Abstract
A well-known principle in optical physics states that power cannot be exchanged between two light waves propagating inside linear homogeneous media. Power exchange between light waves usually occurs with the aid of nonlinearity. Here, we show that this limitation can be overcome using metamaterials. In particular, 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. Our analysis reveals that the magnetic field component of the second wave can now be amplified inside the magnetic metamaterial. We show that the amplification of the magnetic field occurs in frequency regions where the real part of the metamaterial's effective permeability is negative. This type of magnetic amplification, which we call 'metamaterial parametric amplification', is similar in consequence to nonlinear two-beam coupling or optical parametric amplification but is linear and uniquely occurs in metamaterials.