  A well-known principle in optical physics states that power can never be
exchanged between two light waves propagating inside a homogeneous medium when
the medium response is strictly linear. Power exchange between light waves
usually occurs with the aid of nonlinearity. A typical example is nonlinear
optical parametric amplification where net energy can flow from a
high-frequency (high-energy) light wave (pump) to a lower-frequency signal
wave, leading to amplification of the signal wave. Here, we show that this
limitation of ordinary media can be overcome using suitably engineered
metamaterials, which in the recent past have enabled a variety of extraordinary
applications, unattainable using conventional materials, such as perfect
lenses, invisibility cloaks and trapped rainbows. We introduce a blueprint for
magnetic metamaterials that enables, in a totally linear fashion, coherent and
constructive active power flow from one light wave to another (second) light
wave.
