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 nonlinear 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 nonlinear 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 ushers 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.