Metamaterial Moves Photons in Single Direction

A new metamaterial can channel photons in one direction, and could point the way to more efficient and faster computer chips that use light to move data.

Current computer chips use electricity to transmit information. Developing chips that use light to move data more quickly and efficiently has proved difficult because light moving through a waveguide — unlike electrons moving through a wire — can reflect backward, interfering with subsequent transmissions and even disrupting the operation of the emitting laser.

Today’s optical networks employ devices called isolators to prevent the backward reflection of light. Isolators have many of their own challenges: They are made of exotic materials such as yttrium indium garnet, and they operate only when a magnetic field is applied to them; as a result, they are bulky. The disadvantage of isolators is that their ability to absorb photons not only prevents backward scattering, but also hinders forward movement of light, which diminishes signal strength.

To prevent microwaves passing through it from reflecting backward, a new metamaterial uses antennas of alternating orientations (top); the antennas are connected by amplifier circuits (bottom). The technology could lead to more efficient, faster computer chips that use light. (Images: Zheng Wang)

Now, scientists at MIT, the University of Texas at Austin and Zhejiang University in China have created a prototype of a metamaterial that works without a magnetic field to keep photons moving in one direction: It rechannels any stragglers rather than absorbing them. In principle, the prototype could yield optical components much smaller than today’s isolators, and building a chip-scale version would require materials no more exotic than the ones used in microprocessors today.

With isolators, “you may not have reflection, but you lose light as light propagates in your structure,” said Zheng Wang, who led the research as a postdoc and research scientist at MIT and is now an assistant professor of computer and electrical engineering at the University of Texas. “Which is a big deal, because one of the reasons we don’t have large-scale integrated optical devices is that loss limits how many devices we can integrate in the system.”

The new material can channel or herd light with rows of embedded metal antennas with alternating vertical and horizontal orientations. Each antenna is connected by an electrical circuit to an antenna of the opposite orientation on the bottom surface of the material; the direction of light propagation is determined by the direction of current flow through the antenna. Although the prototype's antennas are embedded in a pair of circuit boards, in a chip they could be embedded in silicon.

Now, the challenge lies in making the metamaterial work at visible and near-infrared light frequencies.

“We probably need to use some nonlinear optics,” Wang said. “That’s something we’re still working on.”

The study appeared in the Proceedings of the National Academy of Sciences.

For more information, visit: www.mit.edu