Shrimp: Dinner…and a Movie?
Posted by Lauren Rugani on October 28, 2009
The compound eye of the mantis shrimp (Odontodactylus scyllarus) is among the most complex in the animal kingdom. The Australian reef-dwelling creature can see light across the spectrum, from the infrared to the ultraviolet (humans see only visible light, a small portion in the middle) and process twelve colors (humans process three). They can also detect both linearly and circularly polarized light and convert one type into the other – a feat known to humans only through components in things like DVD players and cameras, which perform very crudely by comparison. Researchers at the University of Bristol in the UK have unlocked the secrets of the inner eye, which could one day lead to ultra-high-def optical technology.
Each eye is made up of three parts: two squished hemispheres on the top and bottom separated by six rows of photoreceptor cell clusters, collectively called the midband. The first four rows are specialized for color vision, while the last two are dedicated to polarized vision.
Polarization is most simply defined as the direction of a light wave’s electric field. The electric field can be broken into two perpendicular components, which are in turn both perpendicular to the direction the wave is traveling. If these components reach their minimum and maximum amplitudes at the same time, the strength and direction of the electric field remain constant, and the wave is linearly polarized. If the components are exactly ninety degrees out of phase (so that one component is at zero while the other is at its maximum), the strength of the electric field changes and it traces a circle around the traveling wave, causing circular polarization.
Special cells in the midband have the ability to turn linearly polarized light into circularly polarized light, and vice versa, by acting like a quarter-wave plate. Essentially, the two perpendicular components of the wave travel at two different speeds through the cells, so that one component remains unchanged and the other is slowed until it becomes ninety degrees out of phase (ninety degrees is one quarter of a circle, or one quarter of a complete wave cycle, hence the “quarter-wave plate” behavior). The linearly polarized light then reaches the brain as circularly polarized light.
Man-made quarter-wave plates perform the same function in CDs, DVDs, and camera filters, but only work well for one color of light; the natural ability of the mantis shrimp works almost perfectly across most of the light spectrum. The relative underperformance of artificial devices is due mainly to manufacturing constraints, but unraveling the construction of a natural, geometrically desirable optical system could inspire solutions to such limitations, as nature often does.
The Nature Photonics study’s lead author, Dr. Nicholas Roberts, suggests that future devices could be made with liquid crystals that are chemically engineered to mimic the properties of the cells in the mantis shrimp eye, and posits that further understanding could open doors to an entirely new class of optical devices and systems.