Gain materials in MEEP

[ovidiopr]

Hi,

I am trying to use MEEP to simulate a SPASER, but I have not been able to figure out how to correctly use materials with gain. Namely, I am trying to reproduce these results: Nature 460 (2009) 1110-1112. Could you please explain me (or tell me where I can find the explanation) how to convert from "real world" units to MEEP units the parameters necessary to represent a 4-level material?

Thanks in advance,

Ovidio