Figure 1.

A classification of biophotonic nanostructural diversity based on the dimensionality of spatial variation in refractive index and its range of periodicity. (a–c) One-, two- and three-dimensional biological photonic crystals with long-range periodic order in refractive index modulations (after Joannopoulos [6]). (d) A chirped lamellar stack, a one-dimensional quasi-ordered nanostructure with short-range spatial periodicity (currently unknown in birds; after Parker [2]). (e) TEM cross section of a two-dimensional amorphous or quasi-ordered nanostructure with short-range order comprising of parallel collagen fibres in a mucopolysaccharide matrix from the green tongue of magnificent bird-of-paradise (Cicinnurus magnificus, Paradisaeidae). (f) TEM cross section of a three-dimensional amorphous or quasi-ordered nanostructure of β-keratin and spheroidal air vacuoles from the spongy medullary cells of the azure blue crown feather barbs of male Blue-crowned Manakin (Lepidothrix coronata, Pipridae) with short-range quasi-periodic order.