Abstract
Semiconducting transition metal dichalcogenides (TMDs) monolayers feature direct bandgap in the visible frequency range, exotic valley physics, strong spin-orbit coupling, and exceptionally strong Coulomb interaction, which has attracted great interest for exploring semiconductor optics and device applications in the atomically thin limit. These monolayers have degenerate band edges located at the Brillouin zone corners, labeled by the valley pseudospin, which is an effective quantum degree of freedom of carriers that can be addressed by electrical and optical controls. Exciton formed by Coulomb binding of an electron-hole pair inherit the valley optical selection rule of the band edges. However, monolayer excitons have short radiative lifetime and fast valley depolarization, which have limited their uses for practical valley-functional devices. Heterobilayers of TMDs provide a way to overcome these limitations. Their type-II band alignment leads to the layer separation of the electron and hole components in the exciton. While retaining the valley optical selection rule, the recombination lifetime and the valley depolarization time are substantially increased because of the quench of the electron-hole wavefunction overlap in the interlayer configuration. Such interlayer excitons have attracted remarkable interest in the exploration of valleytronic and optoelectronic applications based on TMDs heterostructures. In this talk, interlayer excitons and hybridized excitons in TMDs bilayer are discussed. In bilayer TMDs, the constituent monolayer's twin boundary would be magnified into the moire pattern, forming the moire line defect. The configurations of the moire line defect sensitively depend on the layer relative twist angle, lattice mismatch, and the twin boundary. Those configurations can be exploited to engineer various interface moire exciton modes between the R-stacking region and the H-stacking region. Some quantitative examples of the interface mode dispersions are discussed. The topological interface moire exciton states can manifest the phase shift obtained after crossing the twin boundary. Hybridization between the interlayer exciton and the intralayer exciton brings a new kind of excitons in moire pattern and incorporates the large optical dipole of the intralayer exciton and large electric dipole of the interlayer exciton, providing a way to the quantum control of moire excitons. Such hybridized exciton has fine structures in the spectrum, momentum dependent optical properties, and momentum dependent layer. The hybridization enhances the Berry curvature in the moire excitonic bands, which can produce a significant valley Hall effect.
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