Optical control of integer and fractional Chern insulators
Article Date: 28 January 2026
Authors: William Holtzmann et al.
Journal: Nature 649, 1147–1152 (2026)
Summary
This paper demonstrates deterministic optical control of ferromagnetic polarisation, and thereby the sign of the Chern number, in twisted bilayer MoTe2 (tMoTe2). Using circularly polarised optical pumping the authors show two complementary effects: low-power helicity-selective “optical training” that prepares a desired ferromagnetic state during gate sweeps, and higher-power direct optical switching of magnetisation at zero external magnetic field and at temperatures well below the Curie point.
The control is strongest near integer (ν = −1) and fractional (− 2/3) Chern insulator states. The authors attribute this to gap-enhanced valley and trion polarisation lifetimes that amplify the effective optically induced magnetic field. They further demonstrate dynamic switching by modulating pump helicity and spatially resolved optical writing of QAH/FQAH domains, measured via helicity-resolved photoluminescence (PL) and reflective magnetic circular dichroism (RMCD) on devices with ∼3.5°–3.7° twist angles at low temperature (approx. 1.6 K).
Key Points
- Circularly polarised optical pumping can “train” or directly switch ferromagnetic polarisation in tMoTe2, controlling the chirality and sign of integer and fractional Chern insulators at zero magnetic field.
- Optical training: low-power, helicity-selective pumping during gate ramps biases the system into the desired ferromagnetic state; useful when direct switching is not achievable.
- Direct optical switching: higher pump power flips magnetisation on-demand, and switching remains robust well below the Curie temperature.
- Effectiveness peaks near CI and FCI states (notably ν = −1 and − 2/3) because the topological gap enhances valley/trion polarisation lifetimes, increasing the optically induced effective magnetic field.
- Dynamic modulation of pump helicity achieves time-resolved switching; spatially resolved PL maps show optical writing of QAH and FQAH domains, enabling programmable domain patterning.
- Key techniques: helicity-resolved PL, RMCD and spatial PL imaging on moiré devices; devices studied include 3.5° and 3.7° twist-angle samples.
Context and relevance
This work sits at the intersection of moiré engineering, strongly correlated topological phases and ultrafast/optical control of magnetism. Controlling the sign and spatial distribution of Chern numbers optically opens routes to reconfigurable topological circuits, optically addressable topological memories and engineered edge states. It also provides an experimental handle for exploring exotic fractionalised excitations in programmable geometries and could dovetail with efforts to harness non-Abelian physics in engineered platforms. The results are a clear advance in the ongoing push to manipulate correlated topological phases in 2D moiré materials.
Why should I read this
Short version: if you care about quantum materials that you can actually switch with light, this is gold. The authors show you can optically set and reverse the Chern number in a fractional Chern insulator platform, write domains, and flip them dynamically — all at zero external field. That’s not just neat optics; it’s a practical control knob for programmable topological circuitry, spintronic devices and maybe even topological memory prototypes. Read it if you want to know how to use light to manipulate many-body topological order, or if you’d rather save time and let someone else do the heavy reading for you.