Interlayer dark excitons in a van der Waals heterostructure

Abstract: Interlayer excitons in transition metal dichalcogenide (TMD) heterostructures exhibit long lifetimes and long-range transport, making them promising for excitonic devices and quantum many-body phases, such as Bose-Einstein condensates. Achieving these goals requires a precise understanding of spin-allowed bright and nominally spin-forbidden dark excitons, because the lowest-energy exciton species governs population, transport, and condensation. Despite substantial progress, unambiguously distinguishing singlet and triplet interlayer excitons has been challenging, as moiré excitons in these heterostructures can mimic their optical signatures. Here, we report the direct spectroscopic identification of bright (singlet) and dark (triplet) interlayer excitons in high-quality, dual-gated WSe$_2$/hBN/WSe$_2$ homobilayers. Electric-field-dependent photoluminescence and reflectance reveal two momentum-direct interlayer transitions with distinct spin configurations. The interlayer dark excitons obey selection rules that differ from those of bright excitons. Strikingly, interlayer dark excitons retain strong valley polarization, even with their ultralong lifetime exceeding microseconds. Finally, we demonstrate twist-angle control, wherein twist-induced electron-hole momentum mismatch modulates interlayer exciton emission. These results provide critical insights into the electronic and excitonic structure of TMD heterostructures, opening new avenues for excitonic many-body physics and optoelectronic devices.