On the propagation speed of evanescent modes

The group-velocity of evanescent waves (in undersized waveguides, for instance) was theoretically predicted, and has been experimentally verified, to be Superluminal (v_g > c). By contrast, it is known that the precursor speed in vacuum cannot be larger than c. In this paper, by computer simulations based on Maxwell equations only, we show the existence of both phenomena. In other words, we verify the actual possibility of Superluminal group velocities, without violating the so-called (naive) Einstein causality. (Subject classes: General physics, Classical physics, Optics, Special Relativity; PACS nos.: 73.40Gk, 03.80+z, 03.65Bz; Keywords: evanescent waves; tunnelling photons; Hartman effect; group velocity; Superluminal waves; precursors; transient waves; front velocity; Maxwell equations; electromagnetic waves; computer simulations; Special Relativity; Extended Relativity).