Superior valley polarization and coherence of 2s excitons in monolayer WSe₂

We report on the fabrication and spectroscopic measurement of high-quality monolayer WSe₂ crystals. The excitonic emission features in these samples exhibit narrow linewidth. We fully resolve the exchange-interaction-split trion doublet at low temperatures and observe the 2s exciton luminescence up to room temperature. From comparison with absorption spectra, both 1s and 2s luminescence radiation exhibit near-zero Stokes shift, confirming that disorders in these samples are minimal. Remarkably, under similar experimental conditions, the 2s luminescence is found to exhibit much better valley polarization and coherence than 1s. We explain our experimental results in the framework of exchange interaction induced intervalley scattering, the Maialle-Silva-Sham mechanism.