Unhybridized Interlayer Excitons Enabled by Heterostrain and Electric Field in Bilayer WSe₂.

Abstract

Naturally stacked transition metal dichalcogenide (TMD) bilayers offer clean interfaces and simplified fabrication for exciton studies, yet they are dominated by momentum-dark-layer-hybridized excitons, limiting their optical applications. Unhybridized interlayer excitons (IXs) with optical activity and large dipole moments of layer thickness present superior prospects for exploring many-body physics and optoelectronics, but remain unexplored in natural bilayers. Here, we demonstrate efficient IX emission in fully encapsulated WSe₂ natural bilayers, with a threshold electric field 20 times below theoretical predictions. We confirm their unhybridized IX nature by a dipole length of 0.62 nm, characteristic of K-K' interlayer transitions. The heterostructure-like asymmetric linear Stark shift indicates an origin in heterostrain-induced layer degeneracy breaking, which is caused by local stacking disruption. This mechanism explains the notably reduced threshold. These heterostrain-localized unhybridized IXs exhibit superior properties compared to the layer-hybridized excitons, including strong dipolar repulsion (0.47 meV/μW), long lifetime (36 ns), high linear polarization (0.39), and directional long-range propagation (5 μm) with preserved in situ linear polarization and narrow line width. Our findings demonstrate natural bilayers as a field-tunable platform for engineering excitonic states and their interaction with light, which are promising for exploring quantum many-body effects and developing future optoelectronic applications.