Gradient-assisted metasurface absorber with dual-band chiral switching and quasi-linearly tunable circular dichroism

Chiral metasurfaces with tunable or switchable circular dichroism (CD) responses hold great potential for advanced optical devices. In this work, we theoretically propose and numerically demonstrate a chiral metasurface absorber composed of periodically serrated Ge₂Sb₂Te₅ (GST) resonators. By harnessing strong plasmonic resonance using the gradient geometry, we achieve a strongly enhanced chiral response with a CD value of 0.98 at λ₂= 2359 nm and a CD value of 0.7 at λ₁= 2274 nm. Additionally, by controlling the gradient difference in the serrated GST resonator, we can modify the CD intensity in multiple dimensions and near-perfectly optimize the chiral properties. Furthermore, it is worth noting that the CD value can be strongly varied by adjusting the phase transition characteristics of GST in the range of 0.007 to 0.7 at λ₁ and 0.002 to 0.98 at λ₂, corresponding to a switch between "on" and "off" states. The findings give new insight into multi-functional chiroptics and hold wide applications.