The Strong Coupling Characteristics And Manipulation Of Plasmon-exciton Coupling In A WS₂/Ag Nanocavity

In recent years,the strong plasmon-exciton（plexciton）coupling between two-dimensional transition metal dichalcogenides（TMDCs）and plasmonic nanostructures has drawn a lot of attention.When the layer number of TMDCs reduces to single layer（monolayer）,the band gap of TMDCs changes from an indirect band gap to a direct one.Thus,the monolayer of TMDCs has a large exciton binding energy and strong transition dipole moment,which allows for exciton devices at room temperature.Compared with the study of strong coupling with J-aggregates or quantum dots,the study of strong coupling effect with monolayer TMDCs can provide a better application platform for device innovation at room temperature.However,it still faces many challenges to apply in quantum information processing devices.one of the biggest problem is how to enhance the plexciton coupling strength in TMDCs/plasmonic system.In our work,a large Rabi splitting energy up to 266 meV is obtained by immersing in dichloromethane（DCM）.Moreover,we demonstrate a continuously real-time tuning of strong coupling in the monolayer WS₂-Ag nanocavity by immersing in the mixed solution of DCM and ethanol.This has potential applications in quantum information processing,nonlinear optical materials and the possibility of manipulating chemical reaction.The main research results are as follows:（1）The nanoparticle-on-mirror（NPoM）structure is used in the monolayer WS₂-Ag nanocavity system.The plexciton coupling signal between exciton and plasmon is observed by the single particle dark-field scattering spectrum at room temperature.Then we fit the scattering spectra for the coupling strength with the semi-classical coupled oscillator model.We obtain a Rabi splitting of～145 meV.The coupling strength meets the strong coupling criterion of （?）,which indicates that the plasmon-exciton interactions belong to the strong coupling regime.Furthermore,the effective number N of excitons are estimated to be 200 in the system according to the equation of coupling strength.（2）In order to improve the rabi splitting energy of the monolayer WS₂-Ag nanocavity,the system is immersed in DCM solvent.Since the electronegativity of DCM is greater than that of monolayer WS₂,the monolayer WS₂ loses electrons.The density of neutral exciton increases up to～100%.As a result,the interaction between the surface plasmon and exciton in the composite structure is enhanced,and the rabi splitting energy is up to 266 meV.（3）Tuning the electronegativity value of dielectric environment around the system,the density of neutral exciton in monolayer WS₂ can be changed.Therefore,the WS₂-Ag nanocavity is immersed in a mixed solution of ethanol and DCM.By increasing the concentration of DCM,the rabi splitting energy is tuned from 178 meV（in ethanol）to266 meV（in DCM）.DCM and ethanol can be mutually soluble in any radio,and electron transfer is reversible with an ultrafast response.Hence,we can control the strong coupling strength in real-time by manipulating excitons in WS₂ through mixing DCM/ethanol mixture.