The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS₂ on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.

单层过渡金属二硫族化物（TMD）的平衡和非平衡光学性质由强束缚激子决定。 TMD 中的激子弛豫动力学已通过时域光学光谱进行了广泛研究。然而，自由电子-空穴对非共振光激发后激子的形成动力学由于其固有的快速时间尺度而难以直接探测。在这里，我们使用极短的光脉冲非共振激发电子空穴等离子体，并通过诱导光吸收变化在 30 fs 的时间尺度上显示单层 MoS ₂中二维激子的形成。这些形成动力学明显快于传统的二维量子阱，并且归因于二维 TMD 中存在的强烈库仑相互作用。相干偏振的理论模型可以相干并弛豫为不相干激子群，重现了亚 100 飞秒时间尺度上的实验动力学，并揭示了最低能量激子如何产生的基本机制，这对于光电器件来说是最重要的应用，由更高能量的激发形成。重要的是，从较高能态到基激子态的声子介导的激子级联被发现是限速过程。这些结果设定了 TMD 中激子形成的最终时间尺度，并阐明了该过程背后的异常快速的物理机制。