Strain tuning of excitons in monolayer WSe2

We investigate excitonic absorption and emission in monolayer (1L) WSe2 under tensile strain. We observe a redshift of 100 meV in the A exciton energy and a decrease of 25 meV in its estimated binding energy under 2.1% strain. Surprisingly, the linewidth of the A exciton decreases by almost a factor of two under strain, from 42 to 24 meV at room temperature. We explain this effect as the result of suppression of phonon-mediated exciton scattering channels. We show that such a suppression results from the relative shift under strain of a secondary valley in the conduction band that is nearly degenerate with the K valley where the A exciton is formed.