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. Light can be used to drive transient changes in a materials phase; in some cases, breaking material symmetries, in others restoring them. Here, using time resolved Faraday rotation measurements, Seo et al demonstrate that linear polarised light can break time reversal symmetry, leading to a transient magnetization in MoSe2.