Researchers at Washington University in St. Louis have devised an optical resonator system that can be used to turn transparency on and off. The ability to manipulate electromagnetically induced transparency (EIT) without the introduction of an outside influence, such as additional photons, could provide scientists and engineers with more precise control over how much light passes through a medium. The team used whispering gallery mode resonators (WGMRs) for its system. Two WGMRs were indirectly coupled by a fiber optic line. The first resonator was higher in quality, exhibiting just one imperfection. Researcher Changqing Wang added a tiny pointed material to the high-quality resonator that acted like a nanoparticle. By moving this makeshift particle, Wang was able to tune it and thus control the way the light scattered inside the resonator. Wang was also able to tune the resonator to an exceptional point, that is, a point at which only one state can exist. The “state” referred to the direction of light in the resonator, which could be clockwise or counterclockwise. For the experiment, the researchers directed light toward a pair of indirectly coupled resonators. The lightwave entered the first resonator, which was tuned to ensure that light traveled in a clockwise direction. The light bounced around the perimeter of the resonator and then exited, continuing along the optical fiber to the second, lower-quality resonator. When the light entered the second resonator, it was scattered by the resonator’s imperfections. Some of the light began traveling counterclockwise along the perimeter of the WGMR. It then returned to the fiber, but it was headed back in the direction of the first resonator. As the lightwaves propagated back and forth between resonators, an interference pattern formed that allowed light traveling along the optical fiber to get through, making the system transparent. Electromagnetically induced transparency (EIT) is tuned by two particles on the optical resonator. The different locations of particles control the propagation of light in either clockwise or counterclockwise directions, which switch on (upper configuration) or off (lower configuration) the interference of light, leading to controllable brightness (EIT) and darkness in the output. Courtesy of the Yang Lab. EIT has the ability to create “slow light.” Although the speed of light is constant, the actual value of light’s speed can change based on the properties of the medium it passes through. In a vacuum, for example, light always travels at 300 million meters per second. With EIT, researchers have slowed light down to eight meters per second, Wang said. “That can have significant influence on the storage of light information. If light is slowed down, we have enough time to use the encoded information for optical quantum computing or optical communication,” he said. Better control over EIT could enhance the availability of “slow light” for these applications. The ability to manipulate EIT could also be useful in the development of long-distance communications. A tuning resonator could be indirectly coupled to another resonator, kilometers away, along the same fiber optic cable. “You could change the transmitted light down the line,” Yang said. Among other applications, this could be useful for quantum encryption. The research was published in Nature Physics (www.doi.org/10.1038/s41567-019-0746-7).