Aluminum nitride as a new material for chip-scale optomechanics and nonlinear optics

TLDR

Silicon photonics is limited to wavelengths above 1100 nm, suffers from two‑photon absorption and free‑carrier effects, and lacks strong optical nonlinearity, whereas aluminum nitride offers a wide bandgap, low mechanical loss, and high electromechanical coupling. The authors aim to develop an AlN‑on‑silicon platform that enables low‑loss, wide‑band optical guiding and high‑quality optomechanical devices. They fabricate AlN waveguides on silicon and integrate them into optomechanical resonators, exploiting AlN’s second‑order nonlinearity for simultaneous optical and mechanical performance. The platform achieves low‑loss optical guiding, high optical and mechanical quality factors, and demonstrates electro‑optic modulation and efficient second‑harmonic generation, positioning AlN‑on‑silicon as a CMOS‑compatible alternative free from carrier effects.

Abstract

Silicon photonics has offered a versatile platform for the recent development of integrated optomechanical circuits. However, silicon is limited to wavelengths above 1100 nm and does not allow device operation in the visible spectrum range where low noise lasers are conveniently available. The narrow band gap of silicon also makes silicon optomechanical devices susceptible to strong two-photon absorption and free carrier absorption, which often introduce strong thermal effect that limit the devices' stability and cooling performance. Further, silicon also does not provide the desired lowest order optical nonlinearity for interfacing with other active electrical components on a chip. On the other hand, aluminum nitride (AlN) is a wideband semiconductor widely used in micromechanical resonators due to its low mechanical loss and high electromechanical coupling strength. Here we report the development of AlN-on-silicon platform for low loss, wideband optical guiding, as well as its use for achieving simultaneous high optical quality and mechanical quality optomechanical devices. Exploiting AlN's inherent second order nonlinearity we further demonstrate electro-optic modulation and efficient second-harmonic generation in AlN photonic circuits. Our results suggest that low cost AlN-on-silicon photonic circuits are excellent substitutes for CMOS-compatible photonic circuits for building new functional optomechanical devices that are free from carrier effects.