Researchers at École Polytechnique Fédérale de Lausanne (EPFL) and Purdue University demonstrated a hybrid approach to on-chip acousto-optic modulation by combining piezoelectric aluminium nitride technology with ultralow-loss silicon nitride integrated photonics. The hybrid circuit allows wideband actuation on photonic waveguides with ultralow electrical power. A key feature is that it maintains the ultralow loss of silicon nitride circuits. Silicon nitride has emerged as a leading material for chip-scale, microresonator-based optical frequency combs (microcombs). The researchers integrated microelectromechanical systems (MEMS) transducers made of aluminum nitride with a silicon photonic wafer to modulate a soliton microcomb at high frequencies ranging from megahertz to gigahertz. They fabricated piezoelectric aluminium nitride actuators on top of the silicon nitride photonic circuits and applied a voltage signal to them. The signal induced bulk acoustic waves electromechanically. The acoustic waves modulated the microcomb generated in the silicon nitride circuits. By monolithically integrating aluminium nitride actuators on ultralow-loss silicon nitride photonic circuits, the researchers demonstrated voltage-controlled soliton initiation, tuning, and stabilization with megahertz bandwidth. Integrated silicon nitride photonic chips with aluminium nitride actuators. Courtesy of Jijun He, Junqiu Liu (EPFL). Purdue researcher Hao Tian built the MEMS transducers and integrated them with a silicon nitride photonics wafer developed at EPFL. The researchers demonstrated two independent applications using the hybrid system. First, they showed optimization of a microcomb-based massively parallel coherent lidar. This approach could provide a route to chip-based lidar engines driven by CMOS microelectronic circuits. Second, they built magnet-free optical isolators by spatiotemporal modulation of a silicon nitride microresonator. “The tight vertical confinement of the bulk acoustic waves prevents cross-talk and allows for close placement of the actuators, which is challenging to achieve in p-i-n silicon modulators,” Tian said. Microscope image showing the piezoelectric actuators covering the silicon nitride photonic circuits. Courtesy of Junqiu Liu, Rui Ning Wang. The circuit was manufactured using CMOS-compatible foundry processes. The fabrication processes were integrated, which could make the technology more viable commercially. The MEMS transducers were fabricated on top of the silicon nitride photonics wafer with minimal processing, the researchers said. “This achievement represents a new milestone for the microcomb technology, bridging integrated photonics, microelectromechanical systems engineering, and nonlinear optics,” EPFL researcher Junqiu Liu said. “By harnessing piezoelectric and bulk acousto-optic interactions, it enables on-chip optical modulation with unprecedented speed and ultralow power consumption.” New technology uses acoustics to better control a pulse of laser light split into a frequency comb, potentially helping lidar to achieve better detection of nearby high-speed objects. Courtesy of WoogieWorks/Alex Mehler. The new technology could provide the impetus to develop microcomb applications for power-critical systems in space, data centers, and portable atomic clocks, for example, or in extreme environments such as cryogenic temperatures. “As yet unforeseen applications will follow up across multiple communities,” EPFL professor Tobias Kippenberg said. “It’s been shown time and again that hybrid systems can obtain advantages and functionality beyond those attained with individual constituents.” The research was published in Nature (www.doi.org/10.1038/s41586-020-2465-8).