ABSTRACT
Material properties can be determined from the real-time response of the system. For example, for a dielectric, the dipoles constituting the medium take finite time to respond to the quickly oscillating radiation (∼ 1015 Hz in optical domain) passing through the dielectric. We present a brief sketch of the linear response function theory in order to understand the frequency dependence of the dielectric function ǫ(ω). We discuss this for ǫ(ω), while similar arguments can be developed for magnetic response. While the details were worked out for the Lorentz model of the dielectric in Section 2.3.1, here we develop the response for metals (known as the Drude model). In dealing with the susceptibilities, we pay due attention to causality and its manifestation in the form of Kramers-Kronig relations.
