Optical structures with local PT-symmetry

We propose a new class of optical synthetic materials that are described by non-Hermitian Hamiltonians. The building blocks of such systems are coupled PT-symmetric elements (dimers), with coupling t. Despite the lack of global PT-symmetry, these systems have a robust parameter region of real spectra (exact phase) even in cases where the complex refractive index n = beta + i gamma of each PT dimer is random. The validity of our proposition is confirmed for representative cases where we calculate the borders of the exact phase in terms of beta, gamma and t.