Real-Space Imaging of Guided Exciton Polaritons in Free-standing Monolayer WSe2

TL;DR

This study visualizes and analyzes the real-space propagation of exciton-polariton guided modes in a free-standing monolayer WSe2, demonstrating strong light-matter interactions and potential for nanoscale photonics.

Contribution

First direct real-space imaging of exciton-polariton guided modes in monolayer WSe2 using s-SNOM, validated by numerical simulations and theoretical modeling.

Findings

Guided modes exist only under symmetric cladding conditions.

Pronounced back-bending dispersion observed around A exciton.

Confirmed fundamental TE0 exciton polariton mode in monolayer WSe2.

Abstract

Monolayers of transition metal dichalcogenides (TMDCs), known for their strong excitonic states with high binding energies in the visible spectrum at room temperature, offer great potential for polariton-driven devices. While polariton guided modes in bulk TMDCs have been reported the real space experimental observation of 2D exciton-polariton guided modes in a monolayer remains challenging due to various mode cut-off conditions that arise as the TMDC layer becomes thinner, including cut-off frequency, mode confinement and boundary conditions. Here using scanning near-field optical microscopy (s-SNOM), we directly visualized the real-space propagation of these guided modes for the first time in an angstrom-thick, suspended monolayer of WSe2. Through numerical simulations we have also validated that the guided mode can only exist in a monolayer WSe2 when symmetric cladding conditions are closely applied. By tuning the excitation laser energy and analysing the guided mode distribution, we observed a pronounced back-bending dispersion around the A exciton, indicating strong light-matter interactions, and confirmed the existence of the fundamental TE0 exciton polariton (EP) propagation mode. The unique dispersion characteristics of these modes were further validated through theoretical modelling of the mode in free-standing monolayer WSe2. Our findings provide crucial experimental evidence of guided mode EPs in atomically thin TMDCs, opening new possibilities for nanoscale photonic applications.