Studies of double negative materials and of metals at optical frequencies have lead to renewed interest in modelling of materials characterised by Drude or multi-pole Lorentz-Drude frequency dispersion. The auxiliary differential equation method of treatment of Drude dispersion in the finite difference time domain method is revisited, and a substantially more accurate formulation is derived.
Introduction:The finite difference time domain technique (FDTD) [1] has been successfully applied by many authors to analyse problems with heterogeneous, complex materials. In the context of this Letter, of particular interest is the ability of FDTD to model materials characterised by frequency dispersive behaviour approximated by Drude or multi-pole Lorentz-Drude models. Luebbers et al.[2] introduced convolution based extensions of FDTD to simulate a variety of dispersive materials. Sullivan [3] and Rappaport [4] incorporated z transform based techniques to extend FDTD to general dispersive media. However, the auxiliary differential equation (ADE) methods lead, in general, to the most efficient numerical implementations, both in terms of memory and operational count. An excellent review of ADE extensions of FDTD is included in [1], where ADE is also used to model Drude media. The approach followed in [1] leads to a fast and memory efficient formulation for Drude, however the second-order nature of the material is partially lost in the derivation, leading to an implementation that has relatively high computational error. In this Letter, we reformulate the Drude ADE method in a manner similar to that introduced by [5]. The new method shows excellent accuracy; however, while it retains the computational speed of the original formulation, it does require one additional memory location per electric field component.