Cha Soonyoung, Sung Ji Ho, Sim Sangwan, Park Jun, Heo Hoseok, Jo Moon-Ho, Choi Hyunyong
School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea.
Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea.
Nat Commun. 2016 Feb 25;7:10768. doi: 10.1038/ncomms10768.
The 1s exciton--the ground state of a bound electron-hole pair--is central to understanding the photoresponse of monolayer transition metal dichalcogenides. Above the 1s exciton, recent visible and near-infrared investigations have revealed that the excited excitons are much richer, exhibiting a series of Rydberg-like states. A natural question is then how the internal excitonic transitions are interrelated on photoexcitation. Accessing these intraexcitonic transitions, however, demands a fundamentally different experimental tool capable of probing optical transitions from 1s 'bright' to np 'dark' states. Here we employ ultrafast mid-infrared spectroscopy to explore the 1s intraexcitonic transitions in monolayer MoS2. We observed twofold 1s→3p intraexcitonic transitions within the A and B excitons and 1s→2p transition between the A and B excitons. Our results revealed that it takes about 0.7 ps for the 1s A exciton to reach quasi-equilibrium; a characteristic time that is associated with a rapid population transfer from the 1s B exciton, providing rich characteristics of many-body exciton dynamics in two-dimensional materials.
1s 激子（束缚电子 - 空穴对的基态）对于理解单层过渡金属二硫属化物的光响应至关重要。在 1s 激子之上，近期的可见光和近红外研究表明，激发态激子要丰富得多，呈现出一系列类里德堡态。随之而来的一个自然问题是，光激发时内部激子跃迁是如何相互关联的。然而，要研究这些激子内跃迁，需要一种根本不同的实验工具，能够探测从 1s “亮”态到 np “暗”态的光学跃迁。在此，我们利用超快中红外光谱来探究单层 MoS₂ 中的 1s 激子内跃迁。我们观测到在 A 激子和 B 激子内存在双重的 1s→3p 激子内跃迁，以及 A 激子和 B 激子之间的 1s→2p 跃迁。我们的结果表明，1s A 激子达到准平衡大约需要 0.7 皮秒；这一特征时间与从 1s B 激子的快速布居转移相关，揭示了二维材料中多体激子动力学的丰富特性。
