How much can we trust dielectric permittivity ?

By closing a paper based on dielectric permittivities of metals, I realized how much these optical data are badly known. Despite of the extraordinary efforts of Palik and co-workers to collect all together a large amount of good optical data, it appears that no study is able to provide "good" enough values of dielectric permittivity for plasmonic applications. The best would be to verify experimentally optical data of your sample before using it... but who can afford to master this technique AND to perform experiments in his own field?

In addition, as we irradiate the matter with intense lasers, I realize how much we actually change the optical refractive index of matter as a function of temperature, excited electron density, etc...

Everything can become really blurry. And these coefficients drive the linear optical absorption ? How can we model them accurately with so many changes?

Once again, the more we get to know, the more we realize that we actually knew nothing ....

Another solution would be to calculate them from first principles - what I'm planning to do quite soon - but for band gap materials, these optical coefficients rely on the fact that we can properly calculate the electronic band structure.

1. The gap of band gap materials is known to be almost impossible to reproduce from first principles.

2. It was recently found that finding this information is actually a non-decidable mathematicam problem (http://www.nature.com/nature/journal/v528/n7581/pdf/nature16059.pdf).

An exception might be found in my future works at the Max Planck Institute, maybe? Time will tell.