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

Abstract: Emerging technologies based on tailorable interactions between photons and phonons promise new capabilities ranging from high-fidelity microwave signal processing to non-reciprocal optics and quantum state control. While such light-sound couplings have been studied in a variety of physical systems, many implementations rely on non-standard materials and fabrication schemes that are challenging 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 waveguides using electrically-driven surface acoustic waves (SAWs). By co-integrating SAW transducers in piezoelectric AlN with a standard silicon-on-insulator photonic platform, we harness silicon's strong elasto-optic effect to mediate linear light-sound coupling. Through lithographic design, acousto-optic phase and single-sideband amplitude modulators in the range of 1-5 GHz are fabricated, exhibiting index modulation strengths comparable to existing electro-optic technologies. Extending this traveling-wave, acousto-optic interaction to cm-scales, we create electrically-driven non-reciprocal modulators in silicon. Non-reciprocal operation bandwidths of >100 GHz and insertion losses <0.6 dB are achieved. Building on these results, we show that unity-efficiency non-reciprocal modulation, necessary for a robust acousto-optic isolator, is within reach. The acousto-optic modulator design is compatible with both CMOS fabrication and existing silicon photonic device technologies. These results represent a promising new approach to implement compact and scalable acousto-optic modulators, frequency-shifters, and non-magnetic optical isolators and circulators in integrated photonic circuits.