At the Ecology of Vision lab, one of our major fascinations is the way that animals have evolved to use light in many different and novel ways. Not only do they use their eyes to extract optical information from the environment, their bodies are designed to allow them to signal to one another using complex optical manipulations. In this news article, we explore three new studies from our lab and collaborators along this theme.
New type of optical material discovered in the secret language of the mantis shrimp
Mantis shrimp like to keep their conversations private, which is why they communicate using the polarization of light. These animals have evolved bright reflectors that control the polarization of their visual signals, a property of light not commonly used for animal communication. Most eavesdroppers can’t see this type of light information and so animals that use it are less likely to attract the attention of predators or unwelcomed competition.
In a quest to understand how these uncommon light signals are produced in mantis shrimp, researchers from the Ecology of Vision Lab discovered that they use a polarizing structure unlike anything ever seen or developed by humans. The research is published in Scientific Reports.
Using a combination of careful anatomy, light measurements, and theoretical modelling, it was found that the mantis shrimp polarizers work by manipulating light across the structure rather than through its depth, which is how typical polarizers work. Such a photonic mechanism affords the animal with small, microscopically thin and dynamic optical structures that still produce big, bright and colourful polarized signals.
Dr Nicholas Roberts said: “When it comes to developing a new way to make polarizers, nature has come up with optical solutions we haven't yet thought of. Industries working on optical technologies will be interested in this new solution mantis shrimp have found to create a polarizer as new ways for humans to use and control light are developed.”
‘A shape-anisotropic reflective polarizer in a stomatopod crustacean’ by Thomas M. Jordan, David Wilby, Tsyr-Huei Chiou, Kathryn D. Feller, Roy L. Caldwell, Thomas W. Cronin and Nicholas W. Roberts 2016 Scientific Reports [open access]
Under a polarized sky
If you're lucky, when you look up the sky will appear a clear blue. But for many animals, the sky is awash with useful information which they may use to help find their way around.
As light from the sun interacts with particles in the atmosphere, certain angles of polarization make their way to the Earth more easily than others. This results in a complex pattern in the sky which can help some insects to navigate and orient.
Xin Wang is a PhD student visiting the Ecology of Vision group for 2 years from the Hefei University of Technology, China. His research centres on developing technology in order to build a robot which will navigate using the skylight polarization pattern.
On course to this goal, Xin has built a new, computational model of the skylight polarization pattern. Which is published this week in the Journal of Optics.
'An analytical model for the celestial distribution of polarized light, accounting for polarization singularities, wavelength and atmospheric turbidity' by Xin Wang, Jun Gao, Zhiguo Fang and Nicholas W. Roberts 2016 Journal of Optics
Guanine is best known as one of the 4 bases of DNA or as a major constituent of bat droppings! But we know it as a highly capable optical biological material. Its high refractive index and large birefringence (different refractive properties for different polarizations) mean that it appears in reflective structures which need to control their polarization properties, such as the silvery reflections from fish skin.
In a new investigation, Masakazu Iwasaka (HIroshima University) and colleagues found that the application of magnetic fields could manipulate the position of guanine crystals. This allows the manipulation of their reflection of light along particular axes, effectively switching the reflection on and off.
The authors hope that this observation can be applied in the design of a synthetic array of organic micromirrors in future optical technology.
'Magnetic Control of the Light Reflection Anisotropy in a Biogenic Guanine Microcrystal Platelet' Masakazu Iwasaka, Yuri Mizukawa and Nicholas W. Roberts 2016 Langmuir