@article{ISI:000234528900006,
  title = {Sign of the refractive index in a gain medium with negative permittivity and permeability},
  journal = {JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS},
  abstract = {We show how the sign of the refractive index in any medium may be derived using a rigorous analysis based on Einstein causality. In particular, we consider left-handed materials, i.e., media that have negative permittivities and permeabilities at the frequency of interest. We find that the consideration of gain in such media can give rise to a positive refractive index. (c) 2006 Optical Society of America.},
  volume = {23},
  number = {1},
  pages = {45-50},
  year = {2006},
  slug = {isi-000234528900006},
  author = {Chen, YF and Fischer, Peer and Wise, FW}
}