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 circuits^1,2. Essential optical functionalities can be attained by the heterogeneous integration of functional materials into established photonic platforms^3,4. 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 techniques^6-9. 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 θ_F 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 crystals^13,14 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 heterostructures^16,17, leading to new opportunities for advanced hetero-integrated optoelectronic applications and beyond.