We investigate excitonic absorption and emission in bilayer ${\mathrm{WSe}}_2$ under tensile strain. We observe a redshift of 110 meV in the energy of the $A$ exciton absorption peak (at the direct gap at the $K$ point in the Brillouin zone) under 2.1% uniaxial tensile strain. In addition, under the same strain, the spectral linewidth of the $A$ exciton at room temperature decreases by a factor of 2, from 70 to 36 meV. We show that this decrease is a result of suppression of phonon-mediated exciton scattering channels. This suppression is associated with the relative upshift under strain of the $Q$ valley in the conduction band (involved in the indirect exciton emission), which is nearly degenerate with the $K$ valley (involved in the $A$ exciton). We analyze the strain-dependent absorption and photoluminescence spectra to determine the relative positions of these valleys and to infer intervalley scattering rates. Our model describes well the decrease and the distinct trends in the $A$ exciton linewidth of monolayer and bilayer ${\mathrm{WSe}}_2$ under strain. The results show that strain can be used to tune, as well as to probe, the relative energies of band extrema and exciton scattering channels in two-dimensional semiconductors.

• Received 8 December 2019
• Revised 21 January 2020
• Accepted 6 February 2020

DOI:https://doi.org/10.1103/PhysRevB.101.115305