Lin Kai-Qiang, Faria Junior Paulo E, Bauer Jonas M, Peng Bo, Monserrat Bartomeu, Gmitra Martin, Fabian Jaroslav, Bange Sebastian, Lupton John M
Department of Physics, University of Regensburg, Regensburg, Germany.
TCM Group, Cavendish Laboratory, University of Cambridge, Cambridge, UK.
Nat Commun. 2021 Mar 10;12(1):1553. doi: 10.1038/s41467-021-21547-z.
Twist-engineering of the electronic structure in van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/°, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e., the dispersion, of the optical nonlinearity.
范德华层状材料中电子结构的扭曲工程主要依赖于层间的能带杂化。过渡金属二硫属化物半导体中的带边态位于三原子层中心的金属原子周围，因此对扭曲不太敏感。在这里，我们报告双层WSe中高能激子可通过扭曲在235 meV范围内进行调谐，扭曲角敏感度为8.1 meV/°，比带边A激子大一个数量级。这种可调谐性的出现是因为与上导带相关的电子态离域到硫属化物原子中。这种效应可以控制激子量子干涉，这在二次谐波产生中电磁诱导透明（EIT）的选择性激活和失活中得到了体现。在最初建立EIT的传统稀薄原子气体系统中不存在这样的自由度，它使我们能够塑造光学非线性的频率依赖性，即色散。
