Ultrasonic waves can turn optical fiber into a one-way street for photons, a discovery that could aid the development of quantum computers. Researchers from the University of Illinois at Urbana-Champaign demonstrated the effect using a silica resonator adjacent to a microfiber. At its root is the “acousto-optic interaction of light with long-lived propagating phonons” in the resonator, the researcher wrote in a study published in Nature Physics (doi: 10.1038/nphys3236). Called Brillouin scattering-induced transparency (BSIT), the phenomenon permits light to travel in one direction along the fiber while light traveling in the other direction is strongly absorbed. An artist’s visualization of slow light (red), fast light (blue) and one-way light blocking (yellow) using Brillouin scattering-induced transparency in a series of silica microresonators. Courtesy of the University of Illinois at Urbana-Champaign. “The most significant aspect of our discovery is the observation that BSIT is a non-reciprocal phenomenon — the transparency is only generated one way. In the other direction, the system still absorbs light,” said professor Dr. Gaurav Bahl. This non-reciprocal behavior could enable miniaturized optical isolators and circulators. Current isolators and circulators use specialized garnet and ferrite materials to generate the Faraday magneto-optic effect. These materials are challenging to obtain at the chip scale through conventional foundry processes, and magnetic fields can cause interference in applications such as cold atom microsystems. “Brillouin isolators do already exist, but they are nonlinear devices requiring filtering of the scattered light,” Bahl said. “BSIT, on the other hand, is a linear non-reciprocal mechanism.” BSIT also enables the speeding up and slowing down of the group velocity of light, and so could enable slow light devices for quantum information storage and optical buffer applications. “While it is already known that the slow and fast light can be obtained using Brillouin scattering, our device is far smaller and uses far less power than any other previous demonstration, by several orders-of-magnitude. However, we must sacrifice bandwidth to obtain such performance,” said graduate student JunHwan Kim. Funding came from the National Science Foundation and the U.S. Air Force Office for Scientific Research. For more information, visit www.uillinois.edu.