Revealing chirality of light in ultra-thin optical fibers

A group of scientists led by Professor Mark Sadgrove at Tohoku University’s Research Institute for Electrical Communications (RIEC) has revealed a fundamental aspect of chirality of light in ultra-thin optical fiber. Nanofibers using the chiral property may be important components of next generation networks.

Chirality is a property commonly found in nature where the reflected version of an object does not perfectly overlap with the original object when superimposed. Human hands are the best known examples of chiral objects - the left and right hand are mirror images - but chirality is found throughout nature, including at scales as small as molecules, photons of light and even fundamental particles.

Recently, chirality was revealed to play an important role in the propagation of photons in ultra-thin optical fibers (optical nanofibers). In particular, it was found that the left- or right-handed circular polarization of a photon emitted near the fiber surface determines whether it travels left or right down the fiber. This property is expected to play an important role in future networks because it allows single photons to be sent from a sender to a receiver with 100% certainty.

Now, the research group at RIEC - which in addition to Mark Sadgrove, includes Masakazu Sugawara, Yasuyoshi Mitsumori and Keiichi Edamatsu - has measured a fundamental aspect of a very thin optical fiber with this chiral property.

The researchers used a gold nanoparticle as an “optical antenna” to concentrate laser light into a fiber 200 times thinner than a human hair. Such a thin fiber has special polarization properties not found in everyday light.

A gold nanoparticle on the surface of an optical nanofiber is illuminated by a laser, and the scattered light enters the fiber. The output intensity (shown by the spheres at either end of the fiber) depends on the light polarization, and is different for left and right sides of the fiber, that is, it exhibits chirality.
Copyright: Mark Sadgrove

By varying the laser polarization, they demonstrated chirality - difference between intensity of left and right moving light in the fiber - over arbitrary polarizations for the first time. Using these measurements, they were able to show that the property of chirality can be explained by a simple rotating structure in polarization space - a new and revealing characterization of the chirality property.

Chirality was demonstrated in nanofibers a few years ago, but we required a complete characterization of the behavior,” says Sadgrove. “In the future, nanofibers using the chiral property may be important components of next generation networks. Therefore, we should characterize their properties completely, just as we would for a component in an electrical circuit.”

This work was first published in the journal Scientific Reports.”