PerspectiveMaterials Science

Evolutionary Photonics with a Twist

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Science  24 Jul 2009:
Vol. 325, Issue 5939, pp. 398-399
DOI: 10.1126/science.1177729

The visual appearance of many animals is determined not by pigments but by structural processes that allow the animals to manipulate electromagnetic radiation—mostly visible light and color—in courtship, to find prey, or to escape predators. Studies of fish scales (1), insect coatings (2), and bird feathers (3) have revealed a wealth of complex biological structural designs and optical effects that mirror many technological photonic system designs. These photonic technologies are beginning to draw inspiration from the natural world for new generations of devices and products (4). On page 449 of this issue, Sharma et al. (5) add knowledge to this area by elucidating the processes through which the scarab beetle, Plusiotis gloriosa, reflects structural color from its external surfaces (elytra) in the form of left-handed circularly polarized light.

Left-handed circularly polarized reflection means that from the perspective of the observer, the electric field vector of the light reflected from the beetle describes a left-handed corkscrew, or helix, along its direction of propagation. Circularly polarized reflection from specific beetles was first observed nearly a century ago (6). P. gloriosa's tendency to do this is thus not an isolated example, but the attribute is nonetheless rare. It requires a distinct azimuthally twisted—or helical—character in the nanostructure that forms the first few micrometers of its elytra. In these beetles, the spatial pitch of this helix creates the intrinsic periodicity that, to human vision, produces bright iridescent color (an example of this is shown in the figure).

Iridescence with a difference.

What appears to human eyes as green iridescence from certain scarab beetles, such as Plusiotis alphabarrerai shown here, mostly constitutes circularly polarized color.

CREDIT: P. VUKUSIC/UNIVERSITY OF EXETER

Synthetic systems that exhibit strongly circularly polarized color reflections include certain layered mesophases, specifically those associated with cholesteric liquid crystals (also known as chiral nematic liquid crystals). Their circularly polarized optical properties arise because their constituent molecules lack inversion symmetry. This produces intermolecular forces that favor a specific small azimuthal twist through the whole system. In this way, an intrinsic physical helicity is generated that is right-handed or left-handed depending on molecule geometry, yielding right- or left-handed circularly polarized reflection, respectively (7).

Previous studies of circularly polarized colored reflections from beetle elytra revealed the presence of helicity and described the strong analogy with cholesteric liquid crystals (8). Electron micrographs of sections through these beetle elytra revealed “Bouligand structures” (9)—the characteristic series of curves within periodically contrasted layers that indicate the presence of helical symmetry in the constituent material.

In P. gloriosa and several other Rutelinae (10), however, the structural complexity goes beyond mere helically ordered layering. The elytral surfaces of these beetles consist of arrangements of mostly hexagonal micrometer-scale multicolored cells. The different colors are a result of nested close-packed surface concavities that shape the underlying structure, creating a set of color properties that depend on the nature of the illumination.

Using confocal microcopy, Sharma et al. now infer that in cross section, the form and geometry of these surface structures and subsurface features are analogous to the focal conic domains that spontaneously form at the free surface of a cholesteric liquid crystal (11). Given that the dynamics and formation processes of liquid crystals are generally well understood, this association provides new insight, beyond that of earlier cholesteric liquid crystal analogies (8), for the set of variables that may advance our understanding of the self-assembly pathways of this structurally colored insect cuticle. With a few noteworthy exceptions (12), the formation processes of these insect systems are not as well understood as are their photonics.

Helical nanostructure may have different biological functions. Where such helicity is present without accompanying circularly polarized reflection—for example, in some plant epidermi (13)—it may serve to add mechanical strength. This is mimicked effectively at the millimeter scale by processed plywood. However, the beetle helical ultrastructure is arguably too complex and too costly to produce without the benefit of a suitable optical selection advantage, such as effective signaling. The strong circularly polarized reflection observed in the beetles may, for example, play a role in intraspecific communication. This is especially the case for another scarab, Plusiotis resplendens. It exhibits strong broadband reflection of both left- and right-handed circularly polarized light due to the presence of two chirped helical layered regions separated by a half-wave plate (14).

Despite some initial behavioral studies, it remains unknown whether the circularly polarized reflection from these special beetles provides a channel of communication. However, Chiou et al. recently showed that such communication is possible for a marine crustacean: the stomatopod Odontodactylus sp. Not only does this species signal brightly using circularly polarized colored light reflected from two posterior abdominal appendages, but it also responds behaviorally to circularly polarized stimuli (15). Its method for doing so is elegant. Chiou et al. revealed that incident circularly polarized light is converted to linearly polarized light when it is transmitted through a quarter-wave plate in specific cells of the eyes' mid-band region. Upon conversion to linearly polarized light, alignments of conventional microvilli (the basic elements that construct the photoreceptive region of the eye) are used for its photoreception.

This apparently adapted coupling between a circularly polarized light source and a circularly polarized light detection system in stomatopods is unlikely to be the only one in biological systems. Whether circularly polarized reflection in beetles such as P. gloriosa also has such an intraspecific communication purpose remains to be seen.

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