A stable FDTD subgridding method for both spatial and temporal spaces

The proposed FDTD subgridding method is stable in both spatial and temporal spaces. Late-time instability has been resolved by separating the space and time interpolation interfaces from conventional collocated to same position. Numerical experiments conducted in 2D of 10 million time steps have no instability problem after a minimum gap distance for various refinement factors. Compare to existing subgridding methods the new method is not restricted to a single refinement factor and need no special treatment at material interfaces. This should open the FDTD subgridding method to more applications.