Microscopic roundabout directs light without a magnet

Circulators direct light on optical chips, a process essential to communication technology. The component relies on a tiny magnet, but miniaturizing magnets is difficult.

Enter the magnet-free optical circulator. The roundabout can route light without the assistance of a tiny magnet. Researchers say it's the first of its kind.

Using a unique combination of entrance and exit ports, circulators ensure that light is directed to the proper place and that no light -- or information -- is lost on the path from one port to another.

"Light propagation is symmetric in nature, which means if light can propagate from A to B, the reverse path is equally possible. We need a trick to break the symmetry," lead researcher Ewold Verhagen, scientist at the Dutch research institute AMOLF, said in a news release. "Usually this 'trick' is using centimeter-sized magnets to impart directionality and break the symmetric nature of light propagation. Such systems are difficult to miniaturize for use on photonic chips."

Verhagen and his research partners, including scientists from the University of Texas, replaced the magnetized circulator with a microscale glass ring resonator. Structural vibrations control when light can exit and where.

"By shining light of a 'control' laser in the ring, light of a different color can excite vibrations through a force known as radiation pressure, but only if it propagates in the same direction as the control light wave," Verhagen said. "Since light propagates differently through a vibrating structure than through a structure that is standing still, the optical force breaks symmetry in the same way as a magnetic field would."

Scientists still had to find a way to ensure the propagated light exited at the proper port, the next available exit. They realized the control laser can use a phenomenon called optical interference to propagate the light out of a specific exit.

"We demonstrated this circulation in experiments, and showed that it can be actively tuned," said postdoc John Mathew. "The frequency and power of the control laser allow the circulation to be turned on and off and change handedness."

The breakthrough -- detailed in the journal Nature Communications -- could do more than improve communication technologies, it could also power quantum computers.

"The fact that the circulator can be actively controlled provides additional functionality as the optical circuits can be reconfigured at will," Verhagen said.

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