Reconfigurable Quasi‐BIC Terahertz Metasurfaces Through MEMS‐Induced Symmetry Breaking

Abstract Symmetry‐protected bound states in the continuum (BICs) support high‐quality factor (Q) resonances. As a result, realizing tunable devices based on these states requires approaches that break the symmetry. Tunable mode leakage in terahertz BIC metasurfaces is demonstrated through structural symmetry breaking of an in‐plane mirror symmetry. Specifically, a leaky quasi‐BIC mode is created through the introduction of lateral asymmetry of the in‐plane resonator geometry and through asymmetric out‐of‐plane tilting using MEMS cantilever actuation. This provides fine‐tuned and reconfigurable control of the radiative leakage. Although out‐of‐plane deformation can, in principle, induce a BIC‐to‐quasi‐BIC transition, the study focuses on quasi‐BIC‐to‐quasi‐BIC modulation in order to clearly demonstrate leakage control and to facilitate modal analysis. Experimental measurements supported with full‐wave simulations and coupled‐mode theory (CMT) reveal distinct leakage and mode behavior for in‐plane and out‐of‐plane symmetry breaking. Importantly, control experiments using symmetrically tilted cantilevers confirm that radiative leakage to the far‐field arises from symmetry breaking rather than deformation alone. The dual symmetry‐breaking approach enables control over radiative and intrinsic loss through in‐plane and out‐of‐plane symmetry breaking, providing a robust and scalable route toward reconfigurable high‐Q terahertz metasurfaces.