Achieving nonthermally and ultrafast optical control of phase transition is pivotal to developing next-generation optoelectronics. Tin selenide (SnSe) is a promising candidate with ultrafast-pulse tuning functionality due to its multiple lattice instabilities and unique in-plane ferroelectricity. Here, we report two distinct laser fluence-dependent nonthermal phase transition pathways in SnSe by using nonadiabatic molecular dynamics based on real-time time-dependent density functional theory calculation. At low laser fluence, SnSe undergoes an ultrafast all-optical switching of intralayer ferroelectricity ($P\to -P$) for bulk Pnma antiferroelectric state, where the centrosymmetric Cmcm phase acts as a structural intermediary. In contrast, high laser fluence drives a direct transition from Pnma phase to a stable and higher-symmetry Immm paraelectric phase that completely quenches the intralayer ferroelectric order. Moreover, it reveals that photoexcitation of electrons from Se $4p$ to Sn $5p$ orbitals determines the photoinduced phase transition pathways, which reshapes the potential energy surface and coherently excites interlayer shear and intralayer soft phonons. The present work not only unveils the ultrafast nonequilibrium dynamics of photoinduced phase transitions in SnSe, but also provides an efficient strategy to control in-plane polarization through an intermediary phase.

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• Dynamical phase transitions
• Ferroelectricity
• Phonons
• Photoinduced effect
• Ultrafast optics
• First-principles calculations
• Molecular dynamics
• Time-dependent DFT