A Magnetic Field for Photons

A photonic crystal device that tames the flow of free-moving photons with synthetic magnetism could enable scientists to precisely steer light in any direction.

The process breaks a key law of physics known as the time-reversal symmetry of light. Breaking that law introduces a charge on the photons that reacts to an effective magnetic field the way an electron would to a real magnetic field. The discovery could yield an entirely new class of devices that use light instead of electricity for applications ranging from accelerators and microscopes to speedier on-chip communications.

“This is a fundamentally new way to manipulate light flow,” said Shanhui Fan, an electrical engineering professor at Stanford University and a member of the interdisciplinary team that created the device. “It presents a richness of photon control not seen before.”

Using magnetic fields to control electrons is a founding principle of electronics, but a corollary for photons had not previously existed. When an electron approaches a magnetic field, it meets resistance and opts to follow the path of least effort, traveling in circular motion around the field. Similarly, the Stanford device sends photons in a circular motion around the synthetic magnetic field.

Scientists at Stanford University have used synthetic magnetism to control the flow of light at the nanoscale. From left, doctoral candidate Kejie Fang of the department of physics, and professor Shanhui Fan and postdoctoral scholar Zongfu Yu, both of the Stanford School of Engineering. Courtesy of Norbert von der Groeben.

The solution capitalizes on recent research into photonic crystals — materials that confine and release photons. To build its device, the team developed a grid of tiny cavities etched in silicon, forming the photonic crystal. By precisely applying electric current to the grid, the scientists can control, or harmonically tune, the photonic crystal to synthesize magnetism and exert virtual force upon photons.

The radius of the photon’s trajectory was altered by varying the electrical current applied to the photonic crystal and by manipulating the speed of the photons as they entered the system, the scientists said. This dual mechanism provides a great degree of precision control over the photons’ path, allowing the researchers to steer the light wherever they like.

For engineers, breaking time-reversal symmetry means that a photon traveling forward will have different properties than when it travels backward, yielding important technical possibilities.

“The breaking of time-reversal symmetry is crucial, as it opens up novel ways to control light,” Fan said. “We can, for instance, completely prevent light from traveling backward to eliminate reflection.” The device, therefore, solves one major drawback of current photonic systems that use fiber optic cables. Photons tend to reverse course in such systems, causing backscatter.

Essentially, once a photon enters the new device, it cannot go back. This quality, the researchers believe, will be key to future applications of the technology, as it eliminates disorders such as signal loss, which is 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 postdoctoral researcher Zongfu Yu. “We’re excited to see where it leads.”

The study appeared online in Nature Photonics (doi: 10.1038/nphoton.2012.236).

For more information, visit: www.stanford.edu