Natural and nanoengineered chiral reflectors: Structural color of manuka beetles and titania coatings

A parallel study of natural and nanoengineered structurally chiral reflecting coatings is described. It is shown that the nanostructures are different in a minor way but are optically equivalent. Refractive index matching of nanoengineered chiral coatings on a plane substrate is shown to improve the saturation of structural color. Optical and electron microscopies reveal complexity in the multilayered chiral coatings that produce green metallic-like reflections from manuka ( scarab) beetles. In particular, the reflectors are shown to have the form of small concave pits and troughs that are filled with contouring chiral material. Each chiral microreflector presents a range of pitch and tilt to an incident beam of light. Physical properties of the textured coatings are related to optical properties such as spectral reflectance, angle of spread, and perceived color, which has a high degree of saturation due to the filling of the pits. Observations of overlapping chiral mediums in beetle reflectors have inspired nanoengineering of related handed media such as Bragg reflectors for elliptically polarized light.