Fano-modulated chiral metasurface for near-infrared sensing via bound states in the continuum

We report a plasmonic–dielectric metasurface that offers dynamic control over chiroptical properties, adjustable Q-factors, and inter-band chirality manipulation through multiple symmetry-breaking mechanisms. By controlling chiral quasi-bound states in the continuum (BIC) modes and Fano resonances, we enable hybridized and independently tunable spectral responses, offering precise control over resonance amplitude, polarization sensitivity, and refractive index sensing with high flexibility. Chirality is induced by rotating silicon nanofins on a dielectric spacer with a gold backplate, enabling tunable chiroptical responses with controllable spectral linewidth and independent or simultaneous dual-wavelength band excitation. We report a novel phenomenon of controlling interplay of chirality between two closely positioned wavelength bands to tailor symmetry breaking for a specific resonant mode (chiral) while preserving symmetry for the other mode (achiral). Additionally, we report the phase-sensitive evolution of Fano resonance from pure reflectance dip when controlled by dielectric spacer height and further demonstrated phase-insensitive control of Fano resonance amplitude. We divided Fano resonance into a distinct spectral peak and dip to improve light manipulation within the metasurface. Our proposed sensor demonstrates sensitivity and a quality factor of about 435 nm/RIU and 3888, respectively. Furthermore, we compared different phenomena (chiral selectivity, quasi-BIC, Fano) and sensing parameter values for different metasurface configurations (including single nanofin and absence of a spacer) and observed that the configuration without a spacer achieved the highest sensitivity of approximately 640 nm/RIU.