**3.1 Theoretical background**

Pigmentary coloration is based on a spectrally selective absorption of the incident whitelight. For a pigment to be useful, the light which has not been absorbed must be diffused in all directions, providing the same color in all directions. This means that a sheet of material colored by absorption and diffusion will appear with roughly the same color in reflection and transmission. This contrasts structural colors obtained by interference, without absorption, where back- and forward scattering colors tend to be complementary.

The physical description of a selectively absorbing material illuminated by a single frequency needs to extend the concept of the refractive index to include refraction and absorption. A simple way to do this is, at a fixed frequency, to accept to replace its real value by a complex number *n n ik* . A frequency-dependent complex refractive index (or, equivalently) a frequency-dependent complex dielectric constant can explain the optical response of dyes in a homogeneous material. But pigments require to produce diffuse scattering and this will only take place in a random inhomogeneous material or when the absorbing material appears in the form of concentrated granules. This helps providing a distinction between dyes and pigments.

#### **3.2 Pigmentary coloration in plants**

In plants, blue coloration is quite rare. However, it can be seen in some leaves, flowers or fruits. The blue is produced by modified anthocyanin pigments. A wide variety of mechanisms for modifying anthocyanin pigments has been observed in order to get blue or violet colorations. In flowers, they form complexes with flavonoids pigments and are in solution in cellular vacuoles. In leaves, they take place in chloroplasts. The structuration of

How Nature Produces Blue Color 11

To our knowledge, pigmented blue has not been found in mammals and in other insects. However, the presence of blue pigment remains very difficult to prove, partly because of their weak solubility. Extracting and characterizing very weakly soluble pigments is a complex task that restraints the possibilities of analysis. Moreover, determining the

Pigments in bird feathers are assumed to be present since the ages of dinosaurs. Studies on a *Sinosauropterix* (125 million years old), showed that their feathers would be filled with melanosomes and thus should appear dark (Vinther et al. 2008, Zhang et al., 2010). This

One-dimensional, planar or curved, photonic structures are frequent in nature. Insects, in particular, have frequently evolved this kind of structure for the purpose of coloration, as part of signaling or camouflage strategies. The reason may be that the process of fabrication of the outer part of a cuticle by epidermal cells, layer by layer, is compatible with the formation of such structures, even if we cannot claim at the moment that these mechanisms

Many different cases of one-dimensional photonic crystal have been seen in animalia, maybe because this is the most direct way to produce a metallic and/or iridescent color and, in this way, improve specific intra or interspecific functions. We will essentially examine

The multilayer is the most common type of iridescent structure found in beetles and is also very common in butterflies (Kinoshita et al., 2008; Noyes et al., 2007; Parker et al., 1998). In many cases, these multilayers are composed of alternating layers of chitin and air partially filled with a chitinous compound. This produces a high/low index bilayer. Constructive interferences between light reflected by different layers produce one or several colors. The dominant reflected wavelength can be determined by the thicknesses of the layers and the average refractive index (see formula 11). The wavelengths that are not reflected are transmitted and the transmission spectrum is the exact complement of reflection if the system is considered non-absorbing. The color arising from a multilayer also varies with the angle of observation. As the reflection angle increases (starting from normal), the color shifts

The reflectors can be epicuticular (as in cicindelinae or in some chrysomelidae) (Kurachi et

Iridescence in bird feathers comes sometimes from 1D structure. They are located in the barbules like in satin bowerbirds *Ptilonorhynchus violaceus minor* that shows a violet to black iridescence coming from a single layer of keratin on the top of a layer of melanin (Doucet et al., 2006). In European starlings *Sturnus vulgaris*, multiple layers of keratin and melanin give

The single self-supported thin film and the optical overlayer covering a substrate have been known since a long time, in the planar and some other geometry. Constructive and

two cases of physical designs: the single layer film and the Bragg mirrors.

concentrations and localization of pigments within the tissues is still a real challenge.

work was, however, taken with cautious and discussions (Lingham-Soliar, 2011).

**4. One-dimensional photonic structures** 

have been understood in all details.

to lower wavelengths (blue shift).

**4.1 Thin films** 

al., 2002), while others are endocuticular (Hinton, 1973).

a green-blue iridescence (Cuthill et al., 1999, Doucet et al., 2006).

the leave surface cells can help absorption by increasing high angle incident light to be transmitted through the leave or by focusing light in the pigmentary region. For example, in the velvet-leaved anthurium (*Anthurium warocqueanum*), the surface cells are convexly curved to focus light at some internal distance, just onto chloroplasts area (Lee, 2007).
