Light-induced interlayer spacing dynamics via orbital phonon coupling

Imagine a stack of paper so thin that each sheet is only a single layer of atoms. In the world of materials science, these are known as two-dimensional materials. They behave differently than the thick blocks of matter we touch every day because their electrons are confined to a flat plane, creating unique electrical properties. When scientists stack these atomic sheets on top of one another, they do not fuse into a solid block; instead, they rest on each other like a deck of cards, held together by weak forces. The distance between these layers is a critical dial that controls how electricity flows through the material. If the layers move closer together or drift further apart, the material can switch from being an insulator to a conductor, or even change its fundamental quantum nature. For years, researchers have sought a way to control this spacing instantly, using light to push or pull the layers without touching them physically.

A team of researchers has now demonstrated how to achieve this control using ultrafast laser pulses. By firing precise bursts of light at specific colors, they were able to make the layers of a material called tungsten diselenide squeeze together, while a similar material, tungsten disulfide, expanded outward. The key to this behavior lies in the electrons themselves. When the laser hits the material, it excites electrons and moves them from one type of atomic orbit to another. In the first material, this movement clears out electrons from the space between the layers, allowing them to collapse inward. In the second material, the light pushes extra electrons into the gap between the layers, causing them to repel each other and push apart. This discovery reveals that the direction of the movement is not random but is dictated by exactly which atomic orbitals the light energizes.

The scientists focused their study on three-layer stacks of these materials, which are complex enough to show interesting electronic behaviors but simple enough to model in detail. They used powerful computer simulations to watch what happens in the trillionths of a second after a laser pulse strikes. In the case of the tungsten diselenide stack, they fired a laser with a photon energy of 0.6 electron volts. This specific color of light caused the layers to contract, moving closer together in a rhythmic, wave-like motion. The simulations showed that this happened because the light stripped electrons away from the selenium atoms in the space between the layers and moved them onto the tungsten atoms. With fewer electrons in the gap to push the layers apart, the natural attraction between the sheets pulled them tighter. The motion was smooth and predictable, resembling a simple back-and-forth oscillation that grew stronger as the laser intensity increased.

In contrast, when the researchers applied a different laser with a higher energy of 3.0 electron volts to a stack of tungsten disulfide, the result was the exact opposite. The layers moved apart, expanding the gap between them. Here, the light did the reverse: it pushed electrons from the tungsten atoms into the sulfur atoms located in the interlayer space. This accumulation of extra electrons in the gap created a repulsive force that pushed the layers away from each other. Unlike the first material, this expansion did not just oscillate around a single point; the layers shifted to a new, wider resting position. The researchers found that this complex behavior required two different pathways for the excited electrons to relax, rather than just one. One group of electrons moved quickly, while another lingered longer, and the combination of these two different speeds created the shift in the equilibrium position.

To understand why these two similar materials behaved so differently, the team built simplified models of the electron behavior. They found that the tungsten diselenide could be described by a simple system where electrons jump from a ground state to a single excited state, creating a straightforward force that pulls the layers together. However, the tungsten disulfide required a more complex model involving two distinct excited states with different rates of decay. This difference in how the electrons relax explains why one material simply vibrates while the other settles into a new, expanded state. The study confirms that the specific arrangement of electrons around the atoms—their orbital character—determines whether light will cause the material to shrink or swell.

The implications of these findings extend beyond just moving atoms. The researchers calculated what happens to the material's ability to conduct electricity when the layers move. For the tungsten diselenide, squeezing the layers closer together narrows the energy gap that electrons must cross, eventually turning the material into a semimetal. For the tungsten disulfide, pulling the layers apart opens up a gap, turning it into a topological insulator, a special state of matter where electricity flows only on the surface. This suggests that by simply tuning the color and intensity of a laser, scientists could switch a material between different electronic phases on demand. While these results come from computer simulations, the authors note that they can be tested experimentally using ultrafast X-ray diffraction to watch the layers move and specialized spectroscopy to track the electrons.

This work connects the microscopic world of electron orbits to the macroscopic movement of atomic layers. It shows that light is not just a tool for heating or breaking bonds, but a precise instrument for reshaping the internal architecture of a material. By choosing the right color of light, researchers can selectively target specific electron paths, using the resulting charge redistribution to either compress or expand the space between layers. This orbital-selective control offers a new way to tune the properties of layered materials, potentially leading to devices that can be reconfigured instantly with a flash of light. The study establishes a clear link between how electrons are arranged, how they relax after being excited, and how the physical structure of the material responds, providing a roadmap for future experiments in light-driven material science.