  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.
