Researchers from ETH Zürich, University of Washington, University of Basel and National Institute for Materials Science have demonstrated all-optical control over the spin-valley polarization in twisted molybdenum ditelluride (t‑MoTe₂) homobilayers - a step toward dynamically reconfigurable quantum materials and optically defined topological circuits. The work shows how circularly polarized light can reversibly switch the magnetic orientation of a strongly correlated ferromagnetic state, all without changing the sample temperature.
The experiments, led by Prof. Ataç Imamoğlu (ETH Zürich), Prof. Tomasz Smoleński (University of Basel), and colleagues, exploit a system where two atomically thin MoTe₂ layers are stacked with a small twist angle. This twist creates a moiré superlattice with flat, valley‑contrasting Chern bands, giving rise to highly correlated quantum phases - including Chern insulators and ferromagnetic metals - depending on the electron filling. Because the electronic bands are nearly dispersionless, electron-electron interactions dominate, resulting in spontaneous spin alignment even at cryogenic but steady temperatures.
By resonantly driving exciton-polaron transitions with circularly polarized laser pulses, the researchers succeeded in flipping the system’s spin-valley orientation. This optical excitation selectively interacts with one valley pseudospin, leading to an imbalance that reverses the overall magnetization. Crucially, the switching occurs without thermal cycling, in contrast to conventional ferromagnets that must be heated above their Curie temperature to reverse polarity.
To verify that the spin reversal was global and permanent, the team used a weaker probe laser to analyze magneto‑optical Kerr signals from the same spot, confirming that the few‑micrometer‑wide ferromagnetic domain had flipped its collective orientation. As Dr. Smoleński explains, the light not only toggles the polarity but can also write new domain boundaries within the moiré superlattice, effectively patterning regions with distinct topological and magnetic character.
According to Imamoğlu, "What's exciting about our work is that we combine the three big topics in modern condensed matter physics in a single experiment: strong interactions between the electrons, topology and dynamical control". This synergy hints at potential applications like optically writable topological circuits and on‑chip interferometers for detecting minute electromagnetic fields.
The demonstration establishes that non‑thermal, optical switching of ferromagnetic spin states in strongly correlated, topological materials is achievable. More broadly, it underlines that light can serve not only as a probe but as a functional tool for dynamically engineering electronic phases - opening a path toward programmable, light-driven quantum devices.