Acousto-Optic Devices in Hybrid Lithium Niobate on Silicon Nitride Platform

Silicon nitride is a major material platform for photonic integrated circuits. The platform is, however, strictly passive. The hybrid integration of active layers can introduce functionalities which are absent in silicon nitride alone. In this work, we report acousto-optic modulator devices in a commercial hybrid material platform of thin-film lithium niobate on top of silicon nitride. Acoustic waves are launched through piezoelectric actuation in the lithium niobate film and induce photoelastic modulation of guided light in an underlying silicon nitride resonator. The strongest modulation is obtained at 413 MHz frequency. The magnitude of photoelastic modulation is 8.8 × 10^-6 refractive index units per Volt, and the modulation depth of the output power reaches 68%. The efficiency of acousto-optic modulation is orders of magnitude higher than in previously reported thermo-optic actuation devices in standalone silicon nitride. Unlike electro-optic modulation devices in similar hybrid layer stacks, no evanescent overlap is required between the guided optical mode and the lithium niobate film. Constraints on the layer stack and the passive photonic circuits are relaxed accordingly.