Small semiconductor lasers that can be integrated on a chip are essential for a wide range of optical applications, including optical computing, communication and sensing. Practical laser applications have only been developed with direct-bandgap materials because of a general belief that lasing action from indirect-bandgap materials is almost impossible. Here we report unexpected indirect-bandgap transition lasing in an ultra-thin WS₂ disk. We demonstrate that a 50-nm-thick WS₂ disk offers efficient optical gain and whispering gallery modes that are sufficient for lasing action. As a result, the WS₂ disk exhibits indirect transition lasing, even under continuous-wave excitation at room temperature. Our experimental results are in close agreement with theoretical modelling for phonon-assisted photon lasing. The results derived from external cavity-free ultra-thin WS₂ layers offer a new direction for van-der-Waals-material-based nanophotonics and introduce the possibility for optical devices based on indirect-bandgap materials.

可以集成在芯片上的小型半导体激光器对于广泛的光学应用至关重要，包括光学计算、通信和传感。由于普遍认为间接带隙材料的激光作用几乎是不可能的，因此仅使用直接带隙材料开发了实际的激光应用。在这里，我们报告了超薄 WS ₂盘中意外的间接带隙跃迁激光。我们证明了 50 nm 厚的 WS ₂磁盘提供了有效的光学增益和回音壁模式，足以产生激光作用。因此，WS ₂即使在室温下连续波激发下，圆盘也表现出间接跃迁激光。我们的实验结果与声子辅助光子激光的理论模型非常一致。来自无外腔超薄WS ₂层的结果为基于范德华材料的纳米光子学提供了新的方向，并为基于间接带隙材料的光学器件引入了可能性。