  Approximating the frequency dispersion of the permittivity of materials with
simple analytical functions is of fundamental importance for understanding and
modeling the optical response of materials and resulting structures. In the
generalized Drude-Lorentz model, the permittivity is described in the complex
frequency plane by a number of simple poles having complex weights, which is a
physically relevant and mathematically simple approach: By construction, it
respects causality represents physical resonances of the material, and can be
implemented easily in numerical simulations. We report here an efficient method
of optimizing the fit of measured data with the Drude-Lorentz model having an
arbitrary number of poles. We show examples of such optimizations for gold,
silver, and copper, for different frequency ranges and up to four pairs of
Lorentz poles taken into account. We also provide a program implementing the
method for general use.
