Ring resonators proposed to develop micro-optical diodes would replace or be compatible with their electronic counterparts for quantum information. The approach has an advantage over other optical technologies because it works with single photons. The technology is silicon-on-insulator, compatible with CMOS fabrication processes currently used in computer circuits. Scaling the device to fit onto microelectronic chips has proved challenging because of the sizable magnetic field and the difficult integration of magneto-optic crystals onto chips. An optical ring resonator, taken with a scanning electron microscope. (Image: K. Srinivasan, NIST) Scientists from the Joint Quantum Institute (JQI) at the University of Maryland and the Institute for Quantum Optics and Quantum Information discovered that Faraday rotation is not the only way to cause nonreciprocal behavior. They formulated an optomechanical diode, wherein an optical pathway called a waveguide is linked to an optomechanical resonator that resembles a pedestal in appearance. It functions as the optical analog, while the mechanical resonator serves as the tool for creating diodelike behavior in the waveguide. Ring-shaped resonators enable light to bounce around and undergo floppiness. The micro ring can vibrate in varied ways. In their experiment, the scientists used the light within the cavity to make the resonator breathe radially. Light from both directions traveled along the waveguide and was absorbed or transmitted by the ring resonator depending on its wavelength. The found that light, with the right frequency to enter the resonator and stimulate its breathing motion, has a wavelike vibrational motion that interferes with the light wave inside the resonator. The passage of light, even if it travels right or left, is still reciprocal. This is not an optical diode, and light from either direction is slightly affected by the vibrations when not modified. Theoretical calculation of light transmission (Y-axis) through the waveguide when the optical resonator is operated as a diode. For a certain range of laser frequencies, the left moving light is attenuated, while light traveling from the right is transmitted. (Image: Courtesy of the authors; a modified version appears in the article) To get the photons to travel in one direction, the researchers propose injecting intense light, called a “pump,” into one of the resonator pathways. With the pump, the influence of clockwise-moving light on the breathing improves. The pump light, which can now be modulated, enhances the clockwise-moving light’s influence on the breathing. The researchers observed that, at certain wavelengths, the clockwise light transmitted through the waveguide; however, the counterclockwise did not excite vibrations and was blocked or absorbed by the resonator. This is an optical diode. Unlike the macroscopic Faraday rotator, this system can be switched on and off with the help of an optical beam. However, based on the vibration of the light, the clockwise moving light will attain a different phase shift compared with the counterclockwise moving light. This optical isolator can function with single photons, in the quantum limit. “Wave interference (here acoustic vibrations interfering with light waves) is nonquantum,” said Mohammad Hafezi, author of the study. “But, scientists can cool microresonators to a temperature where quantum effects emerge.” An array of these microresonator diodes could be used for stimulating quantum many-body systems. “The outlook of this is an optical isolator that can be used on-chip, which is useful for photonics,” Hafezi said. “On the other hand, it can be used as a nonreciprocal phase shifter so we can explore quantum Hall physics. We can exploit the nonlinearity and nonreciprocity at the same time to simulate different quantum phenomena.” The research appeared in Optics Express. For more information, visit: www.jqi.umd.edu