Comment on "Negative refractive index in gyrotropically magnetoelectric media"

A recent paper [Shen, Phys. Rev. B 73, 045113 (2006)] studied the possibility of realizing negative refraction with gyrotropically chiral media. Formulations have been provided to prove that the magnetoelectric coupling, either isotropic or anisotropic, would favor the realization of negative refraction. We show that the formulations are incorrect in either case, while the conclusion of realizing negative refraction by gyrotropic chiral media can still be drawn.