Localization Manipulation of Skin and in‐Gap States in Nonreciprocal Microring Waveguide Array

ABSTRACT We investigate the localization manipulation of skin and in‐gap states in a one‐dimensional nonreciprocal microring coupled‐resonator optical waveguide array, where topological features of non‐Hermitian phases emerge through modulated next‐nearest‐neighbor hopping. The non‐Hermitian skin effect demonstrates both bidirectional and unidirectional localization behaviors within specific parameter regimes of next‐nearest‐neighbor hopping, as evidenced by the wave function distribution of the system. This distinctive localization facilitates an effective separation between in‐gap states containing zero and nonzero energy edge states from bulk states, establishing a practical approach for the experimental observation of non‐Hermitian topological phases. Through disorder analysis, we reveal the counterintuitive phenomenon that the localization of nonzero energy edge states strengthens with increasing next‐nearest‐neighbor disorder intensity. Additionally, we characterize skin transitions via spectral winding number calculations under periodic boundary conditions and construct a comprehensive skin phase diagram. Our findings not only provide a robust framework for realizing stable and distinguishable topological phases but also pave the way for investigating other complex non‐Hermitian topological phenomena.