Résumé
Several aspects of photonics, such as laser cavities, random lasing, plasmonics, strong coupling with matter etc. requires the description of a medium exhibiting gain. A very simple model consists in using an effective permittivity with a sign changing imaginary part, as compared to a non-active material. Using this model, we investigate the possibility of controlling the closing or opening of a band-gap inside a dielectric metamaterial. Further, we question the model itself and show that it leads to non-physical effects. The conclusion is that the model is not reliable for more than a short time description of the physics at stake.