Non-Hermitian CT-Symmetric Spectral Protection of Nonlinear Defect Modes

We investigate, using a microwave platform consisting of a non-Hermitian Su-Schrieffer-Heeger array of coupled dielectric resonators, the interplay of a lossy nonlinearity and CT-symmetry in the formation of defect modes. The measurements agree with the theory which predicts that, up to moderate pumping, the defect mode is an eigenstate of the CT-symmetric operator and retains its frequency at the center of the gap. At higher pumping values, the system undergoes a self-induced explicit CT-symmetry violation which removes the spectral topological protection and alters the shape of the defect mode.