Significance Photonic integrated circuits (PICs), which manipulate photons on chips, have revolutionized modern information society in both science and applications. Endowed with intrinsic low power consumption, high-speed data transmission, and high-volume communication, integrated photonics has transitioned from academic research in laboratories to industrial deployment in data centers and optical communications. Leveraging the plentiful properties of light such as frequency, polarization, and amplitude, and combining interactions between light and materials, including various nonlinear effects, the photoelectric effect, and the photothermal effect, integrated photonics has produced multiple chip-scale functional devices for spectroscopy, positioning-navigation-timing (PNT), quantum information and optical computation. Compared to other integrated materials, such as silicon, lithium nitride, and various III-V materials, silicon nitride (Si3N4) features comprehensive advantages like a wide transparency window from violet to mid-infrared, high power handling capability, large Kerr nonlinearity, and ultralow linear and nonlinear losses. These properties make it a leading material for Kerr nonlinear integrated photonics and offer the potential for applications beyond traditional materials. Progress Over the past several decades, integrated photonics has evolved into a mature technology that enables the synthesis, processing, and detection of optical signals using PIC. Dating back to the 1980s, silicon-on-insulator (SOI) wafers, initially used for microelectronic circuits, were proposed for photonic circuits—an optical analog of silicon microelectronics that combines photonics and integration. Since then, silicon photonics has developed rapidly and extensively. Today, with heterogeneous and hybrid integration, silicon photonics has become a mature technology used in telecommunication and data centers to process high-data-rate optical signals based on small photonic chips. These chips can be manufactured in high volumes at low cost in well-established CMOS foundries. Despite these major advances, silicon has intrinsic material limitations such as two-photon absorption in the telecommunication bands, which precludes high power handling for nonlinear photonic applications. In the past decade, numerous material platforms have emerged to complement or even replace silicon in specific cases. Among these platforms, Si3N4 has become the leading platform for ultralow-loss integrated photonics. Silicon nitride has a long history of being used as a CMOS material for diffusion barriers, etch masks, and stressor layers in microelectronics. Already in 1987, Si3N4 was proposed and fabricated for low-loss integrated photonics. Its refractive index (n0=2) enables strip waveguides of tight optical confinement using silicon dioxide (SiO2) cladding. Compared with silicon, the smaller difference in refractive indices between the Si3N4 waveguide core and SiO2 cladding reduces scattering losses induced by interface roughness and facilitates fiber-chip interface coupling with reduced mode mismatch. Amorphous Si3N4 has a wide transparency window from visible to midinfrared and a large bandgap of 5 eV, making Si3N4 immune to two-photon absorption in the telecommunication band around 1550 nm, compared to 1.3 eV/1.1 eV/8.9 eV for indium phosphide (InP)/silicon/silicon dioxide (SiO2). In addition, Si3N4 exhibits a dominant Kerr nonlinearity nearly an order of magnitude larger than that of SiO2, with negligible second-order, Raman and Brillouin nonlinearities. These features make Si3N4 an excellent platform for linear and Kerr nonlinear photonics that rely on ultralow optical loss. Frequency comb generation based on Si3N4 Kerr nonlinearity was demonstrated in 2010 with an optical loss of 0.5 dB/cm in thick waveguides. Optical loss below 0.1 dB/m in thin-film planar Si3N4 waveguide and a quality factor higher than 8×107 in thin-film Si3N4 microresonators were demonstrated in 2011 and 2014 respectively. In 2016, the photonic Damascene process was introduced to fabricate thick and high-quality Si3N4 microresonators, demonstrating high-coherent dissipative Kerr solitons (soliton microcombs). Benefiting from the ultra-low loss, battery-driven soliton microcombs were demonstrated in 2018. In 2021/2023, high-yield, wafer-scale fabrication of ultralow-loss, dispersion-engineered silicon nitride photonic circuits was demonstrated using the Damascene process/subtractive process. Recent years have seen progress in integrating Si3N4 with other materials, such as heterogeneous lasers in 2020, Hz-level lasers in 2021, and heterogeneous laser solitons in 2021. Beyond Kerr nonlinearity, multiple nonlinear effects like Raman scattering, Brillion scattering and photogalvanic effect have been observed and utilized in Si3N4. Future applications based on Si3N4 nonlinearities are promising, including photonic microwave generation, astrocomb, continuous light amplification, chip-scale atomic clock lasers, optical computing, and LiDAR (Fig. 8). Conclusions and Prospects In summary, we review the properties and recent progress of ultralow-loss Si3N4 nonlinear integrated photonics. We highlight multiple key results based on the plentiful nonlinearities in Si3N4 for soliton microcomb formation, supercontinuum generation, optical parametric amplification, laser linewidth narrowing, second harmonic generation and more. These advancements in Si3N4 nonlinear integrated photonics are due to significant progress in CMOS fabrication of Si3N4 PIC featuring ultralow-loss, tight-confinement, and the flexibility of dispersion engineering. Active functions are endowed to passive Si3N4 via monolithic, heterogeneous, and hybrid integration. The hybrid, ultralow-loss, Si3N4 nonlinear integrated photonics have already fulfilled numerous promises. Since optical loss in Si3N4 is still dominated by scattering losses, efforts to optimize the fabrication process will continue, potentially leading to nearly an order of reduction in loss over the next decade. So far, demonstrated results on photonic integration with Si3N4 have been limited to one element (lasers, amplifiers, or photodetectors) or one other material (AlN or LiNbO3). As more materials are integrated, future efforts should focus on combining more elements or materials for more sophisticated applications. In this context, micro-transfer printing can be a powerful tool, as elements are fabricated discretely but assembled at wafer scale. Furthermore, by combining electronics, MEMS devices, and hybrid Si3N4 photonics, we can envisage compact, portable, energy-efficient, packaged devices for science and applications. Overall, we are confident in the continuous advancement in design, fabrication, and system architectures of hybrid Si3N4 photonics over the next decade, leading to extensive applications in spectroscopy, metrology, sensing, telecommunications, microwave, quantum science, and photonic computing. © 2024 Chinese Optical Society. All rights reserved.
