Researchers at Rice University have achieved a groundbreaking discovery, demonstrating that certain atom-thin semiconductors, specifically transition metal dichalcogenides (TMDs), can physically shift their atomic lattice when illuminated by light. This novel light-induced atomic response provides a controllable mechanism to precisely tune the behavior and inherent properties of these ultrathin materials, paving the way for revolutionary advancements in optical technologies. The phenomenon is particularly pronounced in a specialized subtype of TMDs known as Janus materials, named for the Roman god of transitions due to their dual-natured composition and response to external stimuli. This newfound light sensitivity holds immense promise for future technological applications that leverage optical signals over traditional electrical currents, potentially leading to significantly faster and more energy-efficient computer chips, highly sensitive sensors capable of detecting minute environmental changes, and sophisticated flexible optoelectronic systems.

As explained by Kunyan Zhang, a Rice doctoral alumna and the first author of the study, the field of nonlinear optics has long explored how light can be manipulated to create new optical functionalities, such as generating different colors, producing faster light pulses, or developing optical switches for signal control. The advent of two-dimensional (2D) materials, which are mere atoms thick, has enabled the miniaturization of these optical tools to an unprecedented scale. The current research delves into how the unique structural attributes of Janus materials contribute to their remarkable optical behavior and, crucially, how light itself can exert a physical force on their atomic structure.

Transition metal dichalcogenides (TMDs) are a class of materials characterized by their layered structure, typically consisting of a transition metal atom sandwiched between two layers of a chalcogen element. This unique composition imbues TMDs with a compelling combination of electrical conductivity, strong light absorption capabilities, and remarkable mechanical flexibility, making them prime candidates for the development of next-generation electronic and optical devices. However, Janus materials distinguish themselves within this family due to their asymmetric atomic structure. Unlike conventional TMDs where both outer layers are chemically identical, Janus materials feature different chemical elements on their top and bottom atomic layers. This inherent asymmetry results in a built-in electrical polarity within the material, which significantly enhances its sensitivity to light and other external forces. This amplified sensitivity is the key to understanding the observed atomic movement.

To meticulously investigate this light-induced atomic motion, the research team employed a sophisticated experimental setup. They directed laser beams of various wavelengths onto a meticulously prepared two-layer Janus TMD material. This specific material was engineered by stacking a layer of molybdenum sulfur selenide on top of a layer of molybdenum disulfide. The researchers then analyzed how the material interacted with and altered the incident light. A crucial technique utilized was second harmonic generation (SHG). In this process, when light interacts with certain materials, it can emit light at precisely double the frequency of the incoming beam. By carefully observing the SHG pattern produced by the Janus material, the team was able to detect subtle distortions. These distortions, particularly when the incoming laser frequency resonated with the material’s natural vibrational modes, provided irrefutable evidence that the atoms within the material were physically shifting their positions.

Zhang elaborated on these findings, stating, "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. 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." This observation is a direct manifestation of light exerting a mechanical influence on the atomic lattice.

The underlying mechanism responsible for this phenomenon was identified as optostriction. Optostriction is a process where the electromagnetic field of light applies a mechanical force on the atoms within a material. In the case of Janus materials, the strong coupling between their distinct atomic layers significantly amplifies this optostrictive effect. This enhanced coupling means that even extremely minuscule forces exerted by the light can produce measurable strain and atomic displacement within the material. "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 further explained. This amplification makes the atomic response detectable and exploitable.

The profound implications of this high sensitivity open up exciting avenues for a wide range of future optical technologies. Devices designed to guide or control light through this light-induced atomic motion could revolutionize photonic chips. By replacing electrical signals with optical ones, these chips could operate at significantly higher speeds and with drastically reduced heat generation, leading to more powerful and energy-efficient computing. Furthermore, similar principles could be applied to create highly sensitive sensors capable of detecting minute vibrations or pressure shifts with unprecedented accuracy. The ability to precisely control light at the atomic level also holds promise for developing adjustable light sources for advanced display technologies and sophisticated imaging systems.

Shengxi Huang, an associate professor of electrical and computer engineering and materials science and nanoengineering at Rice and a corresponding author of the study, highlighted the broader impact: "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." Huang’s affiliations with prominent research institutes at Rice underscore the collaborative and interdisciplinary nature of this groundbreaking work.

In essence, this study underscores a fundamental principle: even subtle structural imbalances within materials can unlock significant technological opportunities. By meticulously demonstrating how the internal asymmetry of Janus TMDs provides a novel mechanism to influence the flow of light, the research highlights the immense potential residing in exploring the nuanced properties of atomically thin materials. This work, supported by a consortium of prestigious funding agencies including the National Science Foundation, the Air Force Office of Scientific Research, the Welch Foundation, and the U.S. Department of Energy, represents a significant leap forward in our understanding and utilization of light-matter interactions at the nanoscale. The findings are solely the responsibility of the authors and do not necessarily reflect the official views of the funding organizations.