Comment on "Total Negative Refraction in Crystals for Ballistic Electrons and Light"

In Phys. Rev. Lett. 91, 157404 (2003) Yong Zhang et al. study light propagation through an interface between two identical but differently oriented uniaxial crystals. The authors show that the light refraction at the interface at some region of incident angles is negative. The observed effect of negative refraction resembles the analogous effect in left-handed materials (LHM) although the physics is different. In LHM the negative refraction is caused by a negative group velocity, whereas in the considered case one deals with the special properties of wave propagation in uniaxial anisotropic media.