Research progress of on-chip nonlinear mid-infrared lasers (invited)

Significance The mid-infrared (MIR) spectral range generally spans from 2 μm to 20 μm (500-5 000 cm⁻¹), which includes numerous vibrational absorption lines of various atoms and molecules, and encompasses several atmospheric windows. Therefore, MIR lasers hold significant application value and potential in the fields such as remote sensing communication, spectroscopic detection, medical applications, and military uses. Miniaturized on-chip integrated devices, which offer substantial advantages in size, power consumption, and large-scale production deployment, are particularly important. Thus, leveraging these characteristics to achieve miniaturized MIR photonic integrated devices is of outstanding significance. In recent years, MIR photonic integrated chips, which possess enormous potential in spectral measurement and biosensing, have become a research hotspot in integrated optics. Various MIR photonic integrated systems, which have been realized using materials such as silicon, germanium, indium phosphide (InP), and chalcogenide glass, demonstrate this potential. Among these systems, MIR light sources, which are crucial components, make the realization of on-chip integrated MIR lasers one of the pressing issues in MIR photonic integration.

Progress Despite recent significant breakthroughs in the direct generation of broadband MIR pulses in quantum cascade lasers (QCL), their performance remains incompatible with some high-power applications, and the spectral bandwidth of these QCL combs is still quite narrow. Therefore, nonlinear frequency conversion techniques, which can achieve ultra-broadband spectra and ultrashort pulse outputs, are considered one of the most promising solutions for realizing miniaturized MIR lasers aside from QCL. Based on the order of nonlinear effects, they can mainly be divided into second-order nonlinear and third-order nonlinear effects. Second-order nonlinear effects include optical parametric generation (OPG), optical parametric amplification (OPA), difference frequency generation (DFG), and optical parametric oscillation (OPO). Third-order nonlinear effects primarily include stimulated Raman-Scattering (SRS), four-wave mixing (FWM), optical frequency comb generation (FC), and supercontinuum generation (SCG). Based on this classification, this paper first introduces the research status of on-chip MIR lasers based on second-order nonlinear effects. It then elaborates on the research progress of four main third-order nonlinear on-chip MIR lasers, summarizing their latest research achievements.　　Based on second-order nonlinear effects, the waveguide materials for on-chip mid-infrared (MIR) lasers mainly include PPLN (periodically poled lithium niobate), OP-GaAs (orientation-patterned gallium arsenide), and ZGP (zinc germanium phosphide). In PPLN waveguides, on-chip gains exceeding 100 dB/cm have been achieved over a bandwidth of 600 nm centered at a wavelength of 2 μm, with experimental reports of OPO (optical parametric oscillation) at a repetition rate of 10 GHz. However, the narrow transparency range of PPLN limits its applications at longer wavelengths. OP-GaAs waveguides have demonstrated continuous output from 4 μm to 9 μm, with mid-infrared outputs extending up to 12 μm. While suitable for longer wavelengths, OP-GaAs waveguides require further efficiency optimization and involve complex manufacturing processes. ZGP waveguides have achieved low-threshold and high-efficiency mid-infrared generation covering a spectrum from 5 μm to 11 μm. However, due to two-photon absorption effects, they still require pump wavelengths greater than 2 μm. These different waveguide materials in on-chip mid-infrared laser research highlight their respective advantages and limitations, offering diverse options for achieving high-efficiency mid-infrared lasers suitable for broader applications.　　SRS (Stimulated Raman-Scattering) does not require phase matching and dispersion control, but its output wavelength is typically shorter, usually less than 5 μm. FWM (Four-Wave Mixing) requires precise phase matching and dispersion management, suitable for broadband conversion. Both FC (Frequency Comb) and SCG (Supercontinuum Generation) can generate broadband mid-infrared outputs, but FC can use continuous-wave pumping without relying on high-power ultrafast pulses as pump sources. Currently, research on on-chip nonlinear mid-infrared lasers mainly focuses on third-order nonlinear waveguide platforms based on materials such as silicon and germanium. These platforms often require complex and precise waveguide dispersion control and high-quality factor microresonators to achieve efficient mid-infrared laser outputs. Therefore, developing a simple and efficient on-chip mid-infrared laser generation based on nonlinear frequency conversion remains a pressing technological challenge.

Conclusions and Prospects The mid-infrared (MIR) spectral range finds critical applications in biomedical, environmental monitoring, and strong-field physics, among many other fields. Miniaturized and efficient MIR lasers have been a focal point in recent years, holding significant research significance for applications such as MIR spectroscopic detection and real-time environmental monitoring. While quantum cascade lasers (QCLs) can directly generate MIR outputs, they are limited by radiation bandwidth and mode-locking mechanisms. Therefore, nonlinear frequency conversion technologies remain the preferred approach for on-chip broadband ultrafast MIR pulse generation. On-chip MIR lasers based on nonlinear frequency conversion offer advantages such as compact structure, wide tunable spectral range, and high stability, leading to rapid development and widespread applications in recent years. This paper first introduces the current research status of on-chip MIR lasers based on second-order nonlinear frequency conversion. It then provides a comprehensive review of four main on-chip third-order nonlinear MIR lasers, including Stimulated Raman-Scattering (SRS), Four-Wave Mixing (FWM), Frequency Comb (FC), and Supercontinuum Generation (SCG). The emergence of mid-infrared waveguides based on ZGP (zinc germanium phosphide) birefringent phase-matching crystals provides a new approach for realizing on-chip mid-infrared nonlinear lasers. Currently, this approach relies on femtosecond laser direct writing to fabricate nonlinear waveguides, which still have significant limitations in roughness and flexibility. Professor Ya Cheng's research group at East China Normal University has developed a technique combining chemical mechanical polishing with femtosecond laser direct writing to fabricate high-quality lithium niobate microresonators. Additionally, Professor Yuanlin Zheng's research group at Shanghai Jiao Tong University has used semiconductor-compatible UV lithography and deep dry etching techniques to fabricate high-quality PPLN (periodically poled lithium niobate) waveguides. These advancements provide new pathways to enhance the fabrication processes of birefringent phase-matching mid-infrared nonlinear on-chip lasers. It is expected that utilizing various birefringent phase-matching nonlinear crystal systems will enable the realization of diverse functional mid-infrared nonlinear on-chip devices.