Integrated photonics enables the synthesis, processing, and detection of optical signals using integrated photonics. It has rapidly transformed numerous fields and applications by enabling faster and more efficient ways to process and transmit information, as well as new techniques for telecommunications, sensing, medical diagnosis, and quantum computing, just to name a few. With its intrinsic high speed, large bandwidth, and unlimited parallelism, integrated photonics plays a critical role in handling high-throughput, data-intensive applications. Over the past few decades, the successful translation from laboratory research to commercial deployment has established integrated photonics as a standard technology widely used in high-data-rate telecommunications and datacenters. The development of heterogeneous integration for various material platforms holds the potential to dramatically improve the performance, cost-effectiveness, and scalability of future optical systems. As such, this special issue features a collection of eight invited papers in the areas of emerging integrated photonics on different materials platforms, covering the latest research and technology development in integrated photonics from device-level innovations to system integration and packaging.

The fast advancement of artificial intelligence is calling for optical interconnects that are dramatically more compact, energy efficient, and faster than traditional optical transceivers. Here, leveraging the power of integrated silicon photonics, Netherton et al. provide a glimpse into future optical interconnect solutions via large-scale silicon photonic integrated circuits [1]. Combining 300-mm silicon photonic components, quantum-dot mode-locked lasers as 20-channel comb source, as well as advanced photonic-electronic integration and packaging technologies, the system architecture could support data transmission at 1 Tbps with sub-pJ/bit energy consumption.

Programmability is a critical characteristic of integrated photonics to enable reconfigurable photonic information processing to handle data-intensive applications. Critical to programmability is the design, fabrication, and integration of a variety of tuning, actuation, and modulation components. Among different approaches, thermal-optic tuners based on metallic heaters are most widely used. Lin et al. have demonstrated reconfigurable six-dimensional linear transformations including cyclic transformations and arbitrary unitary matrices on a novel CMOS-compatible silicon nitride platform, having accomplished an order of magnitude enhancement in power efficiency compared to conventional devices [2]. On the other hand, the silicon nitride platform also features strong optical nonlinearity, enabling nonlinear optical processing to empower integrated photonics with a new degree of freedom to operate and analyze information. Vitali et al. have explored a silicon-rich silicon nitride platform and demonstrated wavelength conversion based on Bragg scattering intermodal four-wave mixing [3]. Phase matching among distinct spatial modes over a large spectral bandwidth has been engineered, allowing for mode conversion, multiplexing and de-multiplexing in different telecommunication bands (separated by 60 nm) with 3 dB bandwidth exceeding 70 nm.

The emergence of diverse material platforms beyond silicon has motivated the development of heterogeneous integration, a powerful technique enabling seamless combination of a series of active materials and components on a monolithic substrate, leading to richer functionalities on-a-chip through the merging and synergy of individual merits from each material. Billent et al. have demonstrated a III–V-on-silicon-nitride mode-locked laser, leveraging micro-transfer printing of a semiconductor optical amplifier on a passive silicon-nitride cavity [4]. With comprehensive investigation of gain voltage, saturable absorber current, and mode overlap with the gain region, the electrically driven lasers can operate stably in the mode-locked regime, featuring an optical spectrum of 23 nm wide in the telecommunication band, while maintaining a defined 10 dB bandwidth for a pulse repetition rate of 3 GHz. Moreover, leveraging ultrafast interaction between the laser gain and external high-Q microcavity, Shen et al. overcome a previous issue of random Rayleigh backscattering and present a practical approach to achieving reliable reflection for laser self-injection locking [5]. The key is to add a Sagnac loop into the external microcavity, enabling strong coupling between the counter-propagating mode, and consequently, robust reflection for each resonance over a broad bandwidth. The work has established an important advancement in design and architecture for narrow-linewidth, mode-locked comb lasers that can be used for field-deployable applications. Leveraging the inherent electrically controlled activity of III–V semiconductors, Cheng et al. present quantum-well-light-emitting transistors (QW-LETs), demonstrating electro-optical sequential logic circuits—set-reset (SR) latches—that enable precise logic operations [6]. These electro-optical SR latches can serve as foundational elements for advanced optoelectronic field-programmable gate arrays, highlighting the versatility of QW-LETs in diverse optoelectronic applications.

Advancing new material platforms beyond the well-established semiconductors is always a key theme for pushing performance boundaries of integrated photonics in various aspects. Zhu et al. have developed nanofabrication techniques to produce microresonators with a record-high Q of twenty-nine million in the thin-film lithium niobate platform [7], which offers unique electro-optic and nonlinear optical properties not commonly available in Si-based platforms. This work features on-chip signal transmission with an ultralow propagation loss of 1.3 dB per meter, potentially paving the way for applications in microwave photonics, quantum computation, and nonlinear optics that were previously unattainable.

As the field of integrated photonics expands to more and more material platforms, each with their distinct advantages, it is becoming increasingly urgent to efficiently interface between different photonic chiplets. Glass waveguides are excellent candidates for such interposer chips as the waveguide geometry can be conveniently controlled and tapered to align different platforms in an arbitrary manner in a three-dimensional space. Here, Kondratyev et al. have developed a programmable eight-port interferometer based on femtosecond laser written glass waveguides, demonstrating the capability of error-tolerant operation within the 920–980 nm wavelength window [8], holding the potential for future quantum and classical information processing applications.