Ultrafast Magneto‐Geometric Control of Terahertz Helicity in Curved Ni Metasurfaces

ABSTRACT Dynamic helicity control in the terahertz (THz) regime is essential for ultrafast polarization optics, chiral spectroscopy, and polarization‐selective photonic devices, yet remains difficult because most THz chiral systems are structurally fixed or rely on slow external tuning. To address this, we report a curved Ni split‐ring metasurface enabling magnetically amplified and ultrafast reconfigurable THz helicity control. Handed curvature breaks mirror symmetry and produces opposite static circular dichroism in left‐handed and right‐handed metasurfaces, accompanied by a near order‐of‐magnitude increase in current asymmetry compared to flat and achiral references. In pump‐free steady‐state THz transmission, in‐plane magnetization increases the static CD contrast from 9.4 to 33.6 (3.57‐fold enhancement). In optical pump–THz probe measurements, magnetization further increases the normalized transient circular dichroism contrast from 0.538 to 1.553 (2.89‐fold enhancement). The magnetized response exhibits switching‐like TRCD reweighting within τ ≈ 0.5 ps, consistent with ultrafast transient helicity control. These results show that femtosecond excitation reweights a pre‐existing magneto‐geometric chiral state rather than creating an independent photoinduced chiral structure. This work provides a lithography‐compatible route to ultrafast, reversible, and magnetically enhanced THz helicity control.