Research On Finite Element Analysis Method Of Optical Force And Photothermal Effects

With the development and maturity of nano-optical device fabrication technology,optical tweezers and optical communication technology have been applied in a wider range of fields,so physical simulation in optics and related fields has had more important significance.Considering that light has the characteristics of electromagnetic waves,the Finite Element Method(FEM)in computational electromagnetics is used into the field of optical calculation.The simulation of multi-physical field coupling of target objects is realized by adopting Maxwell Stress Tensor(MST)and heat conduction equation.Firstly,the electromagnetic field representation of light is given and the whole simulation area is divided by basic geometric meshes.Maxwell differential equations are discretized based on variational method.After determining the boundary conditions,the approximate numerical solution of the spatial electromagnetic field distribution can be obtained by constructing and solving the finite element matrix.Tetrahedron partition and vector basis function make FEM have good adaptability and can simulate the structure of arbitrary shape and arbitrary material properties.Then,the transformation between electromagnetic field and mechanical momentum is established by MST.By determining the closed surface surrounding the object and integrating MST on that,the optical force on the object can be obtained.Optical force simulation is usually used to analyze optical tweezers,especially Surface Plasmon(SP)tweezers.SP optical tweezers form remarkable electric field enhancement in local space.The gradient force generated by steep electric field gradient can be used to capture nanoscale particles.Finally,the coupling of Maxwell equation and heat equation in FEM is realized through the photothermal coupling condition.The interaction of multi-physical fields is reflected in the change of material properties and right-end vector in FEM.By updating and iterating the finite element equation,the simulation of multi-physical field coupling can be realized.Furthermore,the frequency domain equation can be rewritten into the time domain form by using the time difference equation to replace the time differential equation approximately,to obtain the relationship between the multi-physical field and time.