Topological Broadband Receiver Protectors Based on High-Order Degeneracies Enabled by Hidden Symmetries

Recently, we demonstrated the functionality of a nonlinear lossy defect embedded in a Su-Schreiffer-Heeger (SSH) chain as a photonic receiver protector. The transmission of low incident powers is supported by the topologically protected defect mode existing in the gap which is destroyed by the nonlinear perturbation triggered by high incident power. The drawback of this design is the narrow bandwidth associated with the transmission resonance for low incident powers.

Since then, a theory of reflectionless scattering modes (RSMs) has been developed. RSMs are eigenfunctions of a wave operator that describes an auxiliary system. When an RSM is excited at the appropriate eigenfrequency, no power will be reflected at the input ports. When multiple RSMs degenerate, they enforce an anomalous broadened resonance line shape.

We have theoretically constructed a class of one-dimensional coupled resonator microwave waveguides by coupling two single-defect SSH chains; one of which is nonlinear. For low incident powers, the associated auxiliary RSM operator simultaneously respects an anti-linear symmetry and an anti-linear anti-symmetry. Together, these symmetries impose strong constraints on the RSMs, enabling them to display an exceptional point degeneracy of order 6^th and results in an experimentally observable transmission bandwidth broadening. For high incident powers, the nonlinear mechanism kicks in, leading to a self-induced violation of these symmetries and a suppression of the transmittance.