Topological Phase Control via Dynamic Complex Pole-Zero Engineering

Abstract: Precise optical phase control is crucial for innovations in telecommunications, optical computing, quantum information processing, and advanced sensing. However, conventional phase modulators often introduce parasitic amplitude modulation and struggle to provide a full 2{\pi} phase shift efficiently. This work introduces a novel paradigm for complete and robust phase control at constant amplitude by dynamically engineering the pole-zero constellation of resonant photonic systems within the complex frequency plane. We theoretically elucidate and validate two distinct approaches: first, by modulating the complex frequency of an excitation signal to trace an iso-amplitude contour (apollonian circle) around a static reflection zero; and second, by dynamically tuning the physical parameters of the resonator such that its reflection zero encircles a fixed-frequency monochromatic excitation, again constraining operation to an iso-amplitude trajectory. Both methods demonstrate the ability to impart a full 2{\pi} phase shift while maintaining a pre-defined, constant reflection amplitude, thereby eliminating amplitude-to-phase distortion. These results leverage the topological nature of phase accumulation around critical points (poles and zeros), a concept gaining significant traction in non-Hermitian and topological photonics.