Observation of Giant Extrinsic Chirality Empowered by Quasi-Bound States in the Continuum

Without using intrinsic chiral metamaterials with complex structures, here we use achiral metasurfaces to realize giant extrinsic chirality empowered by quasi-bound states in the continuum (quasi-BICs). The giant extrinsic chirality in the proposed achiral metasurface is mainly attributed to the breaking of the mirror symmetry of the whole system, combining the metasurface with incident waves. The transmittance of two different polarization states and the signs of circular dichroism (CD) can be arbitrarily controlled by tilting the plane of the metasurface relative to the incoming waves. Remarkably, we simultaneously select two polarization states at different bands for a certain achiral metasurface based on the symmetry-protected dual bound states in the continuum (BICs). The measured maximum CDs approach \ifmmode\pm\else\textpm\fi{}0.85. In addition, an application of the polarization-multiplexed-field image display assisted by chiral quasi-BICs is demonstrated. This work provides a general method to control extrinsic chirality in achiral metasurfaces and offers possibilities to design high-performance multiband spin-selective transmission, biodetection, chemical analysis, and on-chip chiral manipulation.