Ultra-high CD terahertz chiral metasurface absorber for polarization discrimination and sensing

Terahertz (THz) chiral metasurfaces can discriminate between left- and right-handed circularly polarized (LCP/RCP) waves and are therefore promising for polarization-resolved detection and chiral spectroscopy; however, naturally occurring dielectrics typically exhibit only weak circular dichroism (CD). Here, we numerically demonstrate a reflective metal-insulator-metal (MIM) chiral metasurface absorber composed of a gold-patterned asymmetric dual-slit elliptical split-ring resonator, a polyimide spacer, and a gold ground plane. By breaking in-plane mirror symmetry, the unit cell supports chirality-dependent resonances, giving rise to a pronounced spin-selective absorption peak at 1.648 THz: the absorptance reaches 99.85% for right-handed circularly polarized (RCP) incidence, whereas it remains 3.67% for left-handed circularly polarized (LCP) incidence, corresponding to an absorptance contrast of CD = 0.9618 (CD = A_R - A_L). Oblique-incidence analysis shows that the high CD is preserved over a wide angular range, with CD > 0.9 maintained up to 55.8° and CD > 0.8 up to 70.5°. Near-field and current-distribution analyses indicate that the ultrahigh CD originates from spin-dependent mode matching, which enhances field localization and ohmic dissipation for the target spin state while suppressing coupling for the opposite spin. In addition, by dimerizing the unit cell and introducing an extra dielectric layer, dual-band spin-selective absorption and high-Q narrowband resonances are achieved. Leveraging the high-Q resonance, the structure enables THz refractive-index sensing with a sensitivity of 394 GHz/RIU (with the analyte thickness fixed at 30 µm), suggesting a compact route toward polarization-selective THz components and resonance-based sensing platforms.