Light-induced chiral altermagnets from conventional antiferromagnets
Altermagnets combine compensated collinear magnetic order with momentum-dependent nonrelativistic spin splitting, offering new opportunities for spintronic applications. Here, we propose a route to optically induce altermagnetism through coherent lattice control. Using symmetry analysis, we identify phonon distortions that lift the spin degeneracy of conventional antiferromagnets while preserving the rotational symmetries connecting opposite-spin sublattices. Among these, geometric chiral phonons provide a particularly appealing pathway, linking the emergence of altermagnetism to structural chirality. Taking Fe$_2$TeO$_6$ as a prototype, we demonstrate that a linearly polarized mid-infrared pulse can drive a structural instability through negative biquadratic coupling between an infrared-active phonon and an optically silent, symmetry-breaking mode. First-principles-based lattice dynamics reveal the emergence of a transient chiral distortion, while electronic-structure calculations establish that the light-induced structure exhibits nonrelativistic spin splitting characteristic of altermagnetism. Our results establish a symmetry-guided pathway toward the ultrafast optical control of altermagnetic states through nonlinear phononics.