In a groundbreaking exploration at the intersection of ultrafast optics and two-dimensional quantum materials, researchers have unveiled a spectacular dynamic behavior in moiré superlattices—ultrafast twist and untwist motions triggered within femtoseconds after photoexcitation. This remarkable discovery, chronicled in a recent Nature publication, exposes how the intricate stacking geometry of atomically thin monolayers can be actively manipulated in real time, opening new avenues for the control of correlated and topological quantum phases in two-dimensional materials.
Moiré materials have captivated the condensed matter physics community over the past several years due to their extraordinary ability to engender novel electronic phenomena through the delicate control of atomic registry and stacking angles. By assembling sheets of semiconducting transition metal dichalcogenides (TMDs), such as WSe₂ and MoSe₂, researchers create moiré superlattices with twist angles that determine the material’s emergent optical and electronic properties. These moiré patterns can host strongly correlated insulating states, generalized Wigner crystals, and exotic excitonic and polaronic effects, all reliant on the precise interlayer coupling dictated by atomic registry.
However, prior to this advance, the understanding of how these moiré configurations could be altered dynamically on ultrashort timescales remained elusive. Conventional wisdom suggested that lattice deformations induced by photoexcitation typically lead to incoherent lattice heating and disordering, not coherent modulation of twist angles. This new work overturns that notion by directly observing a coherent twist–untwist oscillation within the moiré superlattice of twisted WSe₂/MoSe₂ heterobilayers, with twist angles initially set at 2° and 57°.
Utilizing state-of-the-art ultrafast electron diffraction techniques with femtosecond temporal resolution, the team captured the time-resolved evolution of the moiré diffraction peaks following above-band-gap optical excitation. Rather than fading monotonically due to thermal disordering, the intensity of the moiré superlattice diffraction features first increased sharply within 1 picosecond, indicative of an enhanced periodic lattice distortion, before gradually diminishing several picoseconds later. This nontrivial behavior signals that an unusual phonon mode associated with lattice twisting is coherently excited.
Detailed kinetic diffraction analysis corroborated by advanced simulations revealed the underlying lattice dynamics—a sub-terahertz frequency oscillation corresponding to a twist-angle modulation of approximately 0.6°, a profound magnitude considering the atomic scale. This twist–untwist motion can be understood as the transient mechanical response of the bilayer heterostructure, where optically generated charge transfer enhances interlayer attraction, effectively pulling the two layers into a slightly altered atomic registry.
This photoinduced lattice motion fundamentally changes the native moiré potential landscape. Since excitons, polarons, and correlated electron behaviors in TMD heterobilayers are sensitively dependent on this periodic potential, the ability to drive and control twist angle oscillations could provide a revolutionary handle for manipulating quantum states of matter dynamically, all on ultrafast timescales previously inaccessible.
The implications are enormous. The rise of controlled moiré dynamics paves the way for engineering tunable quantum phases, where transient manipulations of interlayer twist may switch correlated insulating states on and off or modulate topological properties with a mere optical pulse. Such capabilities would position moiré materials as a new platform for coherent quantum devices, with ultrafast optical control replacing mechanical or static methods.
Furthermore, the discovery highlights the critical importance of coupling electronic and structural degrees of freedom in layered quantum materials. Here, charge redistribution via photoexcitation profoundly modifies interlayer forces and, consequently, lattice geometry. This electron-phonon interplay is a crucial piece of the puzzle in understanding emergent moiré phenomena and could inspire similar studies in other van der Waals heterostructures.
From an experimental standpoint, the use of femtosecond electron diffraction represents a tour de force, directly visualizing atomic-scale lattice motions with unprecedented time resolution. By resolving not only the magnitude but also the direction and frequency of moiré phonons, researchers have opened a window into the transient structural dynamics that underpin optoelectronic functionalities.
Looking forward, one can envision integrating such ultrafast control schemes with other moiré-enabled quantum devices. Combining optical pulses with electrical gating or magnetic field tuning could lead to versatile, multifunctional quantum systems where interactions and topological attributes are modulated on the fly, yielding unprecedented device architectures.
The study also raises intriguing theoretical questions about the stability and nonlinear response of moiré superlattices under strong nonequilibrium perturbations. As photoinduced twist angles oscillate beyond equilibrium regimes, the potential energy landscape may access new metastable configurations, hinting at photoinduced phase transitions or dynamically stabilized states yet to be discovered.
This breakthrough synergizes the rapid advances in ultrafast laser techniques, quantum materials synthesis, and electron microscopy to map dynamic atomic-scale phenomena, promising to propel the field of quantum materials engineering into a new era where static constraints give way to ultrafast tunability.
In essence, the demonstration of photoinduced twist and untwist in moiré superlattices is a vivid testament to the ingenuity of modern materials science. It heralds a future where light can sculpt the quantum landscape in layered materials at will, with profound consequences for next-generation quantum technologies and fundamental condensed matter physics alike.
Subject of Research: Ultrafast dynamics and photoinduced structural modulation in two-dimensional moiré superlattices of twisted WSe₂/MoSe₂ heterobilayers.
Article Title: Photoinduced twist and untwist of moiré superlattices.
Article References:
Duncan, C.J.R., Johnson, A.C., Maity, I. et al. Photoinduced twist and untwist of moiré superlattices. Nature (2025). https://doi.org/10.1038/s41586-025-09707-3
