On a causal dispersion model for the optical properties of metals

A causal model that is fully compliant with the Kramers-Kronig relations (KKRs) is used for characterizing the optical response of metals exhibiting sharp, non-Lorentzian regions of interband absorption. This consistent model also recovers the Gaussian character of the broadened peaks observed in the infrared response of amorphous materials. A reductive procedure for optimal fitting of KKR-compliant composite models to the permittivity data of metals over the intraband and interband energetic regimes is introduced and applied to eleven metals: Ag, Au, Cu, Al, Be, Cr, Ni, Pd, Pt, Ti, and W. We show that results obtained using this optimal procedure outperform-both qualitatively and quantitatively-those obtained by previous non-causal (KKR non-compliant) models that are widely used for approximating the permittivity of metals. A comparative analysis reveals a simultaneous increase in performance (as characterized by the error objective) and a reduction in the cardinality of the parameterization, often yielding more physically meaningful interpretations. These results are obtained without compromising model fidelity in regions of sharp Gaussian interband character, indicating that the proposed model provides an excellent alternative to previously proposed models.