A new microchip generates both light waves and ultrahigh-frequency sound waves — and forces them to interact. “Our breakthrough is to integrate optical circuits in the same layer of material with acoustic devices in order to attain extreme strong interaction between light and sound waves,” said University of Minnesota professor Dr. Mo Li. With nanoscale transducers, the researchers found that acoustic waves with suboptical wavelengths can be excited to induce strong acousto-optic coupling in nanophotonic devices, they wrote in a study. They were also able to achieve “acousto-optic modulation of the resonance modes at above 10 GHz, with the acoustic wavelength significantly below the optical wavelength.” A sound wave passes across an integrated optical waveguide, overlaid with a color map of the light field in it. Courtesy of University of Minnesota. “What’s remarkable is that at this high frequency, the wavelength of the sound is even shorter than the wavelength of light,” said graduate student Semere Tadesse. “This is achieved for the first time on a chip. In this unprecedented regime, sound can interact with light most efficiently to achieve high-speed modulation.” With conventional devices that operate in the megahertz frequency range, the acoustic wavelength is longer than the optical wavelength, and requires a long interaction length to attain significant coupling. The new chip features a silicon base coated with a layer of aluminum nitride that conducts an electric charge. Applying alternating electrical signal to the material causes it to deform periodically and generate sound waves that grow on its surface. This technology already serves as a microwave filter in many wireless communications devices. The researchers are now looking at the interaction between single photons and single phonons, and how the new device could use sound waves as information carriers for quantum computing. It could lead to development of optical systems based on nonlinear Brillouin processes, and potentially offers a direct, wideband link between optical and microwave photons for microwave photonics and quantum optomechanics. The work was funded by the National Science Foundation and the U.S. Air Force Office of Scientific Research. The research was published in Nature Communications (doi: 10.1038/ncomms6402). For more information, visit www.umn.edu.