Modeling of near-field sources in the finite-difference time-domain (FDTD)

The FDTD method has been used extensively in electromagnetic field modeling because of its ability to handle interactions with complex heterogeneous structures robustly. The method has attained success in scattering and absorption predictions restricted to using uniform plane wave sources. There are many cases where the object of interest is located in the radiating near-field of the source, where plane wave approximations are not appropriate. We show how near field sources can be efficiently implemented as spherical waves in the total/scattered FD´I´D method. Given a spherical wave representation for a source, that source can be initiated in the computational domain by time-stepping modal amplitude functions on alternate 1-dimensional radial FDTD grids. By interpolating these modal amplitude functions with stored angular functions, a full set of spherical waves can be initiated on the Huygens surface. To develop the method fully, absorbing boundary conditions must be developed, the method must be synthesized with an existing FDTD program and we must validate the method with a canonical problem involving multiple modes. It should be noted that this paper represents a preliminary validation of the method.