In a groundbreaking discovery, researchers have demonstrated that light can manipulate atomic movement within two-dimensional semiconductors, particularly in a subtype known as Janus materials. Named after the Roman god associated with transitions, these materials exhibit a unique sensitivity to light, paving the way for future technologies that could rely on optical signals rather than electrical currents. This advancement promises innovations such as 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 University 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
Transition metal dichalcogenides (TMDs) are composed of 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 positioned them as key candidates for next-generation electronic and optical devices.
Janus materials, however, stand apart due to their asymmetric structure, where the top and bottom atomic layers consist of different chemical elements. This imbalance results in a built-in electrical polarity, enhancing 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 explained.
Detecting Atomic Motion With Laser Light
To investigate this behavior, the research team employed laser beams of various colors on a two-layer Janus TMD material composed of molybdenum sulfur selenide stacked on molybdenum disulfide. They observed alterations in light through second harmonic generation (SHG), a process where the material emits light at twice the frequency of the incoming beam. When the laser’s frequency matched the material’s natural resonances, the typical SHG pattern was distorted, indicating atomic movement.
“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 noted. “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 attributed the SHG distortion to optostriction, a phenomenon where the electromagnetic field of light exerts a mechanical force on atoms. In Janus materials, the strong coupling between layers amplifies this effect, allowing even minuscule 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 invaluable 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, as light-based circuits produce less heat than traditional electronics. Similar properties could be harnessed to create finely tuned sensors capable of detecting 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 University and a corresponding author of the study.
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 highlights that even tiny structural differences can unlock significant technological opportunities.
The research was supported by the National Science Foundation, the Air Force Office of Scientific Research, the Welch Foundation, the U.S. Department of Energy, 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 the funding organizations and institutions.
