Dual-Band Quasi-Bound States in the Continuum with Opposite Circular Dichroism Supported by Single Metallic Bilayer Twisted Metasurface

The field of optical chirality has achieved remarkable progress with the quick development of artificial metasurfaces. The introduction of quasi-bound states in the continuum (QBIC) into chiral devices offers a groundbreaking and efficient method for modulating chiral responses. However, existing QBIC-based chiral research primarily concentrates on dielectric structures and also encounters challenges such as complex design requirements and limitations to single-frequency band operation. In this work, a metallic double-layer twisted metasurface is proposed. By manipulating a single degree of freedom (DOF), the twist angle, QBIC and chirality are simultaneously induced at two frequency bands. Far-field multipole decomposition and near-field analysis reveal that the two QBICs originated from electric dipole and toroidal dipole moments. Experimental spectra highly agree with simulation results, confirming the efficacy of the design. Remarkably, the structure exhibits opposite chiral responses at different frequency bands. To highlight the device's superiority, a dynamic near-field image display switched by frequency or polarization state is demonstrated in simulation. The proposed structure holds potential applications in chiral enantiomer sensing, chiral lasers, nonlinear filtering, and other related areas.