Emerging altermagnetism provides a crystal symmetry foundation for spin manipulation by understanding the interaction between the altermagnetic ordering and elementary excitations in the 2D limit. In this study, we employ real-time time-dependent density functional theory (rt-TDDFT) and ab initio nonadiabatic molecular dynamics (NAMD) to investigate the out-of-equilibrium laser-induced spin dynamics in an altermagnetic semiconductor MnTe monolayer. During laser irradiation, our findings reveal a negligible electron-phonon coupling (EPC) between opposite spin sublattices. The excited spin-up Mn-$d$ electrons hop to spin-down Mn-$d$ orbitals through spin-orbit coupling (SOC) with a spin flip. Moreover, hexacoordinated $\mathrm{M}{\mathrm{n}}_{\mathrm{B}}$ atoms in the middle layer exhibit a more pronounced EPC compared to tetra-coordination $\mathrm{M}{\mathrm{n}}_{\mathrm{A}}$ atoms in the surface, leading to the photodriven asymmetric demagnetization dynamics and uncompensated net moment. Both EPC and SOC introduce channels for spin transfer from Mn to Te atoms, denoted as the optical-induced intersite spin transfer (OISTR) effect. Driving by asymmetric demagnetization and OISTR effect, our results indicate that altermagnetic state can reversibly transform into a metastable ferrimagnetic state via controlling laser pulse. After laser termination, excited $\mathrm{M}{\mathrm{n}}_{\mathrm{A}}-d$ electrons quickly decay to $\mathrm{M}{\mathrm{n}}_{\mathrm{B}}$ due to opposite spin $d-d$ SOC and then relax to spin-down Te-$p$ orbitals via EPC to recombine with holes. These dual relaxation channels significantly accelerate the spin flipping process and promote the altermagnetic-ferrimagnetic phase transition within a time scale of 246 fs. The reversible switching of the magnetic ordering provides a significant insight for the development of ultrafast optical switches in spintronics.

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• Light-induced magnetic effects
• Magnetic phase transitions
• Ultrafast magnetization dynamics
• 2-dimensional systems
• Altermagnets
• Density functional calculations
• Molecular dynamics