Light–matter Interaction In Two Dimensional Layered Materials

The interaction between light and matter is not only the core of many physical phenomena,it also plays a vital role in modern science and technology,including but not limited to modern spectroscopy,lasers,X-ray sources,light-emitting diodes,photodiodes,solar cells,quantum information processing.This thesis is mainly based on a two-dimensional layered material system,using optical and electrical characterization methods to study the unique physical properties brought about by the interaction of light and matter in two-dimensional materials.The specific research content is as follows:1.Using Raman spectroscopy to study non-adiabatic electron phonon coupling in monolayer graphene.We fabricated ultrathin hexagonal boron nitride encapsulated monolayer graphene devices with very low residual carrier concentrations by"pick up"dry transfer method,such devices meet the conditions for optical measurement.By changing the carrier concentration in the monolayer graphene through the bottom gate,we can achieve the modulation of the electron-phonon interaction in the system.We observe experimentally for the first time the Kohn anomaly due to non-adiabatic electron-phonon coupling in monolayer graphene,demonstrating the existence of very large non-adiabatic electron-phonon interactions in monolayer graphene.2.Spin-layer locking induced second order nonlinear photocurrent response in centrosymmetric crystals.We systematically investigate direct current second-order nonlinear effects(both circular and linear photogalvanic effects)in MoS₂,and demonstrate that second-order nonlinear photocurrent responses can indeed exist in centrosymmetric multilayers as well as bulk MoS₂.We show that the unique spin-layer locking effect in MoS₂ is the fundamental reason for the second-order photocurrent response.Because of the localization of the electron wave function within each layer,multilayer MoS₂ can be viewed as a superposition of each single layer.As a result,multilayer samples can also harbour direct current second-order nonlinear effect as long as the monolayer satisfies the conditions,thus not being inhibited by the global spatial inversion symmetry for the second-order nonlinear effect,and the strength of such direct current second-order nonlinear effect increases monotonically with the number of layers.3.Hybrid interlayer exciton in bilayer MoS₂.We systematically investigate the hybrid interlayer excitons in 2H-phase bilayer MoS₂,demonstrating that such hybrid interlayer excitons can have both the large oscillator strength of the intralayer exciton and the out-of-plane electric dipole moment of the interlayer exciton,so that such interlayer excitons can have a very high electric field modulation while ensuring a large luminescence intensity.In addition,we observe a new set of interlayer excitons in the bilayer MoS₂ under resonant excitation,which exhibits excellent spin-valley properties with a negative and near-unity circular polarization,as well as giant magnetic tunable excitonic valley pseudospin.Such highly tunable bright interlayer excitons in homobilayer MoS₂ is very promising in the field of valley electronics.