Unified causal theory of photonic superluminal delays in dielectric structures

Recent experimental results imply that a single-photon Gaussian wavepacket, which 'tunnels' through a periodic dielectric structure can emerge from the structure as a nearly-undistorted Gaussian wavepacket centered at a much earlier time than its twin-photon wavepacket that has propagated the same distance through air. These results may be viewed as an indirect proof that massive particles tunneling through potential barriers should exhibit 'superluminal' traversal times, by virtue of their analogy to evanescent EM waves in dielectric structures. Here we aim to elucidate the physical origin of such superluminal features by introducing a comprehensive causal description of evanescent EM wavepacket (pulse) propagation in two types of structures that have been studied experimentally: multilayered dielectric structures and waveguides partly filled with a dielectric.