Reconfigurable chiroptical metasurface sensors enabled by bound states in the continuum

Chiral metasurfaces provided a promising platform for realizing high-Q chiral resonances and narrow-band circular dichroism (CD), substantially improving chiral sensing performance. In this study, we introduce a Z-shaped metasurface with C₂ symmetry. By incorporating a single asymmetry parameter, bound states in the continuum (BIC) are converted into chiral quasi-BICs (q-BICs), enabling strong chirality selectivity without additional structural complexity. The proposed design extends the operational spectrum of chiral and refractive index sensing from visible and near-infrared to mid-infrared, achieving both intrinsic and extrinsic transmissive CD values exceeding 0.91 and 0.99. Notably, under in-plane and out-of-plane symmetry breaking, the sensor attains sensitivities of 84.7 nm/RIU (refractive index unit) and 86 nm/RIU with corresponding figure of merit (FOM) of 1,700 and 376, respectively. Furthermore, exploiting the phase-change behavior of Ge₂Sb₂Te₅ (GST-225), the structure allows dynamic resonance wavelength tuning, significantly improving operational adaptability without geometric modifications.