Breakthrough shows light can move atoms in 2D semiconductors

The phenomenon appears in a subtype of TMDs called Janus materials, named for the Roman god associated with transitions. Their light sensitivity could support future technologies that rely on optical signals instead of electrical currents, including faster and cooler computer chips, highly responsive sensors and flexible optoelectronic systems.

"In nonlinear optics, light can be reshaped to create new colors, faster pulses or optical switches that turn signals on and off," said Kunyan Zhang, a Rice doctoral alumna and first author of the study. "Two-dimensional materials, which are only a few atoms thick, make it possible to build these optical tools on a very small scale."

What Makes Janus Materials Different

TMDs are built from stacked layers of a transition metal such as molybdenum and two layers of a chalcogen element like sulfur or selenium. Their blend of conductivity, strong light absorption and mechanical flexibility has made them key candidates for next-generation electronic and optical devices.

Within this group, Janus materials stand apart because their top and bottom atomic layers are composed of different chemical elements, giving them an asymmetric structure. This imbalance produces a built-in electrical polarity and increases their sensitivity to light and external forces.

"Our work explores how the structure of Janus materials affects their optical behavior and how light itself can generate a force in the materials," Zhang said.

Detecting Atomic Motion With Laser Light

To investigate this behavior, the team used laser beams of various colors on a two-layer Janus TMD material composed of molybdenum sulfur selenide stacked on molybdenum disulfide. They examined how it alters light through second harmonic generation (SHG), a process in which the material emits light at twice the frequency of the incoming beam. When the incoming laser matched the material's natural resonances, the usual SHG pattern became distorted, revealing that the atoms were shifting.

"We discovered that shining light on Janus molybdenum sulfur selenide and molybdenum disulfide creates tiny, directional forces inside the material, which show up as changes in its SHG pattern," Zhang said. "Normally, the SHG signal forms a six-pointed 'flower' shape that mirrors the crystal's symmetry. But when light pushes on the atoms, this symmetry breaks -- the petals of the pattern shrink unevenly."

Optostriction and Layer Coupling

The researchers traced the SHG distortion to optostriction, a process in which the electromagnetic field of light applies a mechanical force on atoms. In Janus materials, the strong coupling between layers magnifies this effect, allowing even extremely small forces to produce measurable strain.

"Janus materials are ideal for this because their uneven composition creates an enhanced coupling between layers, which makes them more sensitive to light's tiny forces -- forces so small that it is difficult to measure directly, but we can detect them through changes in the SHG signal pattern," Zhang said.

Potential for Future Optical Technologies

This high sensitivity suggests that Janus materials could become valuable components in a wide range of optical technologies. Devices that guide or control light using this mechanism may lead to faster, more energy-efficient photonic chips, since light-based circuits produce less heat than traditional electronics. Similar properties could be used to build finely tuned sensors that detect extremely small vibrations or pressure shifts, or to develop adjustable light sources for advanced displays and imaging systems.

"Such active control could help design next-generation photonic chips, ultrasensitive detectors or quantum light sources -- technologies that use light to carry and process information instead of relying on electricity," said Shengxi Huang, associate professor of electrical and computer engineering and materials science and nanoengineering at Rice and a corresponding author of the study. Huang is also affiliated with the Smalley-Curl Institute, the Rice Advanced Materials Institute and the Ken Kennedy Institute.

Small Structural Imbalances With Big Impact

By demonstrating how the internal asymmetry of Janus TMDs creates new ways to influence the flow of light, the study shows that tiny structural differences can unlock significant technological opportunities.

The research was supported by the National Science Foundation (2246564, 1943895), the Air Force Office of Scientific Research (FA9550-22-1-0408), the Welch Foundation (C-2144), the U.S. Department of Energy (DE‐SC0020042, DE-AC02-05CH11231), the U.S. Air Force Office of Scientific Research (FA2386-24-1-4049) and the Taiwan Ministry of Education. The content of this article is solely the responsibility of the authors and does not necessarily represent the official views of funding organizations and institutions.