Enhancement and Sensing Performance of Ultra-Narrowband Induced Circular Dichroism in Achiral Nanohole Arrays Based on Bound States in the Continuum

Chiral micro- and nanostructures exhibit strong circular dichroism (CD), demonstrating significant research and engineering potential in chiral molecular recognition, negative refraction material design, and polarization-selective imaging techniques. However, designing chiral structures with powerful CD, extremely narrow bandwidth, and dynamically adjustable performance remains challenging. This work introduces bound states in the continuum (BIC) and magneto-optical effects into achiral hexagonal nanopore arrays (AHNAs), investigating their CD properties under external magnetic fields. Under an applied magnetic field, the symmetric protected BICs within AHNAs are excited into Q-BICs, inducing two remarkably narrow and distinct CD signal responses in the near-infrared region. These signals exhibit a minimum full width at half-maximum of only 1.83 × 10 –5 nm and a maximum Q-factor of 10 7 . Tilting the structure along the x -axis further significantly enhances the CD signal (approaching 1). The formation mechanism of the magnetically induced CD signal is elucidated by combining quantum optical models with analyses of magnetic field and current distributions. The CD response of AHNAs is highly sensitive to structural geometry and can be dynamically modulated by varying the applied magnetic field strength. Additionally, the AHNAs achieve a maximum figure of merit of 10 6 in different refractive index environments and a CD enhancement factor of 10 6 in chiral molecules. These findings not only provide valuable insights for designing ultranarrowband chiral micro- and nanostructures but will also advance their applications in chiral sensing, asymmetric catalysis, and polarization conversion.