Tuesday, November 3, 2009

Nature's almost perfect quarter-wave retarder

On September 28, I reported on the keynote lecture on Why are animals colourful? Sex and violence, seeing and signals Justin Marshall gave at the AIC in Sydney. Prof. Marshall gave a description the mantis shrimp's eye noting how it can see polarization with the help of a structure like a kind of nanotube.

The eye of a particular species of mantis shrimp is well-known to have the most complex vision system in nature

With his colleagues, Prof. Marshall has just published a letter in Nature Photonics 3, 641-644 (2009) with the title A biological quarter-wave retarder with excellent achromaticity in the visible wavelength region.

This paper explains in detail how the mantis shrimp detects polarization. The authors illustrate how a novel interplay of intrinsic and form birefringence results in a natural achromatic optic that significantly outperforms current man-made optical devices. Achromatic here means that polarization detection is independent from the wavelength (±2.7º), which is something we humans do not really know how to build.

a, A frontal view of the compound eye of Odontodactylus scyllarus with the midband rows 5 and 6 highlighted by the greyed out region. VH, ventral hemisphere; DH, dorsal hemisphere; MB, midband. The section A'–A'' is shown schematically in b. Scale bar, 800 m. b, Schematic of a transverse section (A'–A'' in a). This illustrates the arrangement of the 5th and 6th rows of the midband and the location of the 8th retinular cell (R8) quarter-wave retarder and the underlying R1–7 cells. The R8 cell is 150 m long.

A syntectic mantis shrimp eye could be a boon for remote sensing. However, with mother nature being and order of magnitude better than what physicists can build, this will not happen any time soon. It is interesting that this structure has not been repeated in any other animal, as far as we know.