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Photonic integrated circuits are widely used in applications such as telecommunications and data-centre interconnects1-5. However, in optical systems such as microwave synthesizers6, optical gyroscopes7 and atomic clocks8, photonic integrated circuits are still considered inferior solutions despite their advantages in size, weight, power consumption and cost. Such high-precision and highly coherent applications favour ultralow-noise laser sources to be integrated with other photonic components in a compact and robustly aligned format-that is, on a single chip-for photonic integrated circuits to replace bulk optics and fibres. There are two major issues preventing the realization of such envisioned photonic integrated circuits: the high phase noise of semiconductor lasers and the difficulty of integrating optical isolators directly on-chip. Here we challenge this convention by leveraging three-dimensional integration that results in ultralow-noise lasers with isolator-free operation for silicon photonics. Through multiple monolithic and heterogeneous processing sequences, direct on-chip integration of III-V gain medium and ultralow-loss silicon nitride waveguides with optical loss around 0.5 decibels per metre are demonstrated. Consequently, the demonstrated photonic integrated circuit enters a regime that gives rise to ultralow-noise lasers and microwave synthesizers without the need for optical isolators, owing to the ultrahigh-quality-factor cavity. Such photonic integrated circuits also offer superior scalability for complex functionalities and volume production, as well as improved stability and reliability over time. The three-dimensional integration on ultralow-loss photonic integrated circuits thus marks a critical step towards complex systems and networks on silicon.

Integrated photonics has profoundly affected a wide range of technologies underpinning modern society1-4. The ability to fabricate a complete optical system on a chip offers unrivalled scalability, weight, cost and power efficiency5,6. Over the last decade, the progression from pure III-V materials platforms to silicon photonics has significantly broadened the scope of integrated photonics, by combining integrated lasers with the high-volume, advanced fabrication capabilities of the commercial electronics industry7,8. Yet, despite remarkable manufacturing advantages, reliance on silicon-based waveguides currently limits the spectral window available to photonic integrated circuits (PICs). Here, we present a new generation of integrated photonics by directly uniting III-V materials with silicon nitride waveguides on Si wafers. Using this technology, we present a fully integrated PIC at photon energies greater than the bandgap of silicon, demonstrating essential photonic building blocks, including lasers, amplifiers, photodetectors, modulators and passives, all operating at submicrometre wavelengths. Using this platform, we achieve unprecedented coherence and tunability in an integrated laser at short wavelength. Furthermore, by making use of this higher photon energy, we demonstrate superb high-temperature performance and kHz-level fundamental linewidths at elevated temperatures. Given the many potential applications at short wavelengths, the success of this integration strategy unlocks a broad range of new integrated photonics applications.

Integrated-photonics microchips now enable a range of advancedfunctionalities for high-coherence applications such as data transmission,highly optimized physical sensors, and harnessing quantum states, but withcost, efficiency, and portability much beyond tabletop experiments. Throughhigh-volume semiconductor processing built around advanced materials thereexists an opportunity for integrated devices to impact applications cuttingacross disciplines of basic science and technology. Here we show how tosynthesize the absolute frequency of a lightwave signal, using integratedphotonics to implement lasers, system interconnects, and nonlinear frequencycomb generation. The laser frequency output of our synthesizer is programmed bya microwave clock across 4 THz near 1550 nm with 1 Hz resolution andtraceability to the SI second. This is accomplished with a heterogeneouslyintegrated III/V-Si tunable laser, which is guided by dualdissipative-Kerr-soliton frequency combs fabricated on silicon chips. Throughout-of-loop measurements of the phase-coherent, microwave-to-optical link, weverify that the fractional-frequency instability of the integrated photonicssynthesizer matches the $7.0*10^{-13}$ reference-clock instability for a 1second acquisition, and constrain any synthesis error to $7.7*10^{-15}$ whilestepping the synthesizer across the telecommunication C band. Any applicationof an optical frequency source would be enabled by the precision opticalsynthesis presented here. Building on the ubiquitous capability in themicrowave domain, our results demonstrate a first path to synthesis withintegrated photonics, leveraging low-cost, low-power, and compact features thatwill be critical for its widespread use.