Figure 1.

Subfigures (a) and (b), black circles: the measured real $n^{\prime }$ and imaginary parts $n^{″}$ of index of refraction n for gold and silver [34] (Kramers-Kronig relations involved) and (c) and (d): the corresponding real and imaginary parts of the “experimental” function ${\epsilon }^{exp}\left(\hslash \omega \right)={[n^{\prime }\left(\hslash \omega \right)+i{n}^{″}\left(\hslash \omega \right)]}^2$. Real and imaginary parts of ${\epsilon }^{free}\left(\hslash \omega \right)$, ${\epsilon }^{eff}\left(\hslash \omega \right)$ and ${\epsilon }_m\left(\hslash \omega \right)$ (Equations (1)–(3)) and the corresponding real and imaginary parts of the indexes of refraction fitted to both ${\epsilon }^{exp}\left(\hslash \omega \right)$ and $n\left(\omega \right)$ are presented with red, green and blue lines correspondingly. Vertical lines mark the threshold energies for interband transitions for gold [50,52] and silver.