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

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.

References in corpus (4)

• Environmentally-Sensitive Theory of Electronic and Optical Transitions in Atomically-Thin Semiconductors
• Picosecond energy transfer in a transition metal dichalcogenide-graphene heterostructure revealed by transient Raman spectroscopy
• Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time
• Enhanced exciton-exciton collisions in an ultra-flat monolayer MoSe2 prepared through deterministic flattening

Cited by in corpus (1)

• Motion of 2D exciton in momentum space leads to pseudospin distribution narrowing on the Bloch Sphere