Optical Anisotropy Of Two-Dimensional Materials Studied By Scanning Polarization Modulation Microscopy

There is a special category of two-dimensional?2D?materials such as black phosphorus?BP?,ReS₂ and 1T?-MoTe₂,which possess in-plane anisotropy due to their low crystal lattice symmetry.The emergence of in-plane anisotropic properties provides an additional degree of freedom for tuning the optical,electrical and thermal properties,and thus expands the range of possibilities for designing multifunctio nal nanodevices and exploring various applications.Under irradiation,surface plasmon polaritons?SPP?will be generated on metal nanomaterials,which show excellent nano-scale light field regulation and photoelectric transport control ability.The SPP excitation efficiency of 2D metal nanomaterials is anisotropic due to the break of symmetry of structure.These unique characteristics make metal nanomaterials have great application value in the fields of optical switches,polarization control technique,surface plasmon based sensor,optical information processing and ot her micro-nano devices.In this thesis,scanning polarization modulation microscopy?SPMM?was used to investigate the optical anisotropy of anisotropic 2D materials at micron scale and the polarization dependence SPP excitation in a single Au nanoprism.Firstly,we propose and implement a scanning polarization modulation microimaging technique for two-dimensional materials.Based on the theory of polarization imaging technology,we expound the basic theory and key points of polarization modulation microscopic imaging technology and then deduces the differential reflectivity or differential transmittance can be given directly by SPMM.This lays a theoretical foundation for the quantitative study for 2D materials by using scanning polarization modulation microscopy.Experiment showed that the SPMM signal is dependent on the polarization angle of the laser beam in a sinusoidal way and reaches the maximum when the angle between the crystal axis direction and the polarization angle of the laser beam is 45°,by which the crystalline orientation of anisotropic 2D materials was determined.The results are highly consistent with those measured by angle-resolved polarization Raman spectroscopy,which indicates that the SPMM method is a new accurate and reliable method.The sign of the collected differential reflectance signals depends on the thickness of the sample,while the sign of differential transmittance signals does not change with the thickness at a specific wavelength.Therefore,the transmission SPMM can measured the crystal orientation regardless of thickness.By analysing the theoretical expression of SPMM signal,it is found that crystalline orientation can be given by measuring transmission SPMM signals at only two polarization directions,and the whole measurement process will reduce to a few seconds.Benefiting from its simple and fast process,SPMM could be developed as an automatic online probing tool for crystalline orientation in the industrial applications of anisotropic 2D materials.By using home-built SPMM,the optical in-plane anisotropy of few-layer BP,ReS₂ and 1T?-MoTe₂ were studied.Differential reflectance and differential transmittance image of few-layer BP,ReS₂ and 1T?-MoTe₂ with a size of tens of microns were obtained by SPMM.These images have high signal-to-noise ratio and contrast,thus realizing the visualization of optical anisotropy of 2D materials at micron-scale.Thickness and excitation wavelength dependence of differential reflectance and differential transmittance was found after imaging many different BP,ReS₂ and 1T?-MoTe₂ samples with different thickness at 400 nm,532 nm,800 nm and 1064 nm wavelength,and was well explained by considering the anisotropic multi-reflection effect.Using the experimental data of different wavelength and thickness,the anisotropic complex refractive indices of BP at 400 nm,532 nm,800nm and 1064 nm and that of ReS₂ and 1T?-MoTe₂ at 532 nm are obtained.In addition,the influence of wrinkle on the optical anisotropy of 2D materials is quantitatively studied by SPMM.It is found that the optical anisotropy will decrease at the ridge of the wrinkle,while the optical anisotropy will increase at the valley of the wrinkle.The polarization dependence of SPP excitation in single Au nanoprism was studied by using SPMM.SPMM signal represents the differential intensity of SPP excited by horizontal polarization and vertical polarization.Therefore,SPMM images directly reflect the polarization dependence of the excitation efficiency of SPP at different positions of nanoparticles.The far-field scanning imaging of single Au nanoprism were carried out with transmission SPMM.There is basically no signal in the middle part of the Au nanoprism SPMM image,indicating that the SPP excited in the middle part of the Au nanoprism is polarization independent,because the structure in the middle part of the Au nanoprism is completely symmetrical.The SPMM signal on the edge of Au nanoprism changes periodically with the polarization angle of the excitation light,and the variation patterns of the three edges are basically the same only with separated from each other about 60°.The polarization dependence of SPP excitation efficiency on the edge is due to the symmetry destruction on the edge.The numerical simulation based on FDTD reproduces the experimental images of SPMM and obtains that the planar mode and corner mode of SPP in Au nanoprism are both polarization dependent,but the polarization dependent characteristics are just the opposite.