Figure 3.12 — Dielectric function of gold Figure 3.12 — Dielectric function of gold. Real and imaginary parts. The circles and the crosses represent experimental values taken respectively from Johnson and Christy (1972) and Palik (1985). The solid line represents the fitting of the Drude–Sommerfeld model for free electrons [Eq. (3.65)] to the data reported in Johnson and Christy (1972). This model accurately describes the experimental optical properties of gold in the infrared regime, whereas in the visible region it needs to be supplemented with additional terms to take into account the response of bound electrons.