| Light-matter interaction at nanoscale and quantum level is an important research topic,which facilitates the development of future quantum optical devices and inte-grated optical devices.Coupling between plasmonic nanocavities and excitons provides an efficient way to implement this interaction at the nanoscale.Due to the excitation of surface plasmon polaritons,the plasmonic nanocavity with mode volume beyond the diffraction limit can localize the light field to the scale of tens of nanometers,which can not only greatly enhance the interaction of light and matter,but also achieve coupling at the quantum level with few excitons or single excitons.Two-dimensional transition-metal dichalcogenides（TMDs）not only have atomically thin layer thickness,having unique advantages for coupling with light field and photonic integration,but also pos-sess the large exciton binding energy and valley physics,with great potential to achieve novel spintronics and valleytronics devices.More importantly,those layers can gen-erate high-efficiency single photon sources at low temperature due to the existence of the defects.It is of great significance to realize the coupling or even strong coupling between plasmonic nanocavity and those two-dimensional semiconductor excitons for the future practical photonic devices.Up to now,room-temperature strong coupling between nanocavity and two-dimensional valley excitons can be accomplished but it is challenging to reduce the number of excitons involved in coupling to a low number.Fur-thermore,the precise origin of the quantum emitter in monolayer is still unclear,which hinders its further potential applications.Aiming at these problems in the research of in-teractions between plasmonic nanocavity and two-dimensional semiconductor excitons,the main research contents and results of this thesis are as following:1.We obtain high-quality bowtie gap plasmonic nanocavities utilizing the nanofab-rication techniques with high precision.The smallest gap distance of bowties reaches about 20 nm.Utilizing gold-assisted mechanical exfoliation and nondestructive wet transfer techniques,the bowtie nanocavities are successfully covered by large-area Mo S2with different layers.The scattering spectrum measurements show that we obtain a ro-bust and reproducible strong plasmon-exciton coupling.The values of Rabi splitting are 80-110 me V for the systems coupled with different layers.To estimate the number of excitons contributing to the coupling,we calculate the transition dipole moment of excitons in two-dimensional TMDs materials using the quantum well model and ab-sorbance measurements,and correct the wrong values used in the previous literatures.Finally,the number of excitons involved in coupling is estimated to be about 40 to 48,which is the lowest value in similar type of devices.2.We design a chiral plasmonic lattice whose unit cell has C3 symmetry,and obtain a plasmon resonance mode with high quality factor.The calculation of optical chirality shows that there is a strong localized chiral plasmon field for this resonance mode.The low temperature spectral measurements of monolayer WSe2covered on the plasmonic lattice show that the radiations of single quantum emitters enhanced by nanocavity are obtained.The subsequent magneto-optical measurements of the quan-tum emitters show that the two Zeeman splitting branches have the same chirality for some quantum emitters,which shows that the cavity-dependent circularly polarized sin-gle photon output is realized in our system.3.We establish a quantum model to describe the interaction between single in-tervalley defect exciton and chiral plasmonic nanocavity,and solve the model using quantum master equation.In weak coupling regime,the calculated results show that the circularly polarized output of the coupled system mainly comes from the spontaneous emission of quantum emitter which is strongly modified by the chiral plasmon field.This proves that valley polarization effect is completely absent in intervalley defect ex-citons.Our study not only deepens the understanding of the origin of quantum emitters in two-dimensional TMDs materials,but also provides a platform for future research of practical quantum optical devices. |
