Intrinsic abnormal electromagnetic media at optical frequencies based on resonant transitions of electrons

Abnormal electromagnetic media have attracted extensive attention due to their novel electromagnetic properties in comparison with ordinary materials. Abnormal media based on periodic resonant structures (metamaterials) have the disadvantages of complicated fabrication and high loss at optical frequencies, while media based on the intrinsic properties of natural materials exhibit distinct advantages. In this paper, the mechanisms of three kinds of intrinsic abnormal electromagnetic media are described: strong anisotropy, the coupling between spin-waves and plasma, and the resonant transitions of electrons; the third is discussed in detail. Semi-classical theory is used to analyze the electric and magnetic dipole transitions in a gas medium with two or more energy levels, and the expressions for the polarizability, magnetic susceptibility, and abnormal electromagnetic properties in these models are presented. In addition, the relationships between the electronic resonant transitions and extraordinary electromagnetic properties in condensed matter are outlined. Finally, a brief analysis of how to achieve this type of intrinsic abnormal medium experimentally is given.