The emergence of vision∼ 542 million years ago during the Cambrian rapidly led to significant changes in the appearance of animals and plants.[1] Many species evolved structural color in the forms of simple diffraction gratings or more complex hierarchical structures. These features can control the direction, color, and polarization of reflected light, and give many plants and animals vibrant colors that have evolved for warding off predators and attracting mates.[2, 3] One of the most fascinating sources of structural color is the so-called Bouligand, or twisted plywood, structure present in the exoskeletons of some beetles and arthropods.[4] In these structures, chitin fibrils in a matrix of calcium carbonate and/or calcium phosphate and protein are aligned in layers whose orientation rotates through the structure with a characteristic repeating distance, or pitch. The rotation within a domain can theoretically be left-or right-handed, as a cork screw, and this leads the structures to selectively reflect circularly polarized light. There are now several examples of plants and animals that show the Bouligand structure, as exemplified by chitin in the exoskeletons of jewel beetles [5] and peacock mantis shrimps,[6] and cellulose in Pollia fruit shells.[7] As the optical characteristics of these structures are analogous to those of focal conic domains of films formed from chiral nematic (cholesteric) liquid crystals, these structures are often also said to have chiral nematic structure.

An important topic in materials science is the development of photonic structures to manipulate light. Such materials are essential components for optical communication and computing.[8] The natural structures that have evolved in plants and animals to give colors are of great interest to materials scientists who are trying to mimic their unique properties for photonic technologies.[9–12]