Aluminum nitride integration on silicon nitride photonic circuits: a new hybrid approach towards on-chip nonlinear optics

Aluminum nitride (AlN) is an emerging material for integrated quantum photonics with excellent linear and nonlinear optical properties, in particular its χ(2) that allows single-photon generation. Here we demonstrate the integration of AlN on silicon nitride (SiN) photonic chips. Composite microrings are fabricated by sputtering c-axis oriented AlN on top of pre-patterned SiN. This new approach does not require any patterning of AlN and depends only on reliable SiN nanofabrication. Different thicknesses are sputtered on microrings with different radii and different waveguide widths. The fabrication is characterized using XRD, optical reflectometry, SEM, and AFM. The optical properties, such as the quality factor, absorption losses and group index, are obtained. The hybrid resonators can have a one order of magnitude increase in quality factor after the AlN integration, with propagation losses as low as 0.16 dB/cm. This hybrid fabrication introduces an additional degree of freedom for the phase matching, which is important for photon-pair generation.