Electric Quadrupole–Driven Quasi‐BIC Resonances in Terahertz Free‐Standing Metasurface

We report on the design, fabrication, and experimental characterization of a free‐standing terahertz metasurface supporting quasi‐bound states in the continuum (quasi‐BICs). The metasurface, realized by laser cutting double‐slot apertures in a thin brass foil, exhibits a tunable transition from symmetry‐protected BICs to sharp Fano‐type resonances when structural asymmetry is introduced. Terahertz time‐domain spectroscopy measurements confirm high‐Q resonances in good agreement with simulations. Multipole decomposition reveals that the trapped mode response is dominated by the in‐plane electric quadrupole component, which governs the confinement and radiation properties of the quasi‐BIC. The free‐standing design eliminates substrate‐induced losses and opens a robust route toward versatile, high‐performance terahertz devices. These findings demonstrate the potential of quasi‐BIC metasurfaces as compact, tunable high‐Q resonators for applications in sensing, filtering, and terahertz photonics.