A new whispering-gallery microcavity could lead to new and more powerful computers that run faster and at cooler temperatures. The optical diode was created at Washington University in St. Louis (WUSL), where researchers coupled two microresonators using parity-time (PT) symmetry, one with gain and the other with loss, onto a silicon chip. These resonators were positioned to allow light to flow from one to the other. Resonators are in the shape of toroids, one of which has gain and is fabricated from erbium-doped silica film prepared by sol-gel process, while the other has loss and is fabricated from silica. Courtesy of Washington University in St. Louis. When the rate of gain in one resonator exactly equals that of loss in the other, a phase transition occurs at a critical coupling distance between the resonators. "This diode is capable of completely eliminating light transmission in one direction and greatly enhancing light transmission in the other," said lead researcher Bo Peng, a graduate student at WUSL. The loss device is a silica resonator; the gain device incorporates erbium. When erbium interacts with light at 1450 nm, it emits photons at 1550 nm, the researchers said. "As a result, time-reversal symmetry is broken and light is able to move in only one direction — forward," said Dr. Lan Yang, an associate professor of electrical and systems engineering at WUSL. Magneto-optics and high magnetic fields are typically used to break time-reversal symmetry, WUSL scientist Dr. Sahin Kaya Ozdemir added. “Here, we do this using strong non-linearity enabled by broken PT symmetry,” he said. “With an input of only 1 µW, we show seventeenfold enhancement of light transmission in one direction. There is no transmission in the other direction." The new optical diode device could be used in computers and future information processors, as well as other electronics and acoustics. It could also be used to create one-way channels and photonic devices with advanced functionalities. The work was funded by the US Army Research Office and the US Department of Energy. The research is published in Nature Physics (doi: 10.1038/nphys2927). For more information, visit: www.wustl.edu