FDTD dispersion revisited: faster-than-light propagation

The numerical dispersion relation that governs the propagation of fields in a finite-difference time-domain (FDTD) grid was derived several years ago. In this work a different interpretation is given for the governing equation. It is shown that the dispersion relation predicts faster-than-light propagation for coarsely resolved fields. Additionally, some spectral components that were previously believed to have zero phase velocity are shown to propagate, albeit with exponential decay.