Low-Loss Thin Film Lithium Niobate Bonded on Silicon Nitride Waveguides

We demonstrate heterogeneous integration of thin film LiNbO<inf>3</inf> on Si<inf>3</inf>N<inf>4</inf> photonic integrated circuits with SiO<inf>2</inf>, Al<inf>2</inf>O<inf>3</inf> and direct bonding. We report record low losses in bonded ring resonators of 0.4 dB/cm (intrinsic Q = 8.19×10<sup>5</sup>).