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A heterogeneously integrated lithium niobate-on-silicon nitride photonic platform
preprintposted on 2023-06-08, 12:50 authored by Mikhail Churaev, Rui Ning Wang, Viacheslav Snigirev, Annina Riedhauser, Terence Blésin, Charles Möhl, Miles A. Anderson, Anat Siddharth, Youri Popoff, Daniele Caimi, Simon Hönl, Johann Riemensberger, Junqiu Liu, Paul Seidler, Tobias J. Kippenberg
The availability of thin-film lithium niobate on insulator (LNOI) and advances in processing have led to the emergence of fully integrated LiNbO3 electro-optic devices, including low-voltage, high-speed modulators, electro-optic frequency combs, and microwave-optical transducers. Yet to date, LiNbO3 photonic integrated circuits (PICs) have mostly been fabricated using non-standard etching techniques that lack the reproducibility routinely achieved in silicon photonics. Widespread future application of thin-film LiNbO3 requires a reliable and scalable solution using standard processing and precise lithographic control. Here we demonstrate a heterogeneously integrated LiNbO3 photonic platform that overcomes the abovementioned challenges by employing wafer-scale bonding of thin-film LiNbO3 to planarized low-loss silicon nitride (Si3N4) photonic integrated circuits, a mature foundry-grade integrated photonic platform. The resulting devices combine the substantial Pockels effect of LiNbO3 with the scalability, high-yield, and complexity of the underlying Si3N4 PICs. Importantly, the platform maintains the low propagation loss (<0.1 dB/cm) and efficient fiber-to-chip coupling (<2.5 dB per facet) of the Si3N4 waveguides. We find that ten transitions between a mode confined in the Si3N4 PIC and the hybrid LiNbO$_3$ mode produce less than 0.8 dB additional loss, corresponding to a loss per transition not exceeding 0.1 dB. These nearly lossless adiabatic transitions thus link the low-loss passive Si3N4 photonic structures with electro-optic components. We demonstrate high-Q microresonators, optical splitters, electrically tunable photonic dimers, electro-optic frequency combs, and carrier-envelope phase detection of a femtosecond laser on the same platform, thus providing a reliable and foundry-ready solution to low-loss and complex LiNbO3 integrated photonic circuits.