Ultrafast spectroscopy of coherent interlayer couplings in WSe₂/WS₂ heterobilayers

Twisted van der Waals heterostructures have led to emerging layer-dependent correlated physics in moiré potentials [1-3]. While optoelectronic controls over interlayer electronic coupling have been reported, the concomitant interlayer vibration has not yet been to be controlled. Here, we report experimental evidence of ultrafast optical control over amplitude and oscillation period of interlayer breathing phonons in WSe₂/WS₂ heterobilayers. Femtosecond optical excitation above the Mott density in gate-tuned devices shows a strong oscillation amplitude up to 10% of the maximum signal. Furthermore, we find the tunability of the breathing period, i.e. the strength of interlayer interactions, by manipulating the pump fluence and gate voltage. A theoretical model, incorporating both Buckingham and Hartree energies, is presented to elucidate the impact of charge-separated carriers generated by photoexcitation on phonon dynamics. This work, therefore, provides insights for extending optoelectronic engineering into the coherent phonons in moiré systems.

References

[1] Dean, C. et al. “Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices.” Nature 497, 598–602 (2013).

[2] Li, T. et al. “Quantum anomalous Hall effect from intertwined moiré bands.” Nature 600, 641–646 (2021).

[3] Kim, J. et al. “Correlation-driven nonequilibrium exciton site transition in a WSe₂/WS₂ moiré supercell.” Nat. Commun. 15, 3312 (2024).