Exceptional Bound States in the Continuum in a Passive Terahertz Metasurface

Exceptional bound states in the continuum (EP-BICs) are hybrid singularities in non-Hermitian systems that uniquely combine the non-radiating characteristic of bound states in the continuum (BICs) with the enhanced sensitivity of exceptional points (EPs). However, their experimental realization remains challenging because existing proposals typically require precise external excitation to control the loss and coupling strength. Here, we experimentally demonstrate an EP-BIC in a fully passive terahertz metasurface. Through simple geometric rotation, we modulate the internal coupling of two resonator modes to drive an isolated EP to coalesce with a line of BICs and finally form an EP-BIC, as clearly traced by transmission spectra measurements. Near-field terahertz scanning further observes the uniform phase distribution of EP-BIC, confirming its decoupling from incident light. Finally, the sensor performance of EP-BIC is demonstrated in water mist detection and exhibits a distinct dip response that enables unambiguous signal identification, in contrast to the weak and spectrally crowded peak of conventional EP. Our work establishes a practical and robust platform for EP-BICs, opening avenues for ultra-sensitive photonic sensing.