Magnetic-field-enhanced laser-induced electron coherence in 2D magnetic materials

Laser-induced electron coherence is a fascinating behavior in quantum materials. However, to date, viable approaches to control this excited-state behavior remain lacking. Here, we overturn this by realizing magnetic-field-controlled spatial self-phase modulation (SSPM), a third-order non-linear optical response. Light-matter interactions in ferromagnetic VSe₂ and diamagnetic graphene are observed to be precisely controlled by an external magnetic field, achieving an intriguing anisotropic SSPM, accompanied by prominent optical birefringence. We attribute this magnetic-field-controlled SSPM to the magnetic-field-induced broken symmetry in the collective behavior of flakes. Significantly, the non-linear optical coefficient χ ⁽³⁾ is enhanced by 58%, which breaks the record of largest χ ⁽³⁾. Furthermore, we demonstrate a magnetic-field-controlled all-optical switch based on SSPM in VSe₂, achieving an enhancement of 340% in the signal-to-control intensity ratio (manifesting the weak-light-control-strong-light capability). Our findings usher in a new paradigm, controlling (rather than merely monitoring) the laser-induced electron coherence in SSPM, which enables the next generation of all-optical switching based on SSPM.