The Linear Dielectric Susceptibility

InLinear dielectric susceptibility electric dipole approximation, the interaction of electromagnetic radiation with matter results into the formation of an induced electric polarization of the medium. Then, linear optical material properties of a non-magnetic, homogeneous and isotropic medium are completely defined by the complex linear dielectric susceptibility. Its dispersion, i.e. its frequency dependence arises from fundamental physical principles. From the susceptibility, the frequency-dependent complex dielectric function as well as the complex index of refraction may be calculated straightforwardly. The real part of the complex index of refraction (the usual refractive index) and the imaginary part (the extinction coefficient) form the pair of linear optical constants of the medium. The refractive index defines the phase velocity of the electromagnetic wave in the medium, while the extinction coefficient is responsible for the description of damping effects.