Brightening Interlayer Excitons by Electric-Field-Driven Hole Transfer in Bilayer WSe_{2}

We observe the interlayer A_{1s}^{I}, A_{2s}^{I}, and B_{1s}^{I} excitons in bilayer WSe_{2} under applied electric fields using reflectance contrast spectroscopy. Remarkably, these interlayer excitons remain optically bright despite being well separated from symmetry-matched intralayer excitons-a regime where conventional two-level coupling models fail unless unphysically large coupling strengths are assumed. To uncover the origin of this brightening, we perform density functional theory (DFT) calculations and find that the applied electric field distorts the valence-band Bloch states, driving the hole wave function from one layer to the other. This field-driven interlayer hole transfer imparts intralayer character to the interlayer excitons, thereby enhancing their oscillator strength without requiring hybridization with bright intralayer states. Simulations confirm that this mechanism accounts for the major contribution to the observed brightness, with excitonic hybridization playing only a minor role. Our results identify interlayer hole transfer as a key mechanism for brightening interlayer excitons in bilayer transition metal dichalcogenides, especially when inter- and intralayer excitons are energetically well separated.