We present an improved perfectly matched layer (PML) for the analysis of plasmonic structures, based on the manipulation of PML parameters. Two different types of stretched coordinate PML are employed sequentially in the spatial domain: a real stretched coordinate PML to increase the effective buffer space around plasmonic structures and a complex stretched coordinate PML to absorb outgoing waves and terminate the computational domain. Numerical examples show that a significant increase in computational efficiency is obtained because the proposed PML can be placed closer to plasmonic structures than the regular PML without affecting the field distribution of bound modes.

The recently proposed modified PML (MPML) absorbing boundary condition is extended to three dimensions. The performance of the MPML is investigated by FDTD simulation of a typical microstrip line and a rectangular waveguide. The dominant and higher order modes of the microstrip line and the waveguide are excited separately in the computation. In all of the cases of excitation, the reflection properties of the MPML boundaries are examined for the side walls and the end walls, respectively. Various values of the permittivity and permeability of the MPML medium are tested in the computation, and the variation behavior of reflection from the MPML boundaries is examined. The numerical results reveal that by choosing appropriate values of the permittivity and permeability of the MPML, we can realize efficient absorption of both evanescent waves and propagating waves over a wide frequency band.