Liu Xiaoze, Yi Jun, Li Quanwei, Yang Sui, Bao Wei, Ropp Chad, Lan Shoufeng, Wang Yuan, Zhang Xiang
Nanoscale Science and Engineering Center, University of California, Berkeley, Berkeley, California 94720, United States.
School of Physics and Technology, Wuhan University, Wuhan, Hubei, 430072, PR China.
Nano Lett. 2020 Mar 11;20(3):1676-1685. doi: 10.1021/acs.nanolett.9b04811. Epub 2020 Feb 7.
Exciton polaritons (EPs) are partial-light partial-matter quasiparticles in semiconductors demonstrating striking quantum phenomena such as Bose-Einstein condensation and single-photon nonlinearity. In these phenomena, the governing process is the EP relaxation into the ground states upon excitation, where various mechanisms are extensively investigated with thermodynamic limits. However, the relaxation process becomes drastically different and could significantly advance the understanding of EP dynamics for these quantum phenomena, when excited states of EPs are involved. Here, for the first time, we observe nonlinear optical responses at the EP excited states in a monolayer tungsten disulfide (WS) microcavity, including dark excited states and dynamically metastable upper polariton bands. The nonlinear optics leads to unique emissions of ground states with prominent valley degree of freedom (DOF) via an anomalous relaxation process, which is applicable to a wide range of semiconductors from monolayer transition metal dichalcogenides (TMDs) to emerging halide perovskites. This work promises possible approaches to challenging experiments such as valley polariton condensation. Moreover, it also constructs a valley-dependent solid-state three-level system for terahertz photonics and stimulated Raman adiabatic passage.
激子极化激元（EPs）是半导体中的部分光部分物质准粒子，展现出诸如玻色 - 爱因斯坦凝聚和单光子非线性等显著的量子现象。在这些现象中，主导过程是激发后激子极化激元弛豫到基态，人们对其中的各种机制进行了广泛的热力学极限研究。然而，当涉及激子极化激元的激发态时，弛豫过程会变得截然不同，这可能会显著推动对这些量子现象中激子极化激元动力学的理解。在此，我们首次在单层二硫化钨（WS）微腔中观测到激子极化激元激发态的非线性光学响应，包括暗激发态和动态亚稳上极化激元带。非线性光学通过反常弛豫过程导致具有显著谷自由度（DOF）的基态独特发射，这适用于从单层过渡金属二卤化物（TMDs）到新兴卤化物钙钛矿的广泛半导体。这项工作为诸如谷极化激元凝聚等具有挑战性的实验提供了可能的途径。此外，它还为太赫兹光子学和受激拉曼绝热通道构建了一个依赖于谷的固态三能级系统。
