Physical realization of $\mathcal{P}\mathcal{T}$-symmetric potential scattering in a planar slab waveguide

A physical realization of scattering by $\mathcal{P}\mathcal{T}$-symmetric potentials is provided: we show that the Maxwell equations for an electromagnetic wave travelling along a planar slab waveguide filled with gain and absorbing media in contiguous regions, can be approximated in a parameter range by a Schr"odinger equation with a $\mathcal{P}\mathcal{T}$-symmetric scattering potential.