Maximum extrinsic chirality flip and spin-dependent near-field imaging induced by bound state in the continuum.

Effective modulation of circular dichroism (CD) responses is crucial in engineering applications, including polarization manipulation, sensing, and imaging. Here, we propose a dual CD response via bound states in the continuum (BICs) in two-dimensional topologically protected metasurfaces composed of all-dielectric Si structures. By transforming two topologically protected bound states into distinct circular polarization states, the flip of maximum chirality at oblique incidence can be realized without changing the structure. Moreover, the transmittance of two distinct spin states at the multiband, and the signs of CD can be flexibly adjusted by the perturbation in the metasuface. The CD of the two quasi-BICs can achieve perfect ±1. This unprecedented observation of maximum chirality reversal provides more freedom for multi-band chirality control. Finally, the application in polarization multiplexed near-field image display is also exhibited, showing the application potential in pattern encryption and information coding. 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.