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On generation, the excitons relax to the lowest energy 1s state by scattering through various phonon modes in multiple pathways. We use a simple technique in which, by tuning the excitation laser wavelength, the excitons resonantly come down to the 1s state in a single-step manner through scattering with a specific phonon mode. This relaxation process is highly efficient due to the strong resonance of the energy separation between the initial and the final exciton state with a single phonon mode in the system.

We demonstrate that such a single-step relaxation mechanism strongly modulates the 1s exciton PL emission features in a monolayer WS₂ sample sandwiched between few-layer graphene flakes. Using this technique at 5K, we obtain exciton peaks that are: (1) highly luminescent, (2) highly linearly polarized - demonstrating near-perfect valley coherence, and (3) ultra-narrow - due to a reduction in the inhomogeneous broadening.

The lowest linewidth of the 1s exciton peak obtained using this technique is 1.5 meV, which after deconvolution with the excitation laser gives an upper bound of 0.23 meV on the homogeneous broadening of the exciton peak. We show the retention of the above features all the way from cryogenic temperature to room temperature, demonstrating the robustness of the suggested technique with temperature.