"Superluminal" transmission of light pulses through optically opaque barriers

Using simple considerations of causal electrodynamics we analyze the occurrence of superluminal transmission of light pulses through optically opaque barriers. We find that the phenomenon appears 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 sufficiently wide barriers. We discuss a simple theory for the density of states of the barrier system and compare the results of such a theory with exact numerical calculations.