Yang Xu-Chen, Yu Hongyi, Yao Wang
Department of Physics, The University of Hong Kong, Hong Kong, China.
HKU-UCAS Joint Institute of Theoretical and Computational Physics at Hong Kong, China.
Phys Rev Lett. 2022 May 27;128(21):217402. doi: 10.1103/PhysRevLett.128.217402.
Monolayer transition metal dichalcogenides feature tightly bound bright excitons at the degenerate valleys, where electron-hole Coulomb exchange interaction strongly couples the valley pseudospin to the momentum of the exciton. Placed on a periodically structured dielectric substrate, the spatial modulation of the Coulomb interaction leads to the formation of exciton Bloch states with real-space valley pseudospin texture displayed in a mesoscopic supercell. We find this spatial valley texture in the exciton Bloch function is pattern locked to the propagation direction, enabling nano-optical excitation of directional exciton flow through the valley selection rule. The left-right directionality of the injected exciton current is controlled by the circular polarization of excitation, while the angular directionality is controlled by the excitation location, exhibiting a vortex pattern in a supercell. The phenomenon is reminiscent of the chiral light-matter interaction in nanophotonics structures, with the role of the guided electromagnetic wave now replaced by the valley-orbit coupled exciton Bloch wave in a uniform monolayer, which points to new excitonic devices with nonreciprocal functionalities.
单层过渡金属二硫属化物在简并能谷处具有紧密束缚的明亮激子，在那里电子 - 空穴库仑交换相互作用将谷赝自旋与激子的动量强烈耦合。放置在周期性结构的介电基板上，库仑相互作用的空间调制导致在介观超胞中形成具有实空间谷赝自旋纹理的激子布洛赫态。我们发现激子布洛赫函数中的这种空间谷纹理与传播方向模式锁定，通过谷选择规则实现了定向激子流的纳米光学激发。注入的激子电流的左右方向性由激发的圆偏振控制，而角方向性由激发位置控制，在超胞中呈现出涡旋图案。这种现象让人联想到纳米光子学结构中的手性光 - 物质相互作用，现在均匀单层中谷 - 轨道耦合激子布洛赫波取代了导行电磁波的作用，这指向了具有非互易功能的新型激子器件。
