Stanford researchers control light with 'synthetic magnetism'

Shanhui Fan (center) and post-doctoral scholar Zongfu Yu (right), both of the Stanford school of engineering, and doctoral candidate Kejie Fang (left), of the department of physics, have used "synthetic magnetism" to control the flow of light at the nanoscale. (Image: Norbert von der Groeben)

Palo Alto, CA--Researchers at Stanford University have demonstrated a device that produces a so-called "synthetic magnetism" to exert virtual force on photons that is in some ways similar to the effect of magnets on electrons. The advance could lead to photon control that is helpful to integrate photonic circuits. The process, which breaks a key maxim of physics known as the time-reversal symmetry of light (which has been broken before), could yield a new class of devices that use light instead of electricity for applications ranging from accelerators and microscopes to faster on-chip communications.

The Stanford solution capitalizes on recent research into photonic crystals. To fashion their device, the team members created a photonic crystal from silicon; an electric current applied to the device harmonically tunes the photonic crystal to "synthesize" magnetism and exert a virtual force upon photons. The researchers refer to the synthetic magnetism as an effective magnetic field.

The researchers were able to alter the radius of a photon’s trajectory by varying the electrical current applied to the photonic crystal, and by manipulating the speed of the photons as they entered the system. This dual mechanism provides precision control over the photons’ path, allowing the researchers to steer the light. The device sends photons in a circular motion around the synthetic magnetic field. The research was published in a recent issue of Nature Photonics.

For engineers, braking the time-reversal symmetry of light means that a photon traveling forward will have different properties than when it is traveling backward. “The breaking of time-reversal symmetry is crucial, as it opens up novel ways to control light,” said Shanhui Fan, a professor of electrical engineering at Stanford and senior author of the study. "We can, for instance, completely prevent light from traveling backward to eliminate reflection."

The new device solves at least one major drawback of current photonic systems that use fiber-optic cables. Photons tend to reverse course in such systems, causing backscatter. “Despite their smooth appearance, glass fibers are, photonically speaking, quite rough,” says Kejie Fang, a doctoral candidate in the department of physics at Stanford and the first author of the study. "This causes a certain amount of backscatter, which degrades performance."

Breaking time-reversal symmetry, the researchers believe, will be key to future applications as it eliminates disorders such as signal loss common to fiber optics and other light-control mechanisms. “Our system is a clear direction toward demonstrating on-chip applications of a new type of light-based communication device that solves a number of existing challenges,” said Zongfu Yu, a post-doctoral researcher in Shanhui Fan’s lab and co-author of the paper. “We’re excited to see where it leads.”


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