A mantis shrimp’s extraordinary eyes

Photonic devices that can detect and control the polarization of light across a range of wavelengths are rare. More common are materials such as quartz that can be made into monochromatic optical retarders, which through their intrinsic birefringence convert a specific wavelength of linearly polarized light into circularly polarized light, or vice versa. Some multilayered thin films exhibit achromatic retardation through fabricated periodic nanoscale structures that effectively combine the dispersive properties of each layer to achieve wavelength-independent birefringence. But engineering nanoscale structures is tricky, and even the best synthetic achromatic retarders perform poorly across the full visible range, varying by as much as 9.1°. But Nature has already solved the puzzle in animals that have evolved biophotonic structures for signaling, vision, and coloration (see Physics Today, January 2004, page 18). Now, an international team of researchers from the UK, Australia, and the US has discovered a near-ideal achromatic retarder in the eyes of the colorful peacock mantis shrimp, Odontodactylus scyllarus, shown in the image. This mantis shrimp’s biophotonic retarder is the R8 photoreceptor cell—a UV-photopigment-filled lipid bundle with critical radii of 26 nm and 40 nm, which are subwavelength for visible light. When subjected to linearly polarized light, the R8 cell acted as a quarter-wave retarder, converting the incident light to circularly polarized light, as confirmed by close experimental agreement with theoretically determined Stokes parameter values. Moreover, the extent of retardation varied by only 2.7° across the visible spectrum. (N. W. Roberts et al., Nat. Photonics, in press, doi:10.1038/nphoton.2009.189. Image courtesy of Roy Caldwell, University of California Berkeley.)—Jermey N. A. Matthews