Laser pulses are known to induce symmetric demagnetization: equal loss of magnetic moments in the identical sublattices of antiferromagnets and ferromagnets at ultrashort timescales. Using time-dependent density functional theory, we show that linearly polarized laser pulses can drive asymmetric demagnetization between otherwise identical sublattices in the $d$-wave compensated altermagnet (AM) ${\mathrm{RuO}}_2$, resulting in a photoinduced ferrimagnetic state with a strong net magnetization of $\sim 0.2{\mu }_B$ per unit cell. The sign and magnitude of net magnetization are highly controllable by laser polarization. We identify polarization-selective asymmetric optical intersite spin transfer generates an transient spin imbalance between sublattices that is subsequently converted into net magnetization via asymmetric spin-flip processes, both rooted in the characteristic spin-nodal band structure of $d$-wave AMs. Moreover, we demonstrate that this laser-induced magnetization universally emerges in $d$-wave AMs. We uncover a robust route to light-controlled magnetization in AMs on ultrafast timescales.

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• Charge dynamics
• Light-matter interaction
• Magnetism
• Ultrafast demagnetization
• Ultrafast magnetization dynamics
• Ultrafast phenomena
• Density functional theory
• Time-dependent DFT