Controlling exciton transport in monolayer WS₂ through strong coupling to plasmonic nanostructures

Chien-Ju Lee^1*, Hsin-Che Pan¹, Fatemeh HadavandMirzaee², Li-Syuan Lu¹, Fei Cheng³, Tsing-Hua Her², Chih-Kang Shih³, Wen-Hao Chang^1,4,5

¹Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
²Department of Physics and Optical Science, The University of North Carolina at Charlotte, Charlotte, North Carolina, USA
³Department of Physics, The University of Texas at Austin, Austin, Texas, USA
⁴Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
⁵College of Engineering, Chang Gung University, Taoyuan, Taiwan

* Presenter:Chien-Ju Lee, email:chienju1016@gmail.com

Efficient transport of exciton in two-dimensional (2D) semiconductors, such as layered transition-metal dichalcogenides (TMDs), is of great importance for future optoelectronic applications. However, due to its short lifetime, excitons in TMDs can only transport over a few hundred nanometers. In this work, we modify the transport property of excitons by strong coupling monolayer tungsten disulfide (WS₂) to plasmonic nanostructures at room temperature. Monolayer WS₂ were transferred onto periodic arrays of nanogrooves fabricated via focused ion beam. Exciton-polaritons are formed between surface-plasmon-polaritons (SPPs) and tightly bounded excitons with a Rabi splitting reaching 70 meV. The light-mass polariton can travel several micrometers due to its large group velocity. Furthermore, due to the valley contrast optical selection rule in monolayer TMDs, excitons formed at two degenerate but inequivalent K and K’ valleys can couple to light with different helicities. This unique property of spin-valley coupling in 2D TMDs has triggered abundant research activities in valleytronic applications. Here, we combine the spin-orbit coupling of light in plasmonic nanostructures that can separate photons with different spins, and demonstrate the directional routing of exciton-polaritons. Through polarization-resolved photoluminescence imaging of the coupled WS₂-plasmonic nanostructure, we observed valley-dependent directional propagation of polaritons. When SPP modes resonate with WS₂ excitons, polaritons with opposite helicities were routed into different directions, and a polarization contrast of 30 percent was achieved. Our results demonstrate a feasible way to boost and optically control the flow of valley excitons in TMDs and may contribute to the development of valleytronics.

Keywords: Exciton-polariton, TMDs, Exciton transport