Research On Electromagnetic Coating And Absorbing Characteristics Of Targets Based On Transformation Optical FDTD Method

The finite-difference time-domain(FDTD)method has been widely used to solve various electromagnetic problems for modeling complex geometric shapes and nonuniform media targets.However,when the traditional FDTD method simulates multiscale targets which contain both large-scale structure and fine structure,it will occupy large memory and long calculation time.This is because the accurate simulation of the fine structure requires fine space grids throughout the space,which leads to a large increase in the unknown.At the same time,the time step is limited by the CourantFriedrich-Levy(CFL)stable condition.It means that the maximum time step is constrained by the minimum spatial grid size.In the case of fine grids,the time step needs to be small enough to meet the CFL stable condition.Therefore,the calculation efficiency is greatly reduced.Subgridding method is one of the solutions to overcome this problem,but it will bring about late time instability.In this paper,an efficient algorithm based on transformation optics and FDTD method(Transformation Optics-FDTD,TO-FDTD)is proposed to analyze the electromagnetic scattering of thin coating targets of arbitrary shapes and calculate the absorption rate of the periodic absorber based thin graphene layer in the terahertz frequency band,which solves the difficulty of large memory and long calculation time when the traditional FDTD method handles such multi-scale models.The main contents are as follows:(1)The TO-FDTD algorithm is proposed,combined with the NURBS(NonUniform Rational B-Splines)parameter curve,to quickly solve the problem of electromagnetic scattering of thin coating targets.An effective coordinate transformation method is proposed to only increase the thickness of the outermost thin coating layer while keeping the remaining dimensions of the coating target unchanged.At the same time,the parameters of the permittivity and permeability in the transformation space are transformed into non-uniform anisotropic parameters.Therefore,a stable anisotropic FDTD method can be used to simulate the entire computing area in the transformation space with a uniform grid,without the need of fine grids to simulate thin coatings.The problem of the fine grid introduced by the thin coating layer and the reduction of the time step is solved,and the calculation efficiency is effectively improved.(2)For the case where the thin coating layer of target is not located in the outermost layer,applying coordinate transformation only increases the thickness of the inner thin coating layer while keeping the metal target size and other coating thickness unchanged.Although the increase in the total thickness of inner and outer coating layers results in an increase in the surface area of the outermost coating layer,the inverse transformation can be used to obtain the field distribution consistent with the original model after obtaining the field distribution of the model in the transformation space.This conclusion has been demonstrated through the numerical results.The application range of TO-FDTD algorithm in the calculation of electromagnetic scattering of thin coating targets is further expanded.(3)In order to quickly simulate the absorption rate of a periodic absorber with a thin graphene layer in the terahertz frequency band,a method of combining coordinate transformation with periodic boundary conditions is proposed,which only increases the thickness of the thin graphene layer while maintaining the remaining dimensions of the absorber remain unchanged.Numerical examples show that this method greatly improves the calculation efficiency,and provides an efficient calculation method for the electromagnetic simulation of such devices with the thin graphene layer.