Heterogeneous photonic integration of single-crystalline nanomembranes.

Despite the growing maturity of silicon (Si) and silicon nitride (SiN) for integrated photonics, a single material platform is unable to meet the diversifying requirements posed for photonic integrated circuits1,2. Essential optical functionalities can be attained by the heterogeneous integration of functional materials into established photonic platforms3,4. However, stringent lattice-matching constraints severely hinder the quality of heteroepitaxial material grown on dissimilar optical substrates5. Here we obviate this limitation by exploiting free-standing single-crystalline nanomembranes enabled by advanced epitaxy and layer lift-off techniques6-9. We present a versatile framework making use of photonic van der Waals (vdW) integration10 to infuse desired functionalities into Si and SiN photonics. By transferring single-crystalline thin-film barium titanate (BTO) to Si chips, we experimentally demonstrate ultraefficient electro-optical modulation, with a large Pockels coefficient r42 over 1,290 pm V-1 and a 3-dB electro-optical bandwidth of more than 23 GHz, enabled by well-defined BTO crystallographic orientation and quality. We also report ultracompact non-reciprocal magneto-optical isolators by vdW integration of cobalt ferrite (CFO) nanomembranes into Si microrings, with a Faraday rotation coefficient θF of 33,800° cm-1. To exemplify the capability of coalescing diverse functional materials into arbitrary photonic templates, we laterally stitch gallium arsenide (GaAs)11,12 and gallium nitride (GaN) single crystals13,14 on top of SiN photonics to realize expansive photodetection from ultraviolet to near-infrared wavelengths15. Multifunctional ring resonators for simultaneous electro-optical and magneto-optical modulations are also demonstrated by constructing vertical CFO/BTO heterostructures16,17, leading to new opportunities for advanced hetero-integrated optoelectronic applications and beyond.