Such colors are mostly related to multilayer interference, although the structural color of pigeon neck feathers have been discovered to be caused only by the interference from one thin film [10]. Figure 2(A) shows the neck feathers of the domestic pigeon Columba livia domestica, with an iridescent green and purple color. The cross-sectional micrograph of the neck feather taken by the scanning electron microscope (SEM) shows green and purple barbules, both consist of an outer keratin cortex layer surrounding a medullary layer. There is an obvious difference in thickness between green and purple barbules. The interference in the top keratin cortex layer and total thickness of the layers decides the apparent color of the barbule.
A more well-known example of naturally occurring multilayer interference is the brilliant blue color of Morpho butterflies’ wings [8]. Electron microscope observation under high magnification clearly illustrates that a lamellar structure consisting of alternating layers of cuticle and air is present in each ridge (Figure 2(B)). The ridge-lamellar structure formed by discrete multilayers work as an element of quasi-multilayer interference, meaning the narrow width of height-varying ridges causes light diffraction without interference among neighboring ridges. The bright blue color is attributed to a significant difference in the refractive indices between cuticle (n = 1.56) and air (n = 1), with the layer thickness nearly fulfilling the conditions of ideal multilayer interference.Compared to 1D photonic structures, 2D photonic structures in Nature provide richer color.
Zi et al. reported the mechanism of color production in peacock feathers [14], finding that the differently colored barbules contain a 2D PC structure composed of melanin rods connected by keratin (Figure 2(C)). The nearly square lattice structures in the colored barbules differ in characteristics such as lattice constant Cilengitide (rod spacing) and number of periods (melanin rod layers) along the direction normal to the cortex surface. The tunable lattice parameters are the cause of the diverse coloration seen in the barbules. In addition, these 2D gratings exhibit self-cleaning capabilities due to the high fraction of the air trapped in the trough area between melanin rod arrays. Another type of 2D photonic structure is periodic long fibers found in the iridescent setae from polychaete worms (Figure 2(D)) [9].
A 2D hexagonal lattice of voids within the cross-section of each seta creates a
We integrated the components of the IR detector to make it more accurate, convenient and reliable. We used detectors for wavelengths of 3.31 ��m and 3.91 ��m, an IR light source, a circuit board, and a metal net integrated into a gold-plated chamber. The porous gold-plated metal and the metal net allow gas to diffuse into the gas chamber and eliminate the influence of the external environment.