Figure shows a graphical representation of the reflectance minimum, its resonance linewidth in wavenumber units, and resonance center wavenumber extracted from the experimental spectral data. In this analysis, the resonance linewidth was the difference between the maxima and minima of the first derivative of the resonance spectrum. Although the mathematical formula shows that the difference between the maxima and minima of the derivative is slightly larger than the half-width in a bell-shaped function system, this has a great advantage for the analysis since it can be performed with uniform accuracy, including the asymmetric bell-shaped experimental results. The projections of the 3-D plot (Fig. ) are plots of the relationships between the reflectance and resonance linewidth, resonance center wavenumber, resonance linewidth, and reflectance vs. resonance center wavenumber, respectively. The results show that the relationship between the resonance center wavenumber and resonance linewidth is narrower for all the metals at the lower wavenumbers. Furthermore, the resonance line widths of Au, Ag, and Cu narrowed with decreasing reflectivity, while those of HEA, Pt, and Pd widened at low reflectivity conditions. For the SiO 2 films of 100 nm and 200 nm thickness, the same tendency to narrowing of the resonance linewidth was observed as for to Drude-Lorenz model ( j = 1, 2). For Pt and HEA, data can be well fitted with a single Lorenz model ( j = 1).