Silicon photonics based on silicon-on-insulator (SOI) substrates has been rapidly growing over the past two decades, providing various on-chip passive/active photonic functionalities (waveguide-based passive components, modulators, photodetectors, etc.) by taking advantage of mature planar CMOS technology, leading to the development of Si photonic integrated circuits (PICs) [1–4]. Moreover, III–V-on-Si (III-V/Si) heterogeneous integration techniques have been developed to address the laser source issue for Si photonics and have now been adopted in scalable state-of-the-art CMOS-compatible processes [5]. Today, Si photonics is playing a leading role in the community of integrated photonics and has found applications in a large number of areas including optical interconnects [6,7], telecommunications [8], computing [9], and so on [10]. Meanwhile, significant progress spanning the last decade on integrated nonlinear photonics has emerged as a new paradigm for both nonlinear optics research and applications. An intriguing offering of integrated nonlinear photonics is its capability of generating new classes of coherent, ultra-broadband light sources (i.e., microcombs) in nonlinear waveguides [11,12], which is not attainable from linear photonics systems. Microcombs have triggered widespread use of chip-scale nonlinear devices in applications [13] including ultrahigh-capacity coherent telecommunications [14,15], optical frequency synthesis [16], optical atomic clocks [17], quantum optics [18], etc. In the past few years, significant technological advances have enabled ultralow-loss nonlinear waveguides and ultrahigh-quality-factor (Q-factor) microresonators [19–23]. Those breakthroughs have essentially allowed high-efficiency nonlinear processes operating at dramatically reduced power levels of milliwatts or sub-milliwatts [19,21,24–28], eliminating bulky optical equipment such as benchtop pump lasers and amplifiers. The power requirements in nonlinear waveguides are now compatible with co-integrated III-V/Si pump lasers on a single chip [29], which represents a key step towards harnessing the on-chip nonlinear properties from individual and passive-only nonlinear devices, to mass-manufactured system-level architectures and application developments.