Linearly polarized light-induced magnetism in bilayer MoSe₂.

Light-induced symmetry breaking offers a powerful route to manipulate material properties by driving them into non-equilibrium states. Here, we demonstrate that a linearly polarized light transiently breaks time-reversal symmetry in bilayer MoSe2, a nominally non-magnetic van der Waals system. Combined ultrafast measurements and first-principles simulations reveal that linearly polarized excitation launches interlayer-coupled coherent phonons through a displacive mechanism. Time-resolved Faraday rotation measurements confirm that the excitation of two coherent phonons by a linearly polarized light is responsible for breaking time-reversal symmetry. Remarkably, the subsequent beating between the interlayer breathing mode and the in-plane Se-stretching mode, together with asymmetric charge redistribution, phonon dissipation, and spin-layer locking, generates an effective oscillating magnetization. Our findings uncover a new light-induced pathway for interlayer phonon coupling, revealing hidden non-equilibrium functionalities of van der Waals crystals. Our results establish a foundation for light-controlled functionality in stacked two-dimensional systems, advancing the emerging paradigm of materials-on-demand.