  Electromagnetic energy backflow is a phenomenon occurring in regions where
the direction of the Poynting vector is opposite to that of the propagation of
the wave field. It is particularly remarkable in the nonparaxial regime and has
been exhibited in the focal region of sharply focused beams, for vector Bessel
beams, and vector-valued spatiotemporally localized waves. A detailed study is
undertaken of this phenomenon and the conditions for its appearance are
examined in detail in the case of a superposition of four plane waves in free
space, the simplest electromagnetic arrangement for the observation of negative
energy flow, as well as its comprehensive and transparent physical
interpretation. It is shown that the state of polarization of the constituent
components of the electromagnetic plane wave quartet determines whether energy
backflow takes place or not and what values the energy flow velocity assumes.
Depending on the polarization angles, the latter can assume any value from c
(the speed of light in vacuum) to -c in certain spatiotemporal regions of the
field.
