Tapping into the ocean colour scene

04 Jan 2010

The eyes of the mantis shrimp point the way to better optical-storage devices

Here's looking at you

With arguably the most complex eyes in the animal kingdom, the mantis shrimp can convert circularly polarized light into linearly polarized light almost perfectly across the whole visible spectrum. This remarkable ability is leading an international team of researchers to take another look at some of today's man-made polarization optics (Nature Photonics DOI:10.1038/NPHOTON.2009.189).

"We have pinpointed a particular photoreceptor cell in the eye of the Mantis Shrimp that manipulates the polarization of light irrespective of wavelength," Nicholas Roberts, a biologist at the University of Bristol, UK, told optics.org. "If you can chemically engineer the correct optical properties to mimic this cell, then there is no reason why the same optical principles won't apply."

Polarizing wave plates or wave retarders are important optical components used in many common data storage systems such as CDs, DVDs and data projectors. The problem is that designing wave plates to work for many different colours of light rather than for a single colour is exceptionally difficult.

While scientists have managed to fabricate quarter wave plates that contain sub-wavelength gratings to achieve a level of achromaticity, it's still the case that most are monochromatic. The wave plate in the mantis shrimp's eye remains unique in its ability to outperform all manmade systems by operating across the entire visible spectrum.

The secret lies in the mantis shrimp's elegantly designed photoreceptors. The photoreceptor cells contain stacks of cell membranes rolled into tubes. The way the cell membranes are rolled into tubes and packed together influences the refractive indices of the cell.

It is the difference between the refractive indices that splits incident light into two components, both travelling at different speeds. If one component is retarded by one quarter of a wavelength compared to the other, it is turned from circularly polarized light into linearly polarized light.

"In our device, the difference between the refractive indices increases as you go from blue to red," explained Roberts. "This means that for all wavelengths the retardation is always one quarter of a wavelength and we have an achromatic device."

The researchers hope that optical designers will now be able to copy the design to create manmade wave plates with much better performance than is currently available.

The research was carried out by a collaboration involving scientists at the University of Bristol, the University of Maryland Baltimore County, US, and the University of Queensland, Australia.