Theoretical Study On Surface Plasmon Optical Trapping Of Silver Triangular Nanoprism Tetramer

Surface plasmon is the phenomenon that incident light couples with free electrons on the metal surface,and then those electrons collectively oscillate at the metal/dielectric interface.Local surface plasmon is a surface plasmon localized around the metal nanoparticles.This resonance phenomenon can be applied to surface plasmon optical tweezers.Surface plasmon optical tweezers can make up for the shortcomings that traditional optical tweezers can not break the diffraction limit,which make the trapping of nano-scale particles becomes possible.Among metal nanostructures which can excite local surface plasmons,triangular nanoprime can excite multiple plasmon modes due to their anisotropic structure.The sharp vertex of the triangular nanoprime can form a strong local electric field enhancement effect,generating a large gradient force,thereby trapping the particles on the surface.In this dissertation,we demonstrated the absorption spectrum to clarify the surface plasmon modes excited by the silver triangular nanoprime.First,the effects of the incident polarization state,the size of the triangular nanoprime and rounded corners of the triangular nanoprime on the absorption spectrum of the silver triangular nanoprime are studied,in addition,we also studied the effects on the enhancement effect of the local electric field.Compared with single triangular nanoprime,the tetramer has a good structural symmetry,which is of benefit to form a symmetric electric field distribution,so as to improve the trapping propeties.Therefore,we studied the effects of the factors refered before and other factors on the absorption spectrum of silver triangular nanoprime tetramer and the surrounding electric field distribution.The FDTD Solution software was used to caculate the absorption spectrum and electromagnetic field distribution of the silver single triangular nanoprime and tetramers under different conditions.The results show that changing the polarization state of the incident light and the size of the nanostructure can control the position of the local hot spot,so as to control the acquisition performance.The plasmon field is sensitive to changes in factors such as the polarization state of incident light,the apex gap of the triangular plate in the tetramer and rounded corners of the triangular nanoprime.It is necessary to study the influence of these factors on the trapping propeties.This dissertation analyzes the influence of the above factors on the trapping properties of the surface plasmon optical tweezers of the silver triangular nanoprime tetramer structure.Through the FDTD Solution software,the Maxwell stress tensor method was used to calculate the force of polystyrene microspheres on the tetramer surface under different conditions,and the corresponding potential was obtained.The results prove that when linearly polarized light or circularly polarized light is incident,the surface plasmon optical tweezers with silver triangular nanoprime tetramer structure can achieve stable trapping of polystyrene microspheres with a radius of 5 nm.