Relative Orientations of the Poynting and Group Velocity Vectors for Electromagnetic Waves Propagating in an Arbitrary Direction in an Unbounded Bi-Gyrotropic Medium

Using a method based on mathematical operations with Maxwell’s equations in vector form, it is proved that electromagnetic waves propagating in an arbitrary direction in a bi-gyrotropic medium are always characterized by the collinear orientations of the Poynting vector and the group velocity vector. It is shown that the corresponding Cartesian components of these vectors are proportional to each other, which confirms the collinearity of these vectors. It is found that for all types of electromagnetic waves propagating in an unbounded ferromagnetic medium (which is a special case of a bi-gyrotropic medium), these vectors are always codirected.