Strain Tuning of the Excitons of Monolayer WSe₂

Excitons dominate the optical spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) due to their high binding energy and oscillator strength. In this work, we apply tensile strain to monolayer (1L) WSe₂, to manipulate and investigate its excitonic properties. At 2.1% strain, we achieve a redshift of 100 meV in the optical gap and a decrease of 25 meV in the binding energy of the A exciton, as revealed by exciton Rydberg spectroscopy, corresponding to a decrease of 125 meV in the quasiparticle band gap. The B-A exciton splitting increases by 20 meV, mainly due to an increase in the spin-orbit coupling. Surprisingly, the spectral 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 associated with the relative shift under strain of a secondary valley in the conduction band that is nearly degenerate with the K valley involved in the A exciton. Our results help us to understand the excitonic properties of 1L TMDCs and what contributes to the linewidths of these features. In particular, we show that the excitonic linewidths can be strongly manipulated by mechanical means.