Metamaterials with negative refraction and some relativistic effects

The process of transfer of radiation from the emitter to the receiver is considered. It is shown that the mass transferred by radiation is not always associated with portable energy by Einstein's relation $E=mc^2$. A more general relation has the form $E=mV_{gr}V_{ph}$ and includes the dependence on the phase and group velocities of the radiation. This relation implies that in case of a negative refractive index, when the phase and group velocities in opposite directions, the mass is transferred from the receiver to the source, but not vice versa, as usual.