With the acceleration of global informatization, the volume of communication data has been growing exponentially, which puts forward higher requirements for the characteristics of high speed, large capacity, and low cost of information networks. However, the bottleneck of integrated circuits is gradually emerging with the slowdown of Moore’s law. Compared with electronic integrated circuits, photonic integrated circuits (PICs) can significantly reduce the system’s size, weight, operating power, and cost (SWaP-c). A PIC is a miniaturized platform that monolithically combines diverse optical functionalities, including photodetection, signal modulation, spectral filtering, optical routing, and nonlinear optical processes, within a precisely fabricated chip-scale architecture. Its submicron-scale waveguide structures enable deterministic control of light propagation characteristics, while standardized semiconductor manufacturing processes enable cost-effective mass production. While silicon maintains near-total dominance in microelectronics fabrication, PICs demonstrate the versatility of material systems through heterogeneous integration. This is mainly because different material platforms offer various advantages and disadvantages, and therefore, there is no single dominant material platform in the field of integrated photonics. Heterogeneous integration technology can make full use of the advantages brought by multiple materials and structures to realize “complementary material advantages” and “synergistic enhancement of functions”, which can further improve the performance of the device and expand its functions, making it useful in optical communications, computing, lidar, microwave photonics, and other applications. Research in this field not only advances fundamental science but also supports the development of related industries, paving the way for next-generation communication technologies and high-performance computing devices.

In this paper, we focus on an overview of commonly used materials, integration techniques, and application examples in heterogeneous integrated photonics for multi-material systems. In the first section of the review, we briefly describe current bottlenecks in conventional single-material system photonic integration technology. Then, to solve this, heterogeneous integrated photonics technology based on multi-material systems has emerged. To better understand the properties of various photonic integrated materials, we summarize their physicochemical properties as well as material characteristics in the second section, which includes group Ⅳ materials (Si, Ge), group Ⅲ-Ⅴ compound semiconductors, silicon nitride, lithium niobate, two-dimensional materials, and phase change materials. The relevant material parameters are summarized in Table 1. In the third section, we present four heterogeneous integration technology tools, namely inter-chip hybrid integration, wafer bonding, micro-transfer printing, and monolithic integration. The process flow diagram is shown in Fig. 5. The comparison of the four technologies is presented in Table 2. The focus of this review is on the fourth section. During the last decade, heterogeneous integrated photonics for multi-material systems has been applied in various types of photonic devices: 1) Waveguide and passive devices: On-chip optical waveguides and passive components are among the most important components of devices and systems in integrated optics. They are used for device interconnections and information multiplexing and processing, and the overall system performance greatly depends on their basic characteristics. Some recent research on on-chip heterogeneous integrated optical waveguides and passive devices is summarized in Figs. 9 and 10. Heterogeneous integration reduces the complexity of the etching process while expanding the functionality of optical waveguides and passive devices. 2) On-chip lasers: Materials such as silicon, silicon nitride, and lithium niobate lack efficient light sources due to their indirect bandgap. Therefore, to realize integrated on-chip lasers, it is often necessary to introduce other semiconductor materials with a direct bandgap, such as Ⅲ-Ⅴ compound semiconductors, as a gain medium. Several schemes for integrated lasers have been demonstrated, including hybrid integration based on advanced packaging, heterogeneous integration based on wafer bonding, micro-transfer printing, and monolithic integration (Fig. 11). 3) On-chip electro-optic modulators: Electro-optic (EO) modulators play a crucial role in converting high-speed signals from the electrical domain to the optical domain, serving as essential components in long-haul optical communication, microwave photonics, and lidar. By introducing heterogeneous materials with various advantageous properties, the heterogeneous integration approach is worth considering for improving the performance of conventional modulators. For example, integrating germanium or two-dimensional materials on silicon-on-insulator (SOI) platforms enables modulators with high bandwidth [Fig. 12(a)], high linearity [Fig. 12(b)], and low power consumption by utilizing the electro-absorption effect. Pockels modulators, which change the refractive index of the waveguide through an applied voltage, are more suitable for signal modulation in higher-order modulation formats. Lithium niobate wafers can be bonded to planarized optical waveguides via wafer bonding, achieving excellent performance with an EO bandwidth greater than 110 GHz and a half-wave voltage of 3.1 V·cm [Fig. 12(e)]. 4) On-chip photodetectors: An on-chip photodetector (PD) plays a pivotal role in PICs by converting optical signals into electrical ones. The indirect bandgaps of silicon (1.12 eV), silicon nitride (5 eV), and lithium niobate (3.9 eV) are not sufficient for direct absorption and detection in the near- and mid-infrared wavelength ranges. To address this issue, various alternative materials such as germanium, Ⅲ-Ⅴ compound semiconductors, and two-dimensional materials (Fig. 13) are introduced via heterogeneous integration to enable effective absorption at the desired wavelengths. 5) On-chip photonic integrated systems: Optical phased arrays (OPA), microwave photonic systems (MWP), optical frequency combs (OFCs), optical neural networks (ONN), and other function-specific photonic devices or systems can be classified as on-chip photonic integrated systems. These systems are realized by integrating multiple optical functional components (e.g., lasers, modulators, waveguides, passive devices, detectors, etc.) onto a single chip. Heterogeneous integration promotes the evolution of photonic integrated systems toward multifunctionality, high integration, wide bandwidth, and low cost.

In summary, compared to existing single-material photonic integration technology, heterogeneous integration based on multi-material systems can significantly improve device performance and broaden system functionality by making full use of the advantages of different materials. However, several critical issues remain to be addressed. Future development of heterogeneous integrated photonics based on multi-material systems will require careful trade-offs between fabrication cost, device performance, process compatibility, and device size. We believe that multi-material heterogeneous integrated photonics is poised to drive rapid advancements in optical communication, computing, microwave photonics, and lidar.