Conditions for superluminal transmission of evanescent light pulses through optically opaque barriers

The superluminal transmission of evanescent light pulses through optically opaque barriers, is analyzed using simple considerations of causal electrodynamics. By exact numerical calculations, as well as with analytical arguments within the stationary-phase approximation, we show that superluminal transmission occurs whenever the main frequency components of the pulse are confined to frequency regions where the presence of the barrier decreases the density of states of the electromagnetic modes of the system. We also show that these frequency regions correspond to the transmission gaps of wide enough barriers. We discuss a very simple theory for the density of states of the barrier system and compare the results of such a theory with numerical calculations. The results are illustrated with two different models for the barrier, and we find the limits of validity and of occurrence of the phenomenon. We argue that causality is not violated in this type of situations.