Observation and Active Control of a Collective Polariton Mode and Polaritonic Band Gap in Few-Layer WS2 Strongly Coupled with Plasmonic Lattices

Wenjing Liu; Yuhui Wang; Biyuan Zheng; Minsoo Hwang; Zhurun Ji; Gerui Liu; Ziwei Li; Volker J Sorger; Anlian Pan; Ritesh Agarwal

doi:10.1021/acs.nanolett.9b05056

Observation and Active Control of a Collective Polariton Mode and Polaritonic Band Gap in Few-Layer WS₂ Strongly Coupled with Plasmonic Lattices

Nano Lett. 2020 Jan 8;20(1):790-798. doi: 10.1021/acs.nanolett.9b05056. Epub 2019 Dec 23.

Authors

Affiliations

¹ Department of Materials Science and Engineering , University of Pennsylvania , Philadelphia , Pennsylvania 19104 , United States.
² Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, School of Physics and Electronic Science , Hunan University , Changsha , Hunan 410082 , P. R. China.
³ Department of Electrical and Computer Engineering , George Washington University 800 22nd St., Science & Engineering Hall , Washington, D.C. 20052 , United States.

PMID: 31846342
DOI: 10.1021/acs.nanolett.9b05056

Abstract

Two-dimensional semiconductors host excitons with very large oscillator strengths and binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS₂ coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of a complete polariton band gap with energy scale comparable to the exciton-plasmon coupling strength. A coupled oscillator model reveals that the collective mode arises from the cooperative coupling of the excitons to the plasmonic lattice diffraction orders via exciton-exciton interactions, leading to ultrastrong coupling. The emergence of the collective mode is accompanied by a superlinear increase of the polariton mode splitting as a function of the square root of the exciton oscillator strength. The presence of these many body effects, which are enhanced in systems which lack bulk polarization, not only allows the formation of a collective mode with periodically varying field profiles, but also further enhances the exciton-plasmon coupling. By integrating the hybrid WS₂-plasmonic lattice device with a field-effect transistor, we demonstrate active tuning of the collective mode and the polariton band gap. We also report electrically tunable waveguiding in the polariton band gap region through a line defect, which can be turned off with gate bias that can extinguish the collective mode and the polariton band gap. These systems provide new opportunities for obtaining a deeper and systematic understanding of many body cooperative phenomena in two-dimensional materials coupled with periodic photonic systems and for designing more complex and actively controllable polaritonic devices including switchable polariton lasers, waveguides, and optical logical elements.

Keywords: 2D semiconductors; collective polariton mode; exciton−plasmon polaritons; plasmonic lattices; polaritonic band gap.