@article{ISI:000234528900006, title = {Sign of the refractive index in a gain medium with negative permittivity and permeability}, author = {Chen, YF and Fischer, Peer and Wise, FW}, journal = {JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS}, volume = {23}, number = {1}, pages = {45-50}, year = {2006}, doi = {10.1364/JOSAB.23.000045} }