The condition of a propagating surface plasmon wave in a metal film is that the refractive indexes of the metal and surrounding medium are opposite and equal (please correct me!) but this requires that the refractive index of, normally, the metal is negative.
A common example given is that of a gold thin film and the well known Johnson and Christy graph showing the real part of gold refractive index in the optical regime and towards IR too being negative.
When you look up a value for the refractive index of gold, in the same regime, it is positive - between 1 and 3.
How can these two facts be true simultaneously?