Spectrally broad-range tunable perfect intrinsic chirality within Sb₂Se₃ metasurfaces

We propose a metasurface to achieve broadband, tunable, and highly stable perfect circular dichroism (CD) by combining bound states in the continuum (BIC) resonances with the phase-change material Sb₂Se₃. Chiral quasi-BICs are realized by simultaneously breaking in-plane and out-of-plane symmetries, maintaining a high Q-factor while relocating the topological singularity C, which carries a half-integer topological charge, to the Γ point in momentum space to achieve the maximum intrinsic chirality. Subsequently, based on the large refractive index contrast and ultra-low loss characteristics of Sb₂Se₃ before and after the phase transition, a 303 nm large-scale shifting of the CD resonance wavelength is achieved. Notably, the CD values can be well maintained with the intensity very near ±1, and the peak shape remains almost unchanged during the modulation. These impressive performances hold promise for metasurface-based polarization filters, highly saturated and dynamically color-changing chiral anti-counterfeiting labels, and multi-component calibration-free chiral spectral analysis.