Tunable Efficient Doubly Degenerate Dark State and Symmetry Breaking in Non‐Hermitian Metasurface

ABSTRACT Both bound states in the continuum (BICs) and electromagnetically induced transparency (EIT) arise from the destructive interference of two radiative modes in the non‐Hermitian system, yet their fundamental mechanism distinction remains unclear. Additionally, the merging BIC formed by multiple BICs also lacks a mechanistic non‐Hermitian interpretation, and its implementations are confined to the momentum space of photonic crystal slabs, hindering dynamic modulation. Herein, we theoretically establish that designing third‐order Hamiltonian eigenmodes of a graphene‐based non‐Hermitian metasurface enables the merging BIC to be intrinsically considered as a doubly degenerate dark state with orthogonal eigenstates. The resulting merging BIC exists in the Fermi level‐frequency space, distinct from the traditional merging BIC in momentum space, exhibiting dynamic tunability across the full terahertz regime via precise Fermi level modulation. We further demonstrate that breaking eigenmode symmetry prevents complete destructive interference of two radiative modes, thereby converting the BIC into EIT and elucidating their mechanistic relationship. Moreover, by controlling this transition, a hybrid state EIT‐quasi‐BIC (EIT‐q‐BIC) is realized, inheriting the strong dispersion (group delay: 24.61 ps) of EIT and a high quality factor (Q‐factor) (626.1) of BIC. This work establishes a tunable non‐Hermitian framework for biosensing platforms, slow‐light, and next‐generation communication technologies.