Elimination of the band gap of a resonant optical material by electromagnetically induced transparency

We consider the possibility of wave propagation in the normally disallowed (band-gap) region of a resonant optical medium, that is, in the band of frequencies near resonance where the real part of the frequency-dependent dielectric function is negative. We demonstrate that wave propagation can become allowed by the application of a strong electromagnetic field resonant with some additional transition of the material system. The frequencies at which these effects can occur are strongly influenced by local-field effects.