We investigate the propagation of an electromagnetic (EM) wave in a heterostructure formed by spatially modulated density of $\Lambda$ three-level atoms. A new regime of propagation and amplification of EM waves with frequencies lying in the forbidden frequency range is found. An application of this phenomenon is discussed.

• Received 22 December 1997

DOI:https://doi.org/10.1103/PhysRevA.57.4919