State- and Momentum-Dependent Nonlinear Stark Effect of Interlayer Excitons in Bilayer WSe₂

Interlayer excitons in van der Waals heterostructures exhibit rich tunability, with the electric-field response governed by layer localization. Here, in the case of bilayer WSe₂, we reveal how the layer localization of excitons governs their response to an external electric field. Using Many-Body Perturbation Theory, we calculate the exciton dispersion for different stacking symmetries under electric field and/or strain to map the landscape of competing low-energy excitons in four distinct finite-momentum valleys. While intralayer excitons are not affected by the electric field, some interlayer excitons exhibit a nonlinear Stark shift that becomes linear after a critical threshold. The degree of nonlinearity is a direct measure of the layer hybridization of the electronic subcomponents of the exciton. Our results clarify recent experimental observations, including nonlinear Stark shifts, (anti)symmetric spectral behavior near zero field, and dipolar exciton sensitivity to perturbations. These insights are crucial for engineering excitonic condensates, optoelectronic devices, and quantum emitters.