Nature's tough-guy arthropod offers physicists an entirely new sort of optical material.
Image: Roy L Caldwell/NSF
In terms of communications privacy, mantis shrimp are way ahead of humans. Their security measures are built in, biologically, thanks to a peculiar communication strategy based on sending information from shrimp to shrimp through the polarization of light. Eavesdropping predators can't see this information-carrying component, so the mantis shrimp is able to signal to its fellow mantis shrimp invisibly. Evolution is pretty clever.
Researchers from the Ecology of Vision Group at the University of Bristol have been working to understand this system using a combination of light measurements, theoretical modeling, and anatomical observations. In a paper published in this week's Scientific Reports, the group describes a never-before-seen optical material employed by the mantis shrimp that allows it to reflect bright and colorful polarized light using microscopically thin features. As one might imagine, this could potentially be useful for future human communications technologies as it represents an entirely new way of building polarizers.
First, let's pause a moment. Because look at this fucking thing:
OK, now back to physics.
Polarization is a fundamental physical property of many sorts of waves, including waves of light. And if you were to imagine waves of light moving through space, chances are pretty good that you're imagining those waves oscillating "up" and "down." The wave rises and then falls in a direction perpendicular to the wave's direction of travel, like waves on the surface of a sea. However, light as we normally experience it from the Sun or a lamp or candle is unpolarized: It doesn't oscillate in a preferred direction and instead oscillates in all of them equally, with no preference for one particular plane (like a two-dimensional sheet through space).
But light can be polarized such that its waves (or vibrations) do have a preference for a single plane, such as straight up and down or some other direction relative to the waves' direction of travel. This is accomplished using a filter. If we take a filter that only lets through waves that are polarized along a chosen orientation, all of the other waves are blocked. This is polarized light.
As humans with human eyes, we don't really see the polarization component of light. Light mostly just looks like light to us, whether it's up or down or side to side or every direction at once. Many insects and arthropods feature polarization vision as a bonus sense, however, where it's most often employed in environmental sensing. Polarization can carry information about potential threats, for example.
The mantis shrimp is already a classic example of polarized light sensing in nature, as it boasts one of the most complex visual systems ever discovered. The Gonodactylus smithii species is believed to be able to completely detect all possible polarizations, giving it something like perfect polarization vision. It's even been suggested that mantis shrimp can detect cancer and image neural activity.
With the structures characterized by the Bristol group, each a combination of six or so tiny vesicles, a mantis shrimp reflects incoming unpolarized light back out as polarized light, which can then be detected by other mantis shrimp. Within each array of substructures, the raw incoming light becomes coupled to the natural harmonics of the reflective material, with the different vesicles offering different polarization modes. Despite being microscopic in size, the structures are able to emit very bright, powerful light.
Us humans are actively looking for new and better ways of polarization for all kinds of applications, ranging from liquid-crystal displays to characterizing viruses. Simply, shrimp biology could mean improved human technology. As the Bristol study notes, "This control and tuning of the polarization of the reflection using shape-anisotropic hollow scatterers is unlike any optical structure previously described and could provide a new design pathway for polarization-tunability in man-made photonic devices."
Rave on, mantis shrimp. Rave on.