Exploring natural photonic crystals

Many organisms have photonic crystals as part of their ‘bodies’. Recent research into the diversity of photonic crystals in nature has led us to question how such precise nanostructures form, with the hope that answers may lead to breakthroughs in their engineering.

In a study of structural colors in nature, a remarkable convergence emerged in the nanoscale architecture of 2D and 3D periodic photonic crystals between species, families, phyla, and even kingdoms of organisms. Of the many types developed by engineers, living organisms possess only four. These, however, occur in many unrelated species. It is possible that a combination of intra-cellular engineering and molecular self-assembly—as opposed to proportional genetic mutation—is the major factor in the evolution of photonic crystals in nature. By ‘intra-cellular engineering’, I mean using cell membranes or organelles as templates or molds for, or even components of, the final photonic crystals.

Some relatively simple optical reflectors have gradually increased in efficiency over millions of years of evolutionary fine-tuning. For example, the corrugations on the surface of seed-shrimp (cypridinid ostracod) hairs form a diffraction grating and cause iridescence (see Figure 1), which is used as a courtship display.¹ These gratings have improved in optical efficiency throughout the evolution of the group. Conceivably, this is the result of minor changes in the genome, with each change translating to a minor change in the structure. Other simple optical reflectors could be considered to evolve in a similar, gradual manner, where they are manufactured via periodic deposition of materials.

All photonic crystals found in nature, however, show strong architectural parallels with components that are found in cells with nuclei (or, more precisely, eukaryotic cells, which have a membrane-bound nucleus and organelles). Within the general eukaryotic cell, membranes belonging to various organelles are sometimes folded to form intricate lattices, or contain a system of holes. For example, the girdle and the valve (shell) patterns of the single-celled diatom Coscinodiscus granii have the same form as the basic ultrastructure of the intra-cellular membranes (particularly the nuclear membrane) common to all eukaryotic cells. This is not a chance analogy: the intra-cellular membranes probably act as templates during the manufacture of the structures. Further, trans-Golgi-derived vesicles (which are intra-cellular organelles) manufacture the photonic crystals of the single-celled coccolithophores, a type of phytoplankton.³ The organelles within the cell appear to have exact control of photonic-crystal growth (CaCO₃ in the coccolithophores) and packing (SiO₂ in the diatoms). The cell membrane is known also to take the form of hollow, interlocking spheres (e.g. see Figure 2). Similarly, the opal or inverse-opal type of photonic crystal could be manufactured by using such a structure as a template: material fills either the volume inside or outside of the spheres.

Another example is the moth-eye-type antireflector found on the corneas of moths and butterflies, which appears identical to the surface ornamentation of a crab, an unrelated organism. In this case, we know how the mechanism is made: the outer wall of each cell forms hexagonally-arranged microvilli with hemispherical tips, onto which chitin is secreted.⁴ But we also want to determine the effect of molecular self-assembly. (The self-assembly of silica nanostructures has already been examined.⁵) Also, we want to determine the effects of natural phenomena such as cracking through drying or splitting through high-surface-tension effects. Understanding all of these indirect genetic effects is important for our attempts to use genetic manipulation to produce made-to-measure optical devices.

It is possible, consequently, that photonic nanostructures can evolve without passing through the string of intermediate stages one might expect from a complex architecture. This theory may apply only to nanostructures manufactured completely by single cells, in which properties of the cell are exploited. It is the fact that nature's photonic crystals are made by individual cells that is so appealing to the engineer. If we can keep these cells alive in vitro, and induce them to reproduce in some cases, conceivably they will make photonic crystals to order. My research team and I have cells in culture in our labs, gathered from disparate organisms, that are producing photonic crystals before our very eyes. The next step is to farm and manipulate the micron-scale products in appropriate ways, with the ultimate aim of providing an alternative to current engineering techniques. For example, we can already attach antibodies and incorporate dyes into the photonic-crystal tests (cell walls) of diatoms, which we can produce by the ton. We hope to report an application soon.