Optoelectronics could be used to develop fast and wideband information systems. However, the large frequency mismatch between optically synthesized signals and electronic clocks makes it difficult to synchronize optoelectronic systems. We describe an on-chip microcomb that can synthesize single-frequency and wideband signals covering a broad frequency band (from megahertz to hundreds of gigahertz) and that can provide reference clocks for the electronics in the system. Our synchronization strategy, which aligns optically synthesized signals and electronics, can provide signal manipulation precision and data transmission without coherent digital signal processing. To illustrate the capabilities of this approach, we create a wireless joint sensing and communication system based on a shared microcomb-based transmitter. An on-chip microcomb that provides reference clocks for all synthesized signals and electronics within the system can be used to unite time–frequency references in optoelectronics.

Soliton microcombs generated in nonlinear microresonators facilitate the photonic integration of timing, frequency synthesis, and astronomical calibration functionalities. For these applications, low-repetition-rate soliton microcombs are essential as they establish a coherent link between optical and microwave signals. However, the required pump power typically scales with the inverse of the repetition rate, and the device footprint scales with the inverse of square of the repetition rate, rendering low-repetition-rate soliton microcombs challenging to integrate within photonic circuits. This study designs and fabricates silicon nitride microresonators on 4-inch wafers with highly compact form factors. The resonator geometries are engineered from ring to finger and spiral shapes to enhance integration density while attaining quality factors over 10^7. Driven directly by an integrated laser, soliton microcombs with repetition rates below 10 GHz are demonstrated via turnkey initiation. The phase noise performance of the synthesized microwave signals reaches -130 dBc/Hz at 100 kHz offset frequency for 10 GHz carrier frequencies. This work enables the high-density integration of soliton microcombs for chip-based microwave photonics and spectroscopy applications.

Standing-wave supermode microresonators that are created through the strong coupling between counter-propagating modes have emerged as versatile platforms for sensing and nonlinear optics. For example, these microresonators have shown potential in nanoparticle sizing and counting, as well as enhancing the single-photon optomechanical coupling rate of stimulated Brillouin scattering. However, it has been observed that the relation between the mode linewidth and on-resonance transmission of the split supermodes differs obviously from that of the non-split modes. This behavior is typically quantified by the coupling ideality (I), which remains inadequately explored for the standing-wave supermodes. In this study, we theoretically and experimentally investigate the coupling ideality of standing-wave supermodes in a commonly employed configuration involving a SiO2 microresonator side-coupled to a tapered fiber. Our findings demonstrate that, even with a single-mode tapered fiber, the coupling ideality of the standing-wave supermodes is limited to 0.5, due to the strong backscattering-induced energy loss into the counter-propagating direction, resulting in an additional equivalent parasitic loss. While achieving a coupling ideality of 0.5 presents challenges for reaching over-coupled regimes, it offers a convenient approach for adjusting the total linewidth of the modes while maintaining critically-coupled conditions.

Synergistic control of the frequency and orbital angular momentum (OAM) of light offers new opportunities for the generation of spatio-temporal optical waveforms and optical metrology. However, their physical realizations are typically bulky and complex owing to challenges in creating, manipulating and detecting mutually coherent, high-dimensional OAM states. Here we achieve combined control over the frequency and the OAM of a comb structure on a photonic chip. Dissipative optical solitons are formed in a nonlinear ring microresonator and emitted owing to engraved angular gratings, with each comb line carrying a distinct OAM. The beam of such a vortex soliton microcomb manifests dynamically revolving, double-helical intensity profiles. The one-to-one correspondence between the OAM and frequencies features a high extinction ratio of over 18.5 dB, enabling precision spectroscopy of optical vortices. Our work provides an integrated solution for realizing coherent light sources that are multiplexed in the spatial and frequency domains, with the potential to establish a new approach to the generation of high-dimensional structured light. Nonlinear microring resonators can generate a vortex soliton microcomb, that is, a frequency comb with each comb line carrying a distinct orbital angular momentum.

We have designed and fabricated a supermode microresonstor to enhance Brillouin optomechanical coupling, and realized both phonon lasing and optomechanical strong coupling. This system exhibits a single-photon optomechanical coupling rate as high as 12.5 kHz.

Silicon nitride (Si3N4) photonic integrated circuits are rapidly developing in recent decades. The low loss of Si3N4 attracts significant attention and facilitates a wide range of applications in integrated photonics. In this work, we demonstrate the foundry fabrication of a 300-nm-thick 8-in. wafer-scale Si3N4 platform, with a microresonator intrinsic quality factor of up to 15×106, corresponding to an ultralow loss of 2.2 dB/m. Leveraging this platform, we develop a mature process design kit, achieving a single-mode waveguide propagation loss of less than 5 dB/m, an edge coupler loss of 1.3 dB, and an insertion loss of 0.07 dB for multimode interference couplers. Utilizing the processed Si3N4 chip, we realize a hybrid integrated tunable external cavity laser with a tuning range from 1534 to 1602 nm, a record-high side-mode suppression ratio of up to 76 dB, an optical power of 26 mW, and an intrinsic linewidth of down to 314 Hz. Our work lays a solid foundation for the further development of applications, including nonlinear optics, quantum optics, optical communications, and ranging.

Many key functionalities of optical frequency combs such as self-referencing and broad spectral access rely on coherent supercontinuum generation (SCG). While nanophotonic waveguides have emerged as a compact and power-efficient platform for SCG, their geometric degrees of freedom have not been fully utilized due to the underlying nonlinear and stochastic physics. Here, we introduce inverse design to unlock free-form waveguides for coherent SCG. The efficacy of our design is numerically and experimentally demonstrated on Si3N4 waveguides, producing flat and coherent spectra from visible to mid-infrared wavelengths. Our work has direct applications in developing chip-based broadband light sources for spectroscopy, metrology, and sensing across multiple spectral regimes.

Colloidal nanoplatelets (NPLs), a class of semi-conductor nanocrystals, have attracted considerable attention as a promising gain material for their ultralow amplified spontaneous emission (ASE) and lasing thresholds. However, there exist spectral gaps, especially in the green-color range, that NPLs cannot fully cover. The recently developed CdSe/CdSeS core/ alloyed-crown NPLs with excellent tunability across the green-color range offer the possibility to remedy this deficiency. Here, the ASE and lasing characteristics of this new type of NPL are investigated. A remarkably low ASE threshold of 16 mu J/cm2 at 522 nm is measured, the lowest among core/crown NPLs. Microlasers are fabricated by spin-coating them on second-order distributed feedback (DFB) cavities developed in silicon nitride (SiN) substrates. The microlasers exhibit an ultralow lasing threshold of 9 mu J/cm2 at 522 nm. Moreover, they can cover a spectral range of 505-535 nm with all clean single-mode emissions. Picosecond time-and spectral-resolved photoluminescence (PL) spectroscopy reveals that the gain band is determined by the biexciton emission bandwidth. The vigorous development of NPLs with low lasing thresholds in a broad spectral range will greatly facilitate the realization of nanocrystal-based lasers.

Microcombs are revolutionizing optoelectronics by providing parallel, mutually coherent wavelength channels for time-frequency metrology and information processing. To implement this essential function in integrated photonic systems, it is desirable to drive microcombs directly with an on-chip laser in a simple and flexible way. However, two major difficulties have prevented this goal: (1) generating mode-locked comb states usually requires a significant amount of pump power and (2) the requirement to align laser and resonator frequency significantly complicates operation and limits the tunability of the comb lines. Here, we address these problems by using microresonators on an AlGaAs on-insulator platform to generate dark-pulse microcombs. This highly nonlinear platform dramatically relaxes fabrication requirements and leads to a record-low pump power of <1 mW for coherent comb generation. Dark-pulse microcombs facilitated by thermally controlled avoided mode crossings are accessed by direct distributed feedback laser pumping. Without any feedback or control circuitries, the comb shows good coherence and stability. With around 150 mW on-chip power, this approach also leads to an unprecedentedly wide tuning range of over one free spectral range (97.5 GHz). Our work provides a route to realize power-efficient, simple, and reconfigurable microcombs that can be seamlessly integrated with a wide range of photonic systems.

Quantum fluctuations disrupt the cyclic motions of dissipative Kerr solitons (DKSs) in nonlinear optical microresonators and consequently cause timing jitter of the emitted pulse trains. This problem is translated to the performance of several applications that employ DKSs as compact frequency comb sources. Recently, device manufacturing and noise reduction technologies have advanced to unveil the quantum properties of DKSs. Here we investigate the quantum decoherence of DKSs existing in normal-dispersion microresonators known as dark pulses. By virtue of the very large material nonlinearity, we directly observe the quantum decoherence of dark pulses in an AlGaAs-on-insulator microresonator, and the underlying dynamical processes are resolved by injecting stochastic photons into the microresonators. Moreover, phase correlation measurements show that the uniformity of comb spacing of quantum-limited dark pulses is better than 1.2 × 10 −16 and 2.5 × 10 −13 when normalized to the optical carrier frequencies and repetition frequencies, respectively. Comparing DKSs generated in different material platforms explicitly confirms the advantages of dark pulses over bright solitons in terms of quantum-limited coherence. Our work establishes a critical performance assessment of DKSs, providing guidelines for coherence engineering of chip-scale optical frequency combs.

We correlate the dimensions of frequency and orbital angular momentum (OAM) by forming soliton microcombs in an integrated microresonator vortex emitter. One-to-one correspondence between the OAM and frequency is confirmed on 14 distinct comb lines.

We investigate the fundamental timing jitter of dark pulse microcombs generated in an AlGaAs microresonator, where the quantum-limited frequency noise of the 91-GHz repetition rate is measured to be 0.5 Hz 2 /Hz.

We demonstrate coherent microcomb generation based on an AlGaAs-on-insulator microresonator operating at sub-milliwatt pump power regime, that can be directly driven by on-chip laser.