the light wave is transmitted (the metal becomes transparent), since the electrons in the metal are too slow and cannot respond fast enough to screen the field. Figure 8 shows the complex permittivities of gold, silver, copper, aluminum and nickel. The real part of the metal dielectric function is negative due to free electron contributions. At the resonance frequency of a plasmonic structure, the imaginary part of the metal complex permittivity is positive and plays a dominant role in its absorption loss. Figure 8 shows the electrical permittivity and conductivity of all metals take an exponential variation with frequency.