The diverse nodal spin structures in $d/g/i$-wave altermagnets (AMs) may cause distinct light-induced spin responses yet remain poorly understood. Using real-time time-dependent density functional theory, we reveal that laser-induced ultrafast demagnetization dynamics in the $g$-wave AM CrSb are strongly governed by the laser incidence direction. Under normal incidence along the [0001] axis, two Cr sublattices exhibit symmetric temporal demagnetization but with different amplitudes, preserving the zero net magnetization—unlike the behavior in $d$-wave AM. Off-normal incidence, however, induces pronounced asymmetric demagnetization between sublattices, transiently driving the system into a ferrimagnetic state with a sizable net magnetization. Such incidence-dependent magnetization dynamics arises from the characteristic nodal structures in bulk $g$-wave AM's electronic structure, which enable anisotropic optical-induced intersite spin transfer. By comparing $g$-wave and $d$-wave AMs, we propose that light-induced magnetization arises when laser polarization aligns with spin-uncompensated regions in electronic structures. This can be readily determined from the local spin density of states along specific band paths. Our results provide a fundamental understanding of laser-induced ultrafast dynamics in AMs.

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• Ultrafast magnetic effects
• Ultrafast magnetization dynamics
• Altermagnets
• Density functional theory
• Time-dependent DFT