Taming mavericks: Stanford researchers use synthetic magnetism to control light
Stanford researchers in physics and engineering have demonstrated a device that produces a synthetic magnetism to exert virtual
force on photons similar to the effect of magnets on electrons. The advance could yield a new class of nanoscale applications
that use light instead of electricity.
Professor Shanhui Fan (center), postdoc Zongfu Yu (right) and doctoral candidate Kejie Fang (left)
have used synthetic magnetism to control the flow of light at the nanoscale. (Photo: Norbert von der Groeben)
November 1, 2012
By Andrew Myers
Magnetically speaking, photons are the mavericks of the engineering world. Lacking electrical charge, they are free to run even
in the most intense magnetic fields. But all that may soon change. In a
published in Nature Photonics, an interdisciplinary
team of Stanford physicists and engineers reports that they have created a device that tames the flow of photons with synthetic
The process breaks a key law of physics known as the time-reversal symmetry of light and could yield an entirely new class of
devices that use light instead of electricity for applications ranging from scientific devices such as accelerators and microscopes
to speedier on-chip communications.
"This is a fundamentally new way to manipulate light flow. It presents a richness of photon control not seen before,"
a professor of electrical engineering at Stanford and senior author of the study.
The ability to use magnetic fields to redirect electrons is a founding principle of electronics, but a corollary for photons
has 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, this new device sends photons in a circular motion
around the synthetic magnetic field.
The Stanford solution capitalizes on recent research into photonic crystals – materials that can confine and release photons.
To fashion their device, the team members created a grid of tiny cavities etched in silicon, forming the photonic crystal.
By precisely applying electric current to the grid they can control – or "harmonically tune," as the researchers say – the photonic
crystal to synthesize magnetism and exert virtual force upon photons. The researchers refer to the synthetic magnetism as an
effective magnetic field.
The researchers reported that they 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 enter the system. Providing a great degree
of precision control over the photons' path, this dual mechanism allows the researchers to steer the light wherever they like.
In fashioning the device, the team has broken what is known in physics as the time-reversal symmetry of light.
Breaking time-reversal symmetry in essence introduces a charge on the photons that reacts to the effective magnetic
field the way an electron would to a real magnetic field.
For engineers, it means that a photon traveling forward will have different properties than when it is traveling backward,
the researchers said, and this yields promising technical possibilities. "The breaking of time-reversal symmetry is crucial
as it opens up novel ways to control light. We can, for instance, completely prevent light from traveling backward to eliminate
reflection," said Fan.
The new device, therefore, solves at least one major drawback of current photonic systems that use fiber optic cables.
Photons tend to reverse course in such systems, causing a form of reflective noise known as backscatter.
"Despite their smooth appearance, glass fibers are, photonically speaking, quite rough. This causes a certain amount of
backscatter, which degrades performance," said Kejie Fang, a doctoral candidate in physics at Stanford and the first author
of the study.
In essence, 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 common to fiber optics and other
"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 postdoctoral researcher in Fan's lab and co-author
of the paper. "We're excited to see where it leads."
Andrew Myers is associate director of communications at the School of Engineering.