# Enhancing of EM waves through a materal or waveguide

I'm wondering whether there are materials for which an incident EM wave would behave as $\vec E(z,t)=\vec E_0 e^{\kappa z}e^{i(kz-\omega t)}$ where z describes how far from the surface the fields are and $\kappa>0$. Non perfect conductors for example, would have a similar dependency except for the sign in front of $\kappa$, which makes evanescent waves. While in the case I'm looking for, we'd have "enhanced waves".

I also wonder the same thing for waveguides. For example if we consider a rectangular waveguide as well as a TM mode whose frequency is lower than the cutoff frequency, then that TM mode cannot propagage inside the waveguide, the dependency of that mode with respect to "z" (direction alongside the waveguide) is a decaying exponential which is exactly the same than an EM field in a metal, I believe. So I wonder whether there are materials which would make up the waveguide such that the EM fields would get enhanced, at least on a "small distance" (say over a few wavelengths). If so, what kind of materials are they?