Imagine a world where light itself can move matter. That's the groundbreaking discovery Rice University researchers have made, and it could revolutionize technology as we know it. They've found a way to physically shift the atomic structure of certain atom-thin semiconductors, known as transition metal dichalcogenides (TMDs), using just light. This opens up exciting possibilities for controlling the behavior and properties of these incredibly thin materials.
These special materials are called Janus materials, named after the Roman god of transitions, and they're the key to this breakthrough. Their unique structure makes them incredibly sensitive to light. This light sensitivity could pave the way for future technologies that use light signals instead of electricity. Think faster, 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," explains Kunyan Zhang, a Rice doctoral alumna and the study's first author. "Two-dimensional materials, which are only a few atoms thick, make it possible to build these optical tools on a very small scale."
So, what makes Janus materials so special?
These materials are built from stacked layers of a transition metal like molybdenum, combined with two layers of a chalcogen element such as sulfur or selenium. Their unique blend of conductivity, strong light absorption, and mechanical flexibility makes them ideal for next-generation electronic and optical devices. But here's where it gets interesting: Janus materials have an asymmetric structure. Their top and bottom atomic layers are made of different chemical elements. This imbalance creates a built-in electrical polarity and dramatically 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," says Zhang.
Detecting Atomic Motion with Laser Light
The research team used laser beams of different colors on a two-layer Janus TMD material composed of molybdenum sulfur selenide stacked on molybdenum disulfide. They observed how the material alters light through a process called second harmonic generation (SHG). In SHG, the material emits light at double the frequency of the incoming beam. When the incoming laser light matched the material's natural resonances, the 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 explains. "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 team traced the SHG distortion to a phenomenon called optostriction, where the electromagnetic field of light exerts a mechanical force on the atoms. In Janus materials, the strong coupling between layers amplifies this effect, allowing even tiny 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 notes.
The Future is Bright: Potential for Future Optical Technologies
This high sensitivity suggests that Janus materials could become invaluable components in a wide range of optical technologies. Imagine devices that can guide or control light using this mechanism. This could lead to faster and more energy-efficient photonic chips, as light-based circuits generate less heat than traditional electronics. Similar properties could be used to build incredibly precise sensors that detect minute vibrations or pressure changes, or to develop advanced displays and imaging systems with adjustable light sources.
"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," says Shengxi Huang, an associate professor at Rice University.
Small Differences, Big Impact
By demonstrating how the internal asymmetry of Janus TMDs creates new ways to influence the flow of light, the study highlights how tiny structural differences can unlock significant technological opportunities. But what if these materials could be manipulated in even more complex ways? What other applications could emerge from this groundbreaking discovery? Let me know your thoughts in the comments below!
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.
