Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2bilayers

Domain-resolved spectroscopy reveals the impact of local atomic registry and crystal symmetry on the exciton properties of individual domains in near-0 degrees-twist-angle MoSe2/MoSe2. Van der Waals heterostructures obtained via stacking and twisting have been used to create moire superlattices(1), enabling new optical and electronic properties in solid-state systems. Moire lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping(2-5), host Mott insulating and superconducting states(6)and act as unique Hubbard systems(7-9)whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation(10-14). However, due to the nanoscale size of moire domains, the effects of atomic reconstruction on the electronic and excitonic properties have not been systematically investigated. Here we use near-0 degrees-twist-angle MoSe2/MoSe(2)bilayers with large rhombohedral AB/BA domains(15)to directly probe the excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Gamma-K interlayer excitons can be flipped with electric fields, while higher-energy K-K interlayer excitons undergo field-asymmetric hybridization with intralayer K-K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically non-trivial systems(16,17), exotic metasurfaces(18), collective excitonic phases(19)and quantum emitter arrays(20,21)via domain-pattern engineering.