Exciton multipolarity controls coherent and squeezed phonons in van der Waals heterostructures

In the invisible world of atoms, matter is never truly still. Even in a solid block of metal or a thin sheet of crystal, the atoms are constantly jiggling, vibrating in rhythmic patterns known as phonons. For decades, scientists have understood that when light hits a material, it can kick these atoms into a synchronized, marching rhythm called a coherent vibration, much like a crowd of people stepping in unison. This phenomenon is crucial for the future of ultrafast electronics and sensing, as it allows researchers to manipulate the very structure of materials with light. However, a specific type of vibration, one where the atoms do not march in step but instead fluctuate in a more complex, squeezed pattern, has remained difficult to generate and control on demand. The challenge has been finding a reliable way to switch between these two distinct states of atomic motion without changing the material itself.

A team of researchers has now identified a simple switch that can toggle between these two states of atomic vibration in a specific type of artificial material. By stacking thin layers of two different semiconductors, molybdenum diselenide and tungsten diselenide, they created a structure where the behavior of light-absorbing particles, called excitons, dictates how the atoms move. The researchers discovered that the shape of the electric charge within these excitons acts as a master control. When the exciton has a simple, two-sided charge distribution, it pulls the atomic layers into a synchronized, coherent rhythm. But when the charge distribution takes on a more complex, four-sided shape, the atoms enter a different state where their fluctuations oscillate in time, creating what is known as a squeezed state.

The team used detailed computer simulations to map out exactly how these layers interact. They modeled structures ranging from two to four layers thick, alternating the two materials to create a van der Waals heterostructure. In a two-layer stack, the exciton naturally forms a simple dipole, with positive and negative charges separated across the interface. This configuration couples directly to the breathing mode of the layers, causing them to expand and contract in a steady, predictable wave. However, in a three-layer stack, the situation changes. Without any external influence, the exciton forms a quadrupole, a more complex arrangement of charges. In this state, the coupling to the atomic vibrations is different; instead of pushing the atoms into a steady march, it causes their uncertainty in position and momentum to oscillate, creating the squeezed state.

The most significant finding is that this switch is not permanent. The researchers showed that by applying a vertical electric field, they could force the complex quadrupole exciton to transform into a simple dipole. In a three-layer stack of tungsten diselenide, molybdenum diselenide, and tungsten diselenide, the breathing mode vibrates at 1.04 terahertz. At zero electric field, this vibration exists in a squeezed state. When the researchers applied a specific electric field, the exciton changed its character, and the vibration instantly switched to a coherent state. This means the same physical material can be made to produce two fundamentally different types of quantum motion simply by turning a voltage on or off.

The simulations revealed that this control works because the electric field alters the energy landscape for the electrons and holes within the layers. In the three-layer system, the field mixes different possible arrangements of the exciton, effectively converting the four-sided charge pattern into a two-sided one. This transition changes how the exciton interacts with the lattice vibrations. The linear interaction that drives coherent motion replaces the quadratic interaction that drives squeezing. The researchers calculated that the strength of these vibrations and the degree of squeezing are comparable to those observed in well-known bulk materials like bismuth, suggesting that these effects are robust and measurable.

This work provides a clear design principle for engineers who wish to build devices that control light and sound at the atomic scale. Previously, methods to tune these vibrations relied on changing the number of layers or the chemical composition of the material, which are permanent changes. This new approach offers a dynamic, reversible switch. The ability to generate squeezed phonons is particularly valuable because these states can reduce quantum noise, a fundamental limit in precision measurement. If this control can be realized in a laboratory setting, it could lead to new types of sensors capable of detecting incredibly faint signals, or ultrafast switches for computing that operate at terahertz speeds.

The study also clarifies how different layer counts behave. While the three-layer stack showed a dramatic switch from squeezed to coherent states, the four-layer stack exhibited a more complex response with multiple possible charge configurations. This suggests that as materials get thicker, the rules for controlling these vibrations become richer, offering even more possibilities for tuning. The researchers propose that these effects could be observed experimentally using ultrafast X-ray or electron diffraction, which can capture snapshots of the atomic positions, or through transient reflectivity, which measures how the material's surface reflects light as it changes.

By establishing that the multipolarity of an exciton—whether it is a simple dipole or a complex quadrupole—controls the nature of the resulting phonon, the researchers have opened a new pathway for manipulating quantum states in solid materials. This finding moves beyond simply observing these phenomena to actively engineering them. It suggests that the future of quantum technology may lie not just in finding new materials, but in learning how to electrically tune the internal architecture of existing ones to produce the exact type of atomic motion needed for a specific task. The ability to switch between coherent and squeezed states on a chip could eventually enable technologies that operate at speeds and sensitivities previously thought impossible.