Structural engineering of van der Waals heterostructures via stacking and twisting has recently been used to create moiré superlattices 1,2 , enabling the realization of new optical and electronic properties in solid-state systems. In particular, moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) have been shown to lead to exciton trapping 3-8 , host Mott insulating and superconducting states 9 , and act as unique Hubbard systems 10-13 whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures also feature atomic reconstruction and domain formation 14-20. Unfortunately, due to the nanoscale sizes (~10 nm) of typical moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties of these heterostructures could not be investigated systematically and have often been ignored. Here, we use near-0 o twist angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains 21 to directly probe 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 (z) electric dipole moments in opposite directions. The dipole orientation of ground-state -K interlayer excitons (XI,1) can be flipped with electric fields, while higher-energy K-K interlayer excitons (XI,2) undergo fieldasymmetric hybridization with intralayer K-K excitons (X0). Our study reveals the profound impacts of crystal symmetry on TMD excitons and points to new avenues for realizing topologically nontrivial systems 22,23 , exotic metasurfaces 24,25 , collective excitonic phases 26-28 , and quantum emitter arrays 29,30 via domain-pattern engineering. To date, most studies of twisted TMD bilayers have assumed a rigid rotation of layers without atomic-scale rearrangement 3-8. However, recent theoretical 14-16 and experimental 17-20 studies have

Moiré superlattices in twisted van der Waals materials constitute a promising platform for engineering electronic and optical properties. However, a major obstacle to fully understanding these systems and harnessing their potential is the limited ability to correlate the local moiré structure with optical properties. By using a recently developed scanning electron microscopy technique to image twisted WSe2/WSe2 bilayers, we directly correlate increasing moiré periodicity with the emergence of two distinct exciton species. These can be tuned individually through electrostatic gating, and feature different valley coherence properties. Our observations can be understood as resulting from an array of two intralayer exciton species residing in alternating locations in the superlattice, and illuminate the influence of the moiré potential on lateral exciton motion. They open up new avenues for controlling exciton arrays in twisted TMDs, with applications in quantum optoelectronics and explorations of novel many body systems. Engineered moiré superlattices arising from lattice mismatch or relative twist angle between layers can induce periodic potentials for charge carriers and excitons. While

A Wigner crystal, a regular electron lattice arising from strong correlation effects 1-6 , is one of the earliest predicted collective electronic states. This many-body state exhibits quantum and classical phase transitions 7 and has been proposed as a basis for quantum information processing applications 8, 9 . In semiconductor platforms, two-dimensional Wigner crystals have been observed under magnetic field 10-17 or moiré-based lattice potential 18-21 where the electron kinetic energy is strongly suppressed. Here, we report bilayer Wigner crystal formation without a magnetic or confinement field in atomically thin MoSe2 bilayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states formed at symmetric (1:1) and asymmetric (4:1 and 7:1) electron doping of the two MoSe2 layers at cryogenic temperatures. We attribute these features to the bilayer Wigner crystals formed from two commensurate triangular electron lattices in each layer, stabilized via inter-layer interaction 22, 23 . These bilayer Wigner crystal phases are remarkably stable and undergo quantum and thermal melting transitions above a critical electron density of up to 6 ´ 10 12 cm -2 and at temperatures of ~40 K. Our results demonstrate that atomically thin semiconductors provide a promising new platform for realizing strongly correlated electronic states, probing their electronic and magnetic phase transitions, and developing novel applications in quantum electronics and optoelectronics 24-28 .Atomically thin heterostructures made of graphene and transition metal dichalcogenide (TMD) monolayers can host a variety of correlated electronic states [29][30][31][32][33] . Recent advances in materials growth and heterostructure fabrication have enabled the preparation of high-quality heterostructures with minimal disorder [34][35][36][37][38] . The large effective masses of charge carriers 39, 40 and the weak Coulomb screening in TMDs suppress the Fermi energy and enhance electron

The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted WSe 2 =WSe 2 bilayers exhibit a high (>60%) degree of circular polarization (DOCP) and long valley lifetimes (>40 ns) at zero electric and magnetic fields. The valley lifetime can be tuned by more than 3 orders of magnitude via electrostatic doping, enabling switching of the DOCP from ∼80% in the n-doped regime to <5% in the p-doped regime. These results open up new avenues for tunable chiral light-matter interactions, enabling novel device schemes that exploit the valley degree of freedom.

We demonstrate a new approach for dynamically manipulating the optical response of an atomically thin semiconductor, a monolayer of MoSe2, by suspending it over a metallic mirror. First, we show that suspended van der Waals heterostructures incorporating a MoSe2 monolayer host spatially homogeneous, lifetime-broadened excitons. Then, we interface this nearly ideal excitonic system with a metallic mirror and demonstrate control over the exciton-photon coupling. Specifically, by electromechanically changing the distance between the heterostructure and the mirror, thereby changing the local photonic density of states in a controllable and reversible fashion, we show that both the absorption and emission properties of the excitons can be dynamically modulated. This electromechanical control over exciton dynamics in a mechanically flexible, atomically thin semiconductor opens up new avenues in cavity quantum optomechanics, nonlinear quantum optics, and topological photonics.