CT-symmetric Non-linear Topological Defect Modes

Topologically protected states have recently been a subject of increasing interest due to their unique robustness against disorder. Such property is beneficial for a variety of photonics applications ranging from lasers to optical isolators and receiver protectors. On the other hand, an essential piece of information associated with the interplay between optical nonlinearities and topological protection is still missing. Such a question cannot be ignored, since nonlinearity is prevalent in realistic circumstances. Using a non-linear SSH model we have theoretically analyzed the topological nature of an emerging nonlinear defect mode associated with a non-linear defect. To this end, we have devised a theoretical formalism which is based on Green’s functions and prove that for certain type of nonlinearities the nonlinear defect mode is spectrally protected by a charge-conjugation (CT) symmetry. We demonstrate the validity of our study via direct measurements using a CROW SSH array in the microwave domain.