A heterogeneously integrated lithium niobate-on-silicon nitride photonic platform.

The availability of thin-film lithium niobate on insulator (LNOI) and advances in processing have led to the emergence of fully integrated LiNbO 3 electro-optic devices. Yet to date, LiNbO 3 photonic integrated circuits have mostly been fabricated using non-standard etching techniques and partially etched waveguides, that lack the reproducibility achieved in silicon photonics. Widespread application of thin-film LiNbO 3 requires a reliable solution with precise lithographic control. Here we demonstrate a heterogeneously integrated LiNbO 3 photonic platform employing wafer-scale bonding of thin-film LiNbO 3 to silicon nitride (Si 3 N 4 ) photonic integrated circuits. The platform maintains the low propagation loss (<0.1 dB/cm) and efficient fiber-to-chip coupling (<2.5 dB per facet) of the Si 3 N 4 waveguides and provides a link between passive Si 3 N 4 circuits and electro-optic components with adiabatic mode converters experiencing insertion losses below 0.1 dB. Using this approach we demonstrate several key applications, thus providing a scalable, foundry-ready solution to complex LiNbO 3 integrated photonic circuits.