Numerical simulation of a chiral metasurface based on GST for multifunctional terahertz polarization control

Abstract Ge₂Sb₂Te₅ (GST) is a representative chalcogenide phase-change material that has received growing interest due to its non-volatility, rapid nanosecond-scale response and substantial differences in optical constants between its crystalline and amorphous states. This work proposes a bidirectional chiral metasurface based on GST, which comprises two GST-metal composite layers with a U-shaped resonant layer in the middle. With the introduction of mirror symmetry breaking and a Fabry-Pérot cavity cascade design, multifunctional dynamic control of circular dichroism (CD), asymmetric transmission and polarization conversion is achieved in the terahertz band.Geometric optimization via finite element method (FEM) simulations reveals that the metasurface achieves a CD up to ~0.95/0.74 at ~2.72/5.24 THz under the amorphous GST state. Within 3.91-4.3 THz, the corresponding asymmetric transmission parameters under linearly polarized incidence are shown to exceed 0.9, while within 3.94-4.56 THz, the polarization conversion rate (PCR) surpasses 0.95. Modulating the GST crystallization rate is demonstrated to dynamically tune both the CD peak wavelength and intensity while ensuring its excellent polarization response stability across a wide range of incident angles. The present work is insightful for the design of 2 multifunctional chiral devices operating in the terahertz band, which is promising to the applications for next generation optical devices.