Abstract
Controlling quantum matter with light offers a promising route to dynamically tune its many-body properties, ranging from band topology1,2 to superconductivity3. However, achieving such optical control for strongly correlated electron systems in the steady state has remained elusive. Here we demonstrate optical switching of the spin–valley degree of freedom of itinerant ferromagnets in twisted MoTe2 (t-MoTe2) homobilayers. This system uniquely features flat valley-contrasting Chern bands and exhibits a range of strongly correlated phases at various moiré lattice fillings, including Chern insulators and ferromagnetic metals4,5,6,7. We show that the spin–valley orientation of all of these phases can be dynamically reversed by resonantly exciting the exciton–polaron8 transitions with circularly polarized light. These findings not only provide direct evidence for non-thermal optical switching of a ferromagnetic spin state at zero magnetic field but also demonstrate the possibility of dynamical control over a topological order parameter, paving the way for optical generation of chiral edge modes and topological quantum circuits.
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Data availability
The data that support the findings of this study are available from the ETH Research Collection: https://doi.org/10.3929/ethz-c-000784681.
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Acknowledgements
This work was supported by the Swiss National Science Foundation (SNSF) under grant number 200021-204076. E.A., W.L. and X.X. were supported by the US DOE BES (DE-SC0012509) and Vannevar Bush Faculty Fellowship (award number N000142512047). K.W. and T.T. acknowledge support from the JSPS KAKENHI (grant numbers 21H05233 and 23H02052), the CREST (JPMJCR24A5), JST and World Premier International Research Center Initiative (WPI), MEXT, Japan. We thank C. Kuhlenkamp, M. Knap, H. Adlong, A. Christianen and L. Zheng for insightful discussions.
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T.S. designed the experiments and carried out initial observations. T.S. and A.I. supervised the project. O.H., K.K. and T.S. performed the measurements and analysed the experimental data, with assistance from M.K., E.A., W.L. and X.X. W.L. and E.A. fabricated the samples. K.W. and T.T. provided bulk hBN crystals. T.S., O.H. and A.I. wrote the manuscript, with input from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Schematic of the experimental set-up.
For our optical measurements, the light is sent to the sample through the excitation arm, whereas the reflected or emitted signal is collected in the detection arm. All of the lenses are aspheric. BS, beamsplitter; CCD, charge-coupled device used for imaging the sample; LP, linear polarizer; λ/2: half-wave plate; λ/4, quarter-wave plate.
Extended Data Fig. 2 Magnetic-field-dependent PL measurements of device A.
a, Evolution of the PL signal with n at B = 0 T. Cusps corresponding to ICI and FCI states at filling factors ν = −1 and ν = −2/3 are clearly visible. b,c, Evolution of the central energy of a non-dispersive Lorentzian fit to the trion peak in the PL signal with n for B ≥ 0 up to 7 T (b) and B ≤ 0 down to −7 T (c). The grey lines are guides for the eye following the B-dependent cusps and the colour of the line at a fixed B matches that of the corresponding dots in d and e. d,e, Evolution of the cusp charge density with magnetic field for the ICI state at ν = −1 (d) and the FCI state at ν = −2/3 (e). The cusp position for each B was determined using the maximum of the first derivatives of the peak energy with respect to n. The Chern numbers extracted on the basis of the fitted Streda formula at ν = −1 yield C = −1.01 ± 0.05 and C = 0.94 ± 0.08 for ν = −1 at positive and negative B, respectively. At ν = −2/3, the determined Chern numbers are C = −0.63 ± 0.06 (B ≥ 0) and C = 0.62 ± 0.04 (B ≤ 0).
Extended Data Fig. 3 Nearly resonant PL measurement performed with WL excitation in cross-circular polarization setting in device A.
a, PL measurement as a function of charge density n at B = 0 performed using approximately 250-nW PL excitation at 1,090 nm combined with further, resonant, circularly polarized 120-nW WL excitation centred around the AP resonance (about 5 meV FWHM, used for optical spin orientation). b, Trion peak energies extracted from non-dispersive Lorentzian fits to the PL spectra, plotted as a function of charge density n for various magnetic fields B. The cusps observed at filling factors ν = −1 and ν = −2/3 disperse according to the Streda formula, confirming that ICI and FCI states remain robust under circularly polarized WL excitation used for optical spin orientation. The extracted Chern numbers are C = −0.99 ± 0.07 for the ν = −1 state and C = −0.63 ± 0.03 for the ν = −2/3 state.
Extended Data Fig. 4 PL measurement performed after optical spin orientation.
a, Circular-polarization-resolved reflectance contrast spectra showing AP resonance before optical orientation, with the state initialized at B = −120 mT. b, Similar spectra acquired after the optical spin orientation, performed at B = 0. c, PL measurement performed as a function of n at B = 0 after the optical orientation was completed. The cusps corresponding to the incompressible states at ν = −1 and ν = −2/3 states are visible. d, Trion peak energies extracted from non-dispersive Lorentzian fits to the spectra, plotted as a function of charge density n for various magnetic fields B. The cusps observed at filling factors ν = −1 and ν = −2/3 disperse according to the Streda formula, confirming that ICI and FCI states remain robust after optical spin orientation. The extracted Chern numbers are C = −1.04 ± 0.04 for the ν = −1 state and C = −0.65 ± 0.04 for the ν = −2/3 state.
Extended Data Fig. 5 Spin–valley polarization around ν = −1 after optical spin orientation of the ICI state.
a,b, Circular-polarization-resolved reflectance contrast spectra showing the AP resonance before optical orientation, with the state initialized at B = −120 mT (a), and after optical spin orientation, performed at B = 0 (b). c, Spin–valley polarization of the hole system measured as a function of charge density around ν = −1 after optical spin orientation. The data clearly demonstrate that slight underdoping and overdoping of the optically oriented ICI state does not compromise its complete spin–valley polarization.
Extended Data Fig. 6 Schematic of the pump–probe procedure used for optical orientation.
a, The power of the light with about 5 meV FWHM centred on the AP resonance is modulated in time using a fibre optical attenuator. The pump (probe) pulse lasts 3 s (10 s). b, During each pump sequence, the light polarization is set to the desired setting (σ+, σ− or π). During the probe sequence, the excitation is linearly polarized. c, The polarization in the detection path is set to σ− for the initial 5 s of the probe sequence and switched to σ+ for the second half of the probe sequence, allowing us to resolve the hole spin polarization.
Extended Data Fig. 7 Optical spin orientation as a function of excitation power and temperature for ν = −1 and ν = −0.8.
a, For the ICI at ν = −1, efficient optical spin orientation is maintained up to 5 K, although higher excitation powers are required for increasing base temperatures. Between 5 and 7 K, the spin orientation becomes ineffective. b, For the ferromagnetic metal at ν = −0.77, optical spin orientation remains effective up to 1.1 K. Above this temperature, the orientation becomes partial at 1.4 K and is no longer observed at higher temperatures.
Extended Data Fig. 8 Excitation-energy dependence of the optical Chern number orientation.
a–d, Reflectance contrast spectra differentiated with respect to photon energy as a function of displacement field D at a filling factor of ν = −2/3 (a) and ν = −1 (b–d). The spectra were acquired under B = 50 mT in two circular polarizations (σ+: top panels, σ−: bottom panels) using a weak, linearly polarized broadband white-light excitation. Horizontal lines indicate the displacement fields at which the optical switching efficiency is investigated below. e–g, Degree of spin polarization of the AP resonance before (grey circles) and after (coloured circles) illuminating the sample with a narrow-bandwidth (<1 meV FWHM) pump beam as a function of the pump centre wavelength. The wavelength dependence is investigated at the filling factors and displacements fields indicated in the respective reflectance contrast plots above. The pump powers are 90 nW (e), 80 nW (f), 65 nW (g) and 50 nW (h), respectively. In each graph, the spins are first oriented using B = 50 mT, which is then turned off. The spin polarization before and after optical pumping is examined by measuring the helicity-resolved reflection of a linearly polarized white-light probe beam. We pump with σ− (blue circles) and σ+ (red circles) circularly as well as with linearly polarized light (pink circles in g). The spin polarization is inferred by fitting double Lorentzian spectral profiles to the optical resonances in the σ+-polarized and σ−-polarized reflection contrast spectra. The fit parameter corresponding to the amplitude of the stronger Lorentzian is constrained from below and above to reduce fitting uncertainty and an amplitude of zero is assigned to a spectrum when the χ2 of the fit exceeds a global tolerance, that is, when no Lorentzian with a finite amplitude can sensibly be fitted.
Extended Data Fig. 9 Dependence of optical spin switching on pump wavelength and power at ν = −1 and D = 0.
a, Normalized spectra of the pump and probe beams. The centre wavelength of the pump beam can be tuned from 1,080 to 1,125 nm while keeping the output power constant. b–i, Degree of spin–valley polarization of hole system ⟨Sz⟩ at ν = −1 before (grey circles) and after pumping for approximately 5 s with σ+ (red circles) or σ− (blue circles) circularly polarized light at pump powers Ppump between 10 and 510 nW. Before taking each data point, the system is spin-polarized by applying a magnetic field of 50 mT and ramping it back down to zero. The data are analysed as described in the caption of Extended Data Fig. 8.
Extended Data Fig. 10 Optical spin orientation using broadband excitation for device B.
Magnetic hysteresis loop measured at ν = −1 using σ+-polarized and σ−-polarized light with 100 nm power and broad bandwidth spanning from about 1,030 to about 1,120 nm. Even though the hysteresis shows slight dependence on the helicity of the exciting beam, the spin–valley degree of freedom cannot be optically flipped at B = 0, in contrast to the case of narrowband excitation exploited in the main text.
Extended Data Fig. 11 Optical writing of topological edge modes at ν = −2/3 in device A.
a, AP resonance detected with σ− and σ+polarization when the system has been initialized with holes in the K+ valley by applying a finite magnetic field. b, AP resonance detected with σ− and σ+ polarizations after resonant, σ−-polarized excitation with 120 nW excitation power. The vertical dashed lines indicate the optical spot in which the optical spin orientation has been performed.
Supplementary information
Supplementary Information
This file contains Supplementary Information sections 1–5 (including Supplementary Figures 1–7), which provide additional experimental details such as optical spot size determination, stability of the spin states after optical orientation, and the results obtained on another device (device B).
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Huber, O., Kuhlbrodt, K., Anderson, E. et al. Optical control over topological Chern number in moiré materials. Nature 649, 1153–1158 (2026). https://doi.org/10.1038/s41586-025-09851-w
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DOI: https://doi.org/10.1038/s41586-025-09851-w
