  A summation-by-parts simultaneous approximation term (SBP-SAT)
finite-difference time-domain (FDTD) subgridding method is proposed to model
geometrically fine structures in this paper. Compared with our previous work,
the proposed SBP-SAT FDTD method uses the staggered Yee's grid without adding
or modifying any field components through field extrapolation on the boundaries
to make the discrete operators satisfy the SBP property. The accuracy of
extrapolation keeps consistency with that of the second-order finite-difference
scheme near the boundaries. In addition, the SATs are used to weakly enforce
the tangential boundary conditions between multiple mesh blocks with different
mesh sizes. With carefully designed interpolation matrices and selected free
parameters of the SATs, no dissipation occurs in the whole computational
domain. Therefore, its long-time stability is theoretically guaranteed. Three
numerical examples are carried out to validate its effectiveness. Results show
that the proposed SBP-SAT FDTD subgridding method is stable, accurate,
efficient, and easy to implement based on existing FDTD codes with only a few
modifications.
