Excitons in transition metal dichalcogenides (TMDCs)

The strong Coulomb interaction resulting from the reduced screening in two-dimension (2D) leads to tightly bound excitons and even high-order Rydberg excitons. The strong Coulomb interaction also gives rise to many-particle excitonic complexes bound or interacting with other quasiparticles. Excitonic complexes usher in new venues of controlling critical exciton properties such as lifetime and valley degree of freedoms. In this chapter, we present a comprehensive review of the recent studies on the optical properties of various excitonic complexes in TMDCs. We start by introducing the experimental progress on the excitonic Rydberg series, which helps to reveal the effective Coulomb interaction in 2D. We then focus on experimental evidence for exciton complexes in monolayer TMDCs, including charged excitons, biexcitons, dark excitons, and exciton-phonon replicas, with emphasis on their valley and magneto-optical properties. In the third part, we review the recent findings on interlayer excitons in type-II aligned van der Waals hetero-bilayers of TMDCs, which exhibit distinctly different properties than intralayer excitons in exciton dynamics, optical selection rules, and the electric field tunability. Finally, we briefly discuss the moire excitons. We hope this chapter will guide readers interested in the unique excitonic physics in TMDCs and inspire future development of quantum optoelectronics based on excitons in TMDCs.