On reflection from a half-space with negative real permittivity and   permeability: Time-domain and frequency-domain results
  The reflection of a normally incident wideband pulse by a half-space whose permittivity and permeability obey the one-resonance Lorentz model is calculated. The results are compared with those from frequency-domain reflection analysis. In the spectral regime wherein the real parts of the permittivity and the permeability are negative, as well as in small adjacent neighborhoods of that regime, the time-domain analysis validates the conclusion that it is sufficient to ensure that the imaginary part of the refractive index is positive - regardless of the sign of the real part of the refractive index. 
Jianwei Wang and Akhlesh Lakhtakia