Comment on "Resolving the wave vector and the refractive index from the coefficient of reflectance".

In a recent Letter, the reflectance coefficient was used to resolve the sign choice of the wave vector and refractive index in active media. We argue that such a coefficient loses its physical meaning for active media (at real frequencies) when the field amplification is limited only by gain saturation. In this case, the amplitude reflectance coefficient leads to fictitious noncausal reflected fields when the backward Fourier transform is used.