Electrically-driven Acousto-optics and Broadband Non-reciprocity in Silicon Photonics

Emerging technologies based on tailorable interactions between photons and phonons promise new capabilities ranging from high-fidelity microwave signal processing to nonlinear optics and quantum state control. While such light-sound couplings have been studied in a variety of physical systems, many implementations rely on nonstandard materials and fabrication schemes that are nontrivial to co-implement with standard integrated photonic circuitry. Notably, despite significant advances in integrated electro-optic modulators, related acousto-optic modulator concepts have remained relatively unexplored in silicon photonics. In this article, we demonstrate direct acousto-optic modulation within silicon photonic waveguides using electrically-driven surface acoustic waves (SAWs). Electromechanical transduction is achieved through co-integration of piezoelectric SAW transducers in aluminum nitride with a standard silicon-on-insulator optical waveguide platform. These electromechanical transducers emit surface acoustic waves that produce nonlocal light modulation in optical waveguides through silicon's strong elasto-optic effect. Through lithographic control, we separately demonstrate both acousto-optic phase modulation and single sideband amplitude modulation in the range of 1-5 GHz, with refractive index modulation strengths comparable to existing electro-optic technologies. Furthermore, these devices are leveraged to enable non-reciprocal modulation and routing over a 100 GHz optical bandwidth. The modulator design avoids suspended waveguide structures common in many optomechanical devices, and is compatible with both complementary metal–oxide–semiconductor (CMOS) fabrication processes, and with existing silicon photonic device technologies. These results represent a promising new approach to implement compact and scalable acousto-optic modulators, frequency-shifters, and optical isolators and circulators in integrated photonic circuits.