Polarized and narrow excitonic emission from graphene-capped monolayer WS$_2$ through resonant phonon relaxation

TL;DR

This study demonstrates that encapsulating monolayer WS$_2$ with few-layer graphene and tuning phonon resonances significantly narrows excitonic emission linewidths, enhances polarization control, and maintains performance up to 200 K, advancing excitonic optoelectronics.

Contribution

The paper introduces a resonant phonon relaxation technique via graphene encapsulation to achieve narrow, polarized excitonic emission in monolayer WS$_2$, with robustness at elevated temperatures.

Findings

Achieved exciton linewidth of 1.06 meV, reduced further to 0.19 meV after deconvolution.

Enhanced valley polarization from ~40% to ~75% on resonance.

Maintained strong polarization and narrow linewidths up to 200 K.

Abstract

The broadening and polarization of excitonic luminescence in monolayer TMDs largely suffer from inhomogeneity and temperature - an unresolved problem to date. In this work, through few-layer-graphene encapsulation of monolayer WS$_2$, we reduce the inter-excitonic energy separation, which then can have a narrow resonance with a specific phonon mode of our choice. The resulting single-step exciton relaxation with the resonating phonon mode significantly suppresses the inhomogeneous broadening, allowing us to achieve the narrowest exciton linewidth of 1.06 meV (which translates to 0.19 meV after deconvolution with the excitation laser linewidth). The single-phonon resonance helps to achieve a high quantum efficiency despite graphene encapsulation. The technique is powerful in tuning the exciton polarization during relaxation by choosing a specific resonating phonon mode. For example, the valley coherence (polarization) improves from $\sim$68% ($\sim$40%) to $\sim$90% ($\sim$75%) on resonance with 2$A_1$' and $A_1$' modes respectively. We further demonstrate a strong polarization reversal on resonance with a chiral phonon mode. Strikingly, the above features remain robust against temperature (up to 200 K) and sample age (few months in ambient condition). The findings will lead to clean excitonic measurements without requiring cryogenic cooling.