Chrysina Gloriosa and circularly polarized light

2018-02-12

Varunesh Goyal

Light is essentially perpendicularly oscillating electric and magnetic fields in space. Normally the fields are randomly oriented perpendicularly to the direction of propagation too, but when the electric (or magnetic) field oscillates only along a single direction, the resulting light is called polarized. In a linearly polarized light, this direction is fixed while in a circularly polarized light, it keeps on rotating.

Circularly polarized light is widely used in various communication techniques. It is specifically beneficial in satellite communication. If linearly polarized light is used, the orientation of the antenna on ground can greatly affect the received signal strength and it becomes challenging to ensure optimum orientation as the electric field vector can undergo rotation in the atmosphere. Such rotations on the other hand won’t affect circularly polarized light.

In nature, linearly polarized light is easily found - for example whenever light undergoes reflection, the reflected light is partially linearly polarized. But getting a circularly polarized light? Thats a rare thing. We generally produce it using a combination of vertically and horizontally polarized light sources which are 90o out of phase which each other.

Now Chrysina Gloriosa is a type of scarab beetle. And the light reflected off it is unusual as it gets circularly polarized. They look bright metallic green and gold under a left handed circular polarization filter, and dark brown under the opposite filter as not much light can pass through in this case.

FIG: LEFT IMAGE SHOWS CHRYSINA GLORIOSA UNDER NORMAL CONDITIONS. RIGHT IMAGE IS WHEN LOOKED UNDER A RIGHT CIRCULARLY POLARIZED FILTER (IMAGE TAKEN BY J. C. ABBOTT)

This is mainly achieved by microstructures on the surface of the cuticle of the beetle. Chrysina Gloriosa has left-handed twisted structures (helix shape basically) present. A host of other structural aspects are also the cause behind its colors. This is called iridescence (i.e. color is due to fine structures and interference of light that results from it instead of pigments being the cause of color). It is the same phenomenon which gives the wings on butterflies and moths their vibrant colors.

This doesn’t bring an end to the story however. Recent studies have shown that Chrysina Gloriosa are sensitive to circular polarization in light. This makes them only the second species on earth which is known to be sensitive to it after a certain species of shrimp. Which biomolecules enable this and how do they function? To be discussed in a later post ;)