Higher-order chiral exceptional points in asymmetric photonic molecules

Exceptional points (EPs), representing unique non-Hermitian degeneracies in open systems, have been the focus of significant research due to their remarkable phenomena and applications across various fields. While second-order EPs are extensively studied, higher-order EPs—requiring the degeneracy of multiple eigenvalues and corresponding eigenvectors—remain less explored. In this work, we propose a generic approach for realizing, 2N^th-order EP in an N-element photonic molecule (PM). Differing from a common PM consisting of identical microresonators, here a spiral-shaped microresonator is incorporated to form an asymmetric PM, offering unidirectional coupling between eigenmodes and transferring the chirality throughout the whole PM. Moreover, the hierarchical construction of 2N^th-order EPs can be achieved in gain-loss asymmetric PMs, as well as in all-passive ones. In the prototype demonstration based on numerical simulations, the obtained fourth-order EP reveals its superior sensitivity with a three-fold enhancement compared with that of a second-order EP to external perturbations at the single-molecule level. We envisage that the proposed asymmetric PM, circumventing the need for external components or precise control of gain-loss profiles, will pave the way for constructing higher-order EPs in integrated photonic devices and enabling applications in both linear and nonlinear regimes.