Light-Matter Interaction in Flatland: Excitonic Physics in 2D
A CMU-PITT Colloquium
Abstract: The emergence of the two-dimensional (2D) transition metal dichalcogenides (TMDCs) ushers in a new era of light-matter interaction, offering exciting opportunities in quantum information science (QIS). In monolayer TMDCs, the reduced screening enhances the Coulomb interaction and gives rise to strongly bound excitons with the binding energy of hundreds of meV. In addition, the excitons in TMDCs possess a new quantum degree of freedom, valley-spin, which is promising for quantum information processing and can be accessed through chiral light. In this talk, I will discuss our search for valley-contrasting, long-lived quasiparticles through various optical spectroscopy techniques, which advance our understanding of the many-particle excitonic complexes in monolayer WSe2 such as dark excitons, the four-particle biexcitons, five-particle charged biexcitons, and photon replica of dark excitons. We also reveal unique excitonic physics in the presence of Landau quantization introduced by a strong magnetic field.
As atomically thin semiconductors, the TMDCs also offer unprecedented opportunities in bandstructure engineering. A type II alignment can be achieved in TMDC heterobilayers, which hosts long-lived interlayer excitons with electrons and holes residing in different layers. The lattice mismatch or twisted angle between TMDC heterobilayer can lead to moiré superlattices that host flat miniband and give rise to high tunable correlated states in 2D. Recently, we have identified various correlated insulating states in the WS2/WSe2 heterostructure, including Mott insulating state and generalized Wigner crystal states. This unique moiré superlattice provides a new platform for quantum simulation. The correlated electrons also interact with the excitons, providing a readout of the quantum simulator of correlated electrons. This strong interaction also inspires to engineer new correlated excitons for QIS.