State-dependent dissipation and broadband circular-polarization selectivity in a VO 2 -Ag terahertz chiral metasurface

Broadband and tunable discrimination between circular polarizations is important for terahertz polarization control and handedness-selective devices. A reflective chiral metasurface comprising a hybrid VO 2 –Ag top pattern, a TOPAS spacer, and an Au ground plane is proposed. A Python–CST workflow with derivative-free adaptive coordinate search jointly optimizes the peak and contiguous bandwidth of the normalized circular-polarization reflectance contrast C N . The optimized unit cell reaches |C N | ≈ 0.991 near 5.24 THz and maintains |C N | > 0.8 from 4.39 to 6.52 THz, yielding a 2.13 THz bandwidth. The complementary absorption-difference metric reaches |CD A | ≈ 0.889, with |CD A | > 0.8 over 1.62 THz. Mirroring the metasurface interchanges the preferentially absorbed polarization and dominant reflection channels. Furthermore, material-substitution controls and field/current/loss maps support an interpretation based on asymmetric Ag–VO 2 current paths and state-dependent dissipation. Increasing the assumed VO 2 conductivity from 20 S/m to 2 × 10 5 S/m raises |C N | from nearly zero to approximately 0.99. These results indicate a numerically tunable route to broadband circular-polarization-selective terahertz reflection in a compact ground-backed architecture.