Cavity-Induced Coherent Magnetization and Polaritons in Altermagnets

Altermagnets feature antiparallel spin sublattices with d-, g-, or i-wave spin order, yielding nonrelativistic spin splitting without net magnetization. In this Letter, we embed a two-dimensional d-wave altermagnet in a driven terahertz cavity. Because the intrinsic altermagnetic texture renders the electronic sublattices locally inequivalent, coherent cavity driving then couples to them asymmetrically, breaking the spin-sublattice transposing symmetries (such as [C_{2}∥C_{4}] or [C_{2}∥M]), generating a nonequilibrium sublattice population imbalance that produces a steady-state magnetization-a response absent in conventional antiferromagnets with the same lattice geometry. A mean-field Lindblad analysis reveals the dominance of quadratic over linear couplings. In the strong-coupling regime, distinct polariton signatures emerge in the steady state of induced magnetization. This Letter demonstrates cavity control of altermagnets for spintronic applications.