  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.
