Nodal-Loop Engineering of the Second-Order Magneto-Optical Effect in Two-Dimensional Topological Altermagnets.

Abstract

Magneto-optical effects (MOEs) provide powerful noncontact probes of magnetic order and underpin a broad range of photonic and information technologies. However, large and controllable second-order MOEs remain scarce. Here, using two-dimensional topological altermagnet V₂Te₂O as a model system, we demonstrate that nodal-loop engineering offers an effective strategy to enhance and tune second-order MOEs across the infrared-to-terahertz range. Néel-vector rotation selectively gaps one of nodal loops near the Brillouin-zone boundary, activating spin-conserved ladder-like interband transitions that generate a pronounced infrared magneto-optical response, while the terahertz response is dominated by anisotropic intraband dynamics. Carrier doping further shifts the Fermi level relative to the gapped nodal loop, enabling continuous tunability and even sign reversal of the magneto-optical response, and enhancing both infrared and terahertz signals. Our results suggest nodal-loop engineering as a promising strategy for large and tunable second-order MOEs in two-dimensional topological altermagnets, highlighting potential for ultrafast, low-power opto-spintronic applications.