Photonic Structures in the Animal Kingdom: Valuable Inspirations for Biomimetic Applications - Biomimetic Architectures by Plasma Processing - page 7

1.2.2.1 Note on order and disorder

Biological photonic structures are obviously not ideal photonic
structures in the way that the structures are not infinitely large. Yet,
most photonic structures can be adequately described as 1D-, 2D- or
3D-photonic structures due to their local order and the large domain
size compared to the wavelength of visible light.
Most biological photonic structures show a well-tuned interplay
between regularity of the photonic structure and irregularity on
different length scales, for example, to broaden the visibility range of
the reflectance [20,21]. Recently, Trzeciak et al. investigated the role
of order and disorder in the photonic structures of the wing scales
of various papilionid butterflies and showed that a certain amount
of disorder is needed to smoothen the reflectance spectrum of a
photonic structure and thereby, ensure a stable colour contrast [21].

1.3 One-Dimensional Photonic Structures

One-dimensional photonic structures, such as thin films or
multilayers, are probably the most abundant cause of structural
colouration found in nature, creating the iridescent, metallic colours
of many birds, beetles and butterflies (Fig. 1.3). Light reflected by
1D photonic structures is often strongly polarised under high angles
of light incidence, which may potentially serve as a visual signal to
conspecifics (see below).

1.3.1 Thin Films

The simplest case of a 1D photonic structure is a thin film, that is, a
single layer of material. The most extreme case of a biological thin
film can be found not only in the clear wings of many flies, but also
in the so-called glass scales of the Common Bluebottle,

Graphium

sarpedon , a butterfly native to Southern and Southeast Asia. The

glass scales, that cover large patches of the wing underside, are
shaped into a single layer of chitin with a thickness of approx.
400 nm (see Fig. 1.3d) [22]. For a thin film, the reflectance is
determined by constructive interference of light reflected at the two
air-material interfaces. The reflectance of such a thin film can be
calculated analytically via Airy's formula: