A mantis shrimp’s extraordinary eyes

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. Through their intrinsic birefringence, those devices retard the phases of a specific wavelength of incident light, converting the light from linearly to circularly polarized 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 on the nanoscale is tricky, and even the best synthetic achromatic retarders perform poorly across the full visible range, straying from the expected retardation by as much as 2.5%. Nature, though, has already solved the puzzle in animals that have evolved biophotonic structures for signaling, vision, and coloration (see Physics Today, January 2004, page 18 ). 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. The 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 acts 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 retardation varied by only 0.8% from ideal values across the visible spectrum. (N. W. Roberts et al., Nat. Photonics 3 , 641, 2009 .)