ABSTRACT
In the case of negative real part of ε and positive μ (second quadrant), the corresponding materials are called epsilon-negative (ENG) media; this quadrant includes all the materials able to provide plasmonic e«ects (e.g., noble metals at optical frequencies). In particular, noble metals possess large negative permittivity (real part) in a wide frequency band, and usually quite large imaginary part (i.e., high losses). ¡e permittivity of metals at optical frequencies can be described, for example, by the Drude model [19-24] ε ε ω ω ω γm p i= − +∞ 2 /[ ( )], where ε∞ is a high-frequency ¢tting parameter, ωp is the plasma frequency of the metal (expressed in radian per second), and γ is the damping factor (expressed per second). ¡ese parameters can be chosen to match experimental data, such as the ones reported in [24,25]. However, the Drude model may underestimate the metal losses compared to those in [24,25] in some frequency range, where more sophisticated models may be adopted to obtain better agreement (see for example [26-28]). In the case of positive real part of ε and negative μ (fourth quadrant), the corresponding materials are called mu-negative (MNG) media. Notice that the media pertaining to the second and fourth quadrants are de¢ned together as single negative (SNG) media, and that electromagnetic waves are evanescent inside SNG media. In case the real part of either ε or μ is near zero, they are called epsilon near zero (ENZ) or mu near zero (MNZ) media, respectively. Also, if both ε and μ are equal to zero, then they are called zero index media [29]. It is noteworthy that SNG media can be designed in an easier way than DNG media.
