Optically controllable magnetism in atomically thin semiconductors*

Correlated phases including Wigner-crystals, Mott insulators, charge density waves, and superconductivity have been demonstrated in 2D semiconductors. Magnetic phases have also been theoretically predicted and experimentally demonstrated under magnetic fields. Here, we experimentally demonstrate that a mesoscopic ferromagnetic order can be generated and controlled by local optical pumping in WSe2 at zero applied magnetic field. Under a circularly-polarized, off-resonance optical pump, a mesoscopic magnetic order that extends as large as 8 µm x 5 µm, bounded by sample edges and folds, is built up and probed by circular dichroism(CD) in reflectivity with CD magnitude exceed 20% at excitonic resonances. The helicity of the optical pump determines the orientation of the magnetic order, allows optical control of the magnetic order. The research demonstrates a versatile way to manipulate long-range magnetic order in 2D semiconductors, enables sophisticated control of correlated electron phases in two-dimensional electron gases (2DEGs). The convenient manipulation of the magnetic order and the large CD amplitude also unlocks new applications of monolayer semiconductors in spintronics and nanophotonics.