Extracting an accurate model for permittivity from experimental data :   Hunting complex poles from the real line
  In this letter, we describe a very general procedure to obtain a causal fit of the permittivity of materials from experimental data with very few parameters. Unlike other closed forms proposed in the literature, the particularity of this approach lies in its independence towards the material or frequency range at stake. Many illustrative numerical examples are given and the accuracy of the fitting is compared to other expressions in the literature. 
Mauricio Garcia-Vergara, Guillaume Dem\'esy and Fr\'ed\'eric Zolla
physics.optics