Researchers at Rice University have achieved a groundbreaking discovery, demonstrating that certain atom-thin semiconductors, specifically transition metal dichalcogenides (TMDs), possess the remarkable ability to physically shift their atomic lattice when illuminated by light. This newly observed light-induced atomic response offers an unprecedented level of controllable tuning for the behavior and properties of these ultrathin materials, paving the way for revolutionary advancements in optics, electronics, and sensing technologies. The phenomenon is particularly pronounced in a specialized subtype of TMDs known as Janus materials, named for the Roman god of transitions, whose inherent asymmetry imbues them with exceptional sensitivity to light and external forces. This light-driven atomic manipulation promises to enable future technologies that harness optical signals as efficiently, or even more so, than traditional electrical currents, leading to the development of significantly faster and cooler computer chips, highly responsive and precise sensors, and adaptable, flexible optoelectronic systems.

"In the realm of nonlinear optics, light possesses the remarkable ability to be reshaped and manipulated, allowing for the generation of new colors, the creation of ultrafast pulses, and the development of optical switches that can precisely control signal flow – turning them on and off with exquisite accuracy," explained Kunyan Zhang, a Rice doctoral alumna and the first author of the groundbreaking study. "The development of two-dimensional materials, which are astonishingly thin, only a few atoms thick, makes it feasible to engineer these sophisticated optical tools on an incredibly small and manageable scale."

The Unique Architecture of Janus Materials: A Foundation for Light-Induced Atomic Motion

TMDs, as a class of materials, are constructed from layered arrangements of a transition metal, such as molybdenum, sandwiched between two layers of a chalcogen element, like sulfur or selenium. Their unique combination of desirable properties – including excellent electrical conductivity, strong light absorption capabilities, and impressive mechanical flexibility – has positioned them as prime candidates for the development of next-generation electronic and optical devices. However, it is within the specific subgroup of Janus materials that the most profound light-induced phenomena are observed.

What distinguishes Janus materials is their inherently asymmetric atomic structure. Unlike conventional TMDs, where both the top and bottom atomic layers are chemically identical, Janus materials feature different chemical elements in their uppermost and lowermost atomic planes. This structural imbalance is not merely a cosmetic difference; it leads to the creation of a built-in electrical polarity within the material. This intrinsic polarity, in turn, significantly amplifies their sensitivity to external stimuli, including light and mechanical forces.

"Our research meticulously investigates how the specific, asymmetric structure of Janus materials profoundly influences their optical behavior," Zhang elaborated. "Furthermore, we are exploring the fundamental mechanisms by which light itself can exert a tangible physical force on the atoms within these materials, leading to their displacement."

Unveiling Atomic Motion: The Power of Laser Light and Second Harmonic Generation

To meticulously investigate and quantify this light-induced atomic motion, the research team employed a sophisticated experimental approach. They subjected a two-layer Janus TMD material, specifically composed of molybdenum sulfur selenide stacked upon molybdenum disulfide, to precisely controlled laser beams of varying wavelengths. The team then analyzed the material’s response by examining how it modified the incident light through a phenomenon known as second harmonic generation (SHG). SHG is a nonlinear optical process where a material, when exposed to incident light of a certain frequency, emits light at precisely twice that frequency.

The critical observation occurred when the frequency of the incoming laser beam was carefully tuned to match the material’s natural optical resonances. Under these specific conditions, the standard SHG pattern, which typically exhibits a high degree of symmetry, became noticeably distorted. This distortion served as a clear and compelling indicator that the atoms within the material were physically shifting their positions.

"We made a remarkable discovery: when we shine light onto Janus molybdenum sulfur selenide and molybdenum disulfide, it generates subtle, yet distinctly directional forces within the material," Zhang recounted. "These internal forces manifest themselves as observable changes in the material’s SHG pattern. Typically, the SHG signal forms a beautiful, six-pointed ‘flower’ shape, which directly mirrors the inherent symmetry of the crystal structure. However, when the incident light exerts a physical push on the atoms, this delicate symmetry is broken – the petals of the SHG pattern shrink unevenly, providing a visual signature of the atomic displacement."

Optostriction and Layer Coupling: The Synergy Driving Atomic Response

Through their detailed analysis, the researchers were able to definitively attribute the observed distortion in the SHG signal to a phenomenon called optostriction. Optostriction is a fascinating process where the electromagnetic field of light exerts a mechanical force on the atoms within a material. In the context of Janus materials, this optostrictive effect is significantly amplified due to a strong coupling between their different atomic layers. This enhanced inter-layer coupling allows even extremely minute forces, generated by the light, to produce measurable strain within the material.

"Janus materials are ideally suited for observing and exploiting this phenomenon precisely because their uneven chemical composition leads to an enhanced coupling between their atomic layers," Zhang explained. "This enhanced coupling makes them exquisitely sensitive to the incredibly tiny forces exerted by light. While these forces are so small that they are difficult to measure directly, we can effectively detect their presence and magnitude through the subtle yet distinct changes they induce in the SHG signal pattern."

The Dawn of Advanced Optical Technologies: Harnessing Light for Control and Sensing

The exceptional sensitivity demonstrated by Janus materials to light-induced atomic motion holds immense promise for a wide array of future optical technologies. Devices designed to guide, manipulate, or control light using this novel mechanism could revolutionize the field of photonics. The development of photonic chips that rely on light for data processing and transmission, rather than electricity, could lead to significantly faster and more energy-efficient computing, as light-based circuits generate substantially less heat than their traditional electronic counterparts.

Beyond computing, these unique properties could be leveraged to create highly sensitive sensors capable of detecting extremely subtle vibrations, minute pressure shifts, or even minute changes in their environment with unprecedented accuracy. Furthermore, the ability to precisely control light-matter interactions opens doors for the development of advanced adjustable light sources, crucial for next-generation display technologies and sophisticated imaging systems.

"The ability to actively control atomic behavior with light opens up exciting avenues for designing next-generation photonic chips, ultrasensitive detectors, and even quantum light sources," commented Shengxi Huang, an associate professor of electrical and computer engineering and materials science and nanoengineering at Rice University and a corresponding author of the study. Huang, who is also affiliated with prestigious Rice institutes including the Smalley-Curl Institute, the Rice Advanced Materials Institute, and the Ken Kennedy Institute, emphasized the paradigm shift this research represents. "These are technologies that fundamentally rely on light to carry and process information, offering a compelling alternative to traditional reliance on electricity."

The Profound Impact of Subtle Structural Imbalances: A Testament to Precision Engineering

This seminal study underscores a critical principle in materials science: even seemingly small structural differences within a material can unlock significant and transformative technological opportunities. By clearly demonstrating how the inherent internal asymmetry of Janus TMDs provides novel pathways to influence and control the flow of light, the research highlights the immense potential of precisely engineered nanomaterials. The findings not only advance our fundamental understanding of light-matter interactions at the atomic scale but also provide a concrete roadmap for the development of future technologies that are faster, more efficient, and more sensitive than ever before.

The research was generously supported by numerous funding organizations, including the National Science Foundation (grants 2246564, 1943895), the Air Force Office of Scientific Research (grant FA9550-22-1-0408 and FA2386-24-1-4049), the Welch Foundation (grant C-2144), and the U.S. Department of Energy (grants DE‐SC0020042, DE‐AC02‐05CH11231). Additional support was provided by the Taiwan Ministry of Education. It is important to note that the content of this article represents the sole responsibility of the authors and does not necessarily reflect the official viewpoints of the funding organizations and institutions involved.