of spherical centers found in scales of the Malaysian longhorn *Pseudomyagrus waterhousei*. The compactness of the structure leads to the production of short-wavelength scattering, providing a slightly desaturated blue-violet color.
