Strained bilayer WSe₂ with reduced exciton-phonon coupling

We investigate excitonic absorption and emission in bilayer (2L) WSe₂ under tensile strain. We observe a redshift of 110 meV in the energy of the A exciton absorption peak (direct gap) under 2.1% strain. The photoluminescence (PL) spectrum exhibits multiple peaks under 0% strain, corresponding to the indirect and direct gaps. Surprisingly, the spectral linewidth of the A exciton decreases by almost a factor of two under strain, from 70 to 36 meV at room temperature. We explain this effect as the result of the suppression of phonon-mediated exciton scattering channels. That suppression is associated with the relative shift under strain of the Q valley in the conduction band (involved in the indirect PL), which is nearly degenerate with the K valley (involved in A exciton). We analyze the strain-dependent absorption and PL spectra to determine the relative positions of those valleys and to calculate the intervalley scattering rates. Our model describes well the strain-induced linewidth decrease of both monolayer (1L) and 2L WSe₂ and helps us understand what contributes to the linewidths. The results show that strain tuning can be employed to probe the band structures and the excitonic properties of 1L and 2L TMDCs.