Credit: Roy Caldwell / UC Berkeley
The mantis shrimp—an animal that has received press for its spectacular color vision and its faster-than-a-speeding-bullet death punch—may have proven itself even more amazing with a potentially novel optical polarizer that it uses for visual communication.
An international research team led by Nicholas Roberts from the Ecology of Vision Group at the University of Bristol’s School of Biological Sciences, U.K., has reportedly demonstrated the structure and function of the stomatopod’s 3-D reflective polarizer (Sci. Rep., doi: 10.1038/srep21744). In short, they showed that the polarizers manipulate light across the structure, rather than through it as most other biophotonic polarizers do. These findings could be a first step toward the design of new man-made 3-D photonic structures.
Secret conversations
Mantis shrimp, like some species of octopus and fish, can communicate with each other using visual signals created by reflective polarizers on their bodies. (For the mantis shrimp, these reflective polarizers are located on their maxillipeds—the bright blue/green structures under the eyes.) Most animals can’t see these visual signals, which keeps the conversation concealed from possible prey and predators. Roberts and his colleagues wanted to test previous hypotheses that say the mantis shrimp reflective polarizers, unlike typical 2-D biophotonic polarizers, have a 3-D architecture.
To do so, the researchers used transmission electron microscopy, optical measurements and theoretical modelling to validate the polarizer’s 3-D structure. Their results confirmed that the mantis shrimp’s visual signals are reflections of incident light by a polarizer consisting of 6 – 8 layers of hollow, oval-shaped vesicles that “exhibit degrees of both positional and orientational order.”
In-plane coupling
The researchers’ theoretical model dissected the optical response of the mantis-shrimp polarizer into contributions from different Bragg harmonics. They found that the polarizing reflections are created by a resonant coupling between light and the first-order, in-plane Bragg harmonics. This in-plane coupling is, according to the authors, a novel reflection mechanism for a biophotonic structure, which is “unlike any optical structure previously described.” The authors say that their findings could provide a new design pathway for polarization-tunability devices.