PL Characteristics Of Tmdcs Based On Cylindrical Vector Beams

Due to the large exciton binding energy,two-dimensional(2D)transition metal dichalcogenides(TMDCs)provide an ideal platform for studying excitonic states and related photonics and optoelectronics.Exciting materials with different polarization states of light leads to distinct light-matter interactions.A deep understanding of these mechanisms is of great significance for the application of devices.The normally used linear polarized laser beam in optical microscopy create only in-plane electric fields at the focus,which limits the study of different excitation conditions.Therefore,a suitable method is needed to study the physical phenomena such as light emission in2D TMDCs excited with the out of plane electric field.In this thesis,we use two kinds of cylindrical vector beams,radially polarized light with strong out-of-plane polarization and azimuthally polarized light with pure in-plane polarization at the focal fields,to explore the light-matter interactions of 2D TMDCs under different polarization states.The main research achievements are as follows:(1)The realization of optical excitation and detection system based on cylindrical vector beam.Two kinds of cylindrical vector beams,radially and azimuthally polarized light,have been successfully realized by using liquid crystal polarization converters.On this basis,a polarized microscopy optical imaging and spectral measurement system with an inverted optical microscope was built.The photoluminescence(PL)images were obtained by using a single Cd Se nanowire under the excitation of the different polarized laser beams,which demonstrated the different polarization characteristics with the cylindrical vector beams.(2)The optical polarization characteristics of 2D WS₂,WSe₂,Mo S₂,Mo Se₂ were studied with the cylindrical vector beams.Strong PL emission is observed with the excitation of in-plane electric field.The peak positions of PL spectrum based on radial and azimuthal polarization modes are consistent,indicating the same excitonic origin of the PL emission.Under two polarized modes,the fluorescence intensity ratio of excitons varies with the material type and the number of layers.According to the normalized spectra,we found the same in-plane and out-of-plane polarization behavior from the intralayer and indirect excitons.The excitation and emission polarization behaviors under linear,radial and azimuthal polarized laser in monolayer TMDCs are further studied.(3)The optical polarization characteristics of 2D heterojunction based on TMDCs were studied with the cylindrical vector beams.The PL spectrum of 1L WS₂/1L WSe₂heterojunction was detected with the cylindrical vector beams to investigate the effect of the in-plane and out-plane polarized beams on the out-plane electric dipole moment.From the absolute emission intensity,spectra taken with azimuthal mode are still stronger than those with radial mode.However,the interlayer exciton peak is obviously stronger under the excitation of the radial mode based on the relative intensities after the spectral normalization.In addition,we found that the relative intensity ratio of interlayer excitons under two polarization modes also varies locally due to the different local morphology and interlayer coupling in heterojunctions.