Mid-infrared light sources serve as a key to accessing the “invisible world.” A broad range of applications, including gas detection, molecular spectroscopy, medical diagnostics, and free-space optical communications, rely on semiconductor lasers operating in the 2–5 μm wavelength range. For decades, high-performance lasers in this band have largely depended on GaSb-based material systems. However, their high costs, limited thermal management capabilities, and incompatibility with the established InP-based photonics platform have become major bottlenecks for the advancement of mid-infrared light sources. Although the InP-based semiconductor lasers offer lower cost and greater fabrication maturity, both InP-based quantum-well and quantum-dash structures face common challenges: high threshold current densities and restricted operating temperatures.
Recently, the research team led by Professor Huiyun Liu at University College London reported the first demonstration of InAs/InP quantum-dot lasers in the mid-infrared 2 μm band . The device incorporates a five-stack InAs/InP quantum-dot active region and achieves a low threshold current density of 118 A/cm² per layer at room temperature. This achievement not only represents a critical milestone in the development of low-cost, high-performance mid-infrared light sources but also establishes a viable path for deploying InAs/InP quantum-dots in mid-infrared optoelectronic applications. The study, entitled “ Mid-infrared InAs/InP quantum-dot lasers ,” is published in Light: Science & Applications . Yangqian Wang (PhD candidate, UCL), Dr. Hui Jia, and Dr. Jae-Seong Park are co-first authors, with Dr. Hui Jia and Dr. Jae-Seong Park serving as corresponding authors.
Mid-infrared quantum-dot lasers
Quantum-dot lasers utilize nanoscale “quantum dots” as the active region. These quantum dots are three-dimensionally confined nanostructures that can be considered “artificial atoms,” where carriers are confined in all spatial directions and form discrete energy levels. Compared with conventional quantum wells (2D confinement) and quantum dashes (1D confinement), quantum dots offer remarkable advantages: lower threshold current, enhanced temperature stability, broader gain bandwidth, and stronger defect tolerance. These properties provide greater flexibility for realizing high-performance devices on heterogeneous platforms such as silicon.
However, extending quantum-dot laser technology into the mid-infrared region (λ > 2 μm) has long remained challenging. In the InAs/InP material system, the lattice mismatch is only 3.2%, making it difficult to form high-density, uniform quantum dots. To achieve emission beyond 2 μm, the quantum dots must be enlarged, which increases the risk of introducing crystal defects. Meanwhile, indium adatoms on the InP surface exhibit strong anisotropic diffusion, making it easier to form elongated quantum-dash-like structures instead of compact quantum dots. These elongated structures provide weak carrier confinement, undermining the low threshold and high thermal stability traditionally associated with quantum-dot lasers.
Precise control of morphology in weakly strained systems
To address the “morphology instability” inherent in weakly strained systems, the UCL team analysed the diffusion behaviour of indium adatoms and proposed a carefully engineered strategy that includes:
1. Using As₂ instead of As 4 : Eliminating the process of cracking As 4 on the surface and providing stable As-terminated atomic steps along the [1 1 0] direction, fundamentally reducing diffusion anisotropy.
2. Controlling growth rate and temperature: Adopting a high growth rate combined with low-temperature epitaxy limits the diffusion length of indium adatoms, preventing their anisotropic migration.
3. Optimizing deposition conditions: Precisely regulating the InAs coverage and V/III ratio—specifically at 7.5 ML under optimized V/III conditions—produces a high-density, uniform, and dislocation-free quantum-dot ensemble (Fig. 1).
Five-stack InAs/InP quantum-dot lasers achieving room-temperature 2 μm lasing
Based on this precise control strategy, the team successfully developed a five-stack InAs/InP quantum-dot laser structure and achieved the first InP-based 2 μm mid-infrared quantum-dot lasing at room temperature . The device emits at 2.018 μm and exhibits a threshold current density of 118 A/cm² per layer , representing the lowest threshold reported to date for InP-based lasers operating in the 2–2.5 μm wavelength range (Fig. 2).
Summary and outlook
This work demonstrates that InAs/InP quantum dots provide a transformative mid-infrared gain medium, offering significantly lower power requirements compared with traditional quantum-well and quantum-dash lasers in the 2 μm band. Leveraging the mature InP platform, these results pave the way for a new class of low-cost, high-performance mid-infrared light sources using InAs quantum dots, opening the door to a broader family of mid-infrared quantum dot-based optoelectronic devices.
Light Science & Applications
Mid-infrared InAs/InP Quantum-Dot Lasers: Opening a New Era for Mid-Infrared Light Sources