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Perturbative Unidirectional Invisibility

We outline a general perturbative method of evaluating scattering features of finite-range complex potentials and use it to examine complex perturbations of a rectangular barrier potential. In optics, these correspond to modulated refractive index profiles of the form $n(x)=n_0+f(x)$, where $n_0$ is real, $f(x)$ is complex-valued, and $|f(x)|\ll1\leq n_0$. We give a comprehensive description of the phenomenon of unidirectional invisibility for such media, proving five general theorems on its realization in ${\cal PT}$-symmetric and non-${\cal PT}$-symmetric material. In particular, we establish the impossibility of unidirectional invisibility for ${\cal PT}$-symmetric samples whose refractive index has a constant real part and show how a simple scaling transformation of a unidirectionally invisible ${\cal PT}$-symmetric index profile with $n_0=1$ may be used to generate a hierarchy of unidirectionally invisible ${\cal PT}$-symmetric index profiles with $n_0>1$. The results pertaining unidirectional invisibility for $n_0>1$ open up the way for the experimental studies of this phenomenon in a variety of active material. As an application of our general results, we show that a medium