The world is experiencing an unprecedented information explosion. The rapid development of high-performance computing (HPC), the Internet of Things (IoT), and artificial intelligence (AI) has introduced new demands for transmission bandwidth and information capacity. However, the bottleneck of integrated circuits is gradually emerging with the slowdown of Moore’s law. Compared with traditional integrated electric circuits, photonic circuits stand out due to their unique advantages such as low power consumption, high operating speed, and multi-lane processing capability. They are regarded as a key technology in the “post-Moore era.” Photonic integrated circuits (PICs), utilizing photons as the information carrier, have emerged as a crucial technology to overcome the communication capacity crunch in modern information society. High-quality optical materials and advanced integration strategies are essential cornerstones for photonic circuits. Silicon, as a dominant semiconductor material, is a popular photonic platform owing to its large refractive index and good compatibility with the CMOS processing procedure. However, silicon exhibits a relatively high propagation loss in the communication band and strong two-photon absorption (TPA) and free-carrier absorption (FCA) effects, hindering its further applications in large-scale integrated circuits and nonlinear photonics. In recent years, a variety of alternative materials have emerged, including silicon nitride, thin film lithium niobate (TFLN), aluminum nitride, silicon carbide, and chalcogenide glasses (ChGs). Key parameters of common photonic materials are summarized in Table 1. It can be seen that the refractive index of the ChGs can be flexibly tuned over a broad range. In addition, ChGs have been extensively used in optical signal processing due to their considerable photoelastic coefficients, low propagation loss, broad transparency window, and good compatibility with various material platforms. Achieving multifunctional PICs on a single chip has become a hotspot for researchers. However, no single material can fulfill all the requirements ranging from signal generation, modulation, transmission, to detection. Therefore, heterogeneous integration is considered the optimal approach for the future evolution of integrated photonics.

Progress In this paper, we review three applications of heterogeneous chalcogenide photonics based on the “ChGs+X” material platform: high-efficiency acousto-optic modulation, on-chip nonlinear parametric frequency conversion, and rare-earth ion-doped waveguide amplification (Fig. 1).

1) Acousto-optical modulation: Current commercial acousto-optic modulators (AOMs) are typically made from bulk piezoelectric crystal materials like tellurium dioxide (TeO₂) or lithium niobate, but their high power consumption and large volume limit their application in photonic circuits. With the rapid development of “ion cut” technology and the success of TFLN, on-chip acousto-optic modulators based on TFLN have been reported in recent years (Fig. 2). However, dry etching lithium niobate smoothly is challenging due to its chemical inertness. In addition, isolating the TFLN from the bottom SiO₂ substrate is difficult due to its fragility. To address these issues, heterogenous waveguide structures are designed to achieve high-efficiency on-chip AOMs by utilizing the soft chalcogenide waveguide loaded on the low-loss TFLN. This strategy enables the creation of high-efficiency acousto-optic modulators without the need for etching or suspending the TFLN (Figs. 3-4).

2) Parametric frequency conversion: The χ⁽²⁾-based nonlinear optical effect has been extensively studied. Various material platforms have been proposed to achieve efficient parametric frequency conversion, including lithium niobate, some Ⅲ-Ⅴ materials with intrinsic χ⁽²⁾ nonlinearity, as well as silicon and silicon nitride with externally induced χ⁽²⁾ properties. Among them, lithium niobate has been employed to achieve high-efficiency χ⁽²⁾ nonlinearity by dry etching and periodical domain engineering of lithium niobate. However, this fabrication process is complex and not compatible with the CMOS procedure. Recently, bound states in the continuum (BICs) have been suggested for obtaining second harmonic generation (SHG) via modal phase matching without the need for etching the TFLN. However, the conversion efficiency is low and not suitable for wideband applications. In our work, we propose a heterogeneous integration strategy by integrating chalcogenide strip waveguide with TFLN slab (Fig. 5). This approach has enabled the realization of on-chip high-efficiency SHG and observation of broadband parametric conversion efficiency via the effect of cascaded second-harmonic generation and difference-frequency generation (cSHG-DFG).

3) Optical waveguide amplification: Erbium-doped waveguide amplifiers (EDWAs) have become indispensable components in large-scale photonic circuits. To date, different material platforms and fabrication methods have been utilized to obtain efficient EDWAs such as erbium-doped Al₂O₃ via atomic layer deposition (ALD), erbium-doped TFLN, silicon nitride with erbium ion implantation, and rare-earth-doped chalcogenide films. However, the gain properties of ChGs-based waveguide amplifiers are lackluster for practical applications due to their intrinsically low solubility of rare-earth ions, low luminous efficiency of chalcogenide hosts, and increased etching complexity when introducing erbium ions into chalcogenide films. To address these challenges, we propose an efficient waveguide amplifier prototype without the need to dope the chalcogenide films directly (Fig. 8). The waveguide consists of a low-loss chalcogenide waveguide on a highly-doped erbium-doped Al₂O₃ thin film. This work facilitates the development of an efficient waveguide amplifier based on integrated chalcogenide photonics.

In summary, ChGs have emerged as promising candidates in PICs. Enhancing the functionalities of ChGs by adopting integrated “chalcogenide+X” heterogeneous platforms offers valuable insights for the future development of PICs in various research fields, including optical computing, optical memory, and integrated optical engines.