is the imaginary part of the permittivity, which is related to the dissipation (or loss) of energy within the medium. When the real part is positive, the phase shift is positive and the material is said to have an intrinsically capacitive (current leads voltage) optical response. When the real part is negative, the phase shift between electric and magnetic fields is negative and the material is said to have an inductive (voltage leads current) optical response. Using the same analogy, we can say that when the imaginary part is