Modelling the dielectric function of Au-Ag alloys

Two analytical models are proposed to represent the dielectric function of the AgxAu1-x alloys. The intraband effects are represented by the Drude model whereas interband transitions are modelled by means of either Lorentzian-pole pairs or critical-point terms. Quantitative comparisons are presented to show that, for all the alloy compositions, both models describe very well the experimental data reported by Peña-Rodríguez et al. [Opt. Mater. Express 4 (2014) 403–410] in the energy region between 0.65 and 6.6 eV. Accuracy of the critical-point terms is considerably better but Lorentzian-pole pairs can be easier to integrate in existing FDTD software.