Spin-Sensitive Optical Anisotropy Controlled by Pump Polarization in the Kagome Antiferromagnet FeSn.

Antiferromagnetic (AFM) materials offer ultrafast spin dynamics, negligible stray fields, and robustness against magnetic perturbations, yet their vanishing net magnetization renders optical detection and control of the Néel vector fundamentally challenging. Here, we demonstrate pump-polarization-written optical and magneto-optical (MO) anisotropy in the Kagome AFM FeSn by using pump-probe ultrafast spectroscopy. The experimental findings reveal that the principal axes of both optical and MO anisotropies are dictated exclusively by the pump polarization but remain insensitive to sample rotation, ruling out lattice birefringence as the dominant origin. Linearly polarized excitation generates a photoinduced electronic polarization that couples efficiently to AFM order via spin-layer-locked Dirac Fermions. In parallel, a cooperative anisotropic deformation potential mechanism produces bond-selective transient strain that modifies the dielectric tensor through elasto-optic and spin-orbit coupling, becoming increasingly prominent at higher fluence. These results establish a deterministic and nonthermal control of spin-sensitive optical anisotropy by pump polarization in Kagome AFMs.