Perfect broadband invisibility in isotropic media with gain and loss

We outline a method for constructing effectively two-dimensional isotropic optical media that are perfectly and omnidirectionally invisible for both TE and TM waves provided that their wavenumber does not exceed a preassigned value $\alpha$. This relies on the observation that a complex scattering potential $v(x,y)$ displays perfect invisibility for wavenumbers $k\leq\alpha$ provided that its Fourier transform with respect to $y$, which we denote by $\tilde v(x,\mathfrak{K}_y)$, vanishes for $\mathfrak{K}_y\leq 2\alpha$. We use this result to construct the permittivity profile for a realistic optical slab that is perfectly invisible in the wavenumber window $[0,\alpha]$.