Researchers at Rice University have unveiled a remarkable phenomenon where certain atom-thin semiconductors, specifically transition metal dichalcogenides (TMDs), exhibit a tangible physical response to light, causing their atomic lattice to shift. This groundbreaking observation provides a novel, light-driven method for precisely controlling the intrinsic behavior and tunable properties of these ultrathin materials, paving the way for next-generation optical technologies.

The newly identified light-induced atomic motion is particularly pronounced in a specialized subclass of TMDs known as Janus materials. These materials derive their name from the Roman god of transitions, Janus, symbolizing their dual-natured response. Their heightened sensitivity to light suggests a future where optical signals, rather than electrical currents, could power advanced technologies. This includes the potential for significantly faster and more energy-efficient computer chips, highly sensitive sensors capable of detecting minute changes, and versatile, flexible optoelectronic systems that seamlessly integrate light and electronics.

"In the realm of nonlinear optics, light possesses the remarkable ability to be manipulated and reshaped, enabling the creation of new spectral hues, the generation of ultrafast pulses, and the development of optical switches that can precisely control the flow of signals, turning them on and off," explained Kunyan Zhang, a distinguished alumna of Rice University and the first author of the study. "The advent of two-dimensional materials, characterized by their astonishing thinness – merely a few atoms thick – has revolutionized the possibility of miniaturizing these sophisticated optical tools to an unprecedented scale."

The Unique Characteristics of Janus Materials

Transition metal dichalcogenides (TMDs) are fundamentally constructed from layered arrangements. At their core lies a transition metal atom, such as molybdenum, flanked by two layers of a chalcogen element, like sulfur or selenium. This atomic architecture endows TMDs with a compelling combination of desirable properties, including excellent conductivity, potent light absorption capabilities, and remarkable mechanical flexibility. These attributes have propelled them to the forefront as prime candidates for the development of electronic and optical devices that will define the next generation of technology.

Within the broader family of TMDs, Janus materials distinguish themselves through a unique structural asymmetry. Unlike conventional TMDs where both the top and bottom atomic layers are chemically identical, Janus materials feature distinct chemical elements on their opposing surfaces. This inherent imbalance creates a built-in electrical polarity within the material, significantly amplifying its responsiveness to external stimuli, including light and mechanical forces.

"Our investigation delves into the intricate relationship between the structural characteristics of Janus materials and their optical behavior," Zhang elaborated. "Crucially, we sought to understand how light itself can exert a physical force within these materials, inducing a measurable response."

Detecting Atomic Displacement Through Laser Illumination

To meticulously investigate this light-induced atomic motion, the research team employed a sophisticated experimental setup. They directed laser beams of varying wavelengths onto a carefully engineered two-layer Janus TMD sample. This specific sample was composed of molybdenum sulfur selenide stacked atop molybdenum disulfide. The researchers then analyzed how this material interacted with and altered the incident light. Their primary analytical tool was second harmonic generation (SHG), a nonlinear optical process where the material, when illuminated by a laser beam, emits light at precisely double the frequency of the incoming beam.

The critical observation occurred when the frequency of the incoming laser beam was precisely tuned to match the material’s natural resonant frequencies. Under these conditions, the typically predictable SHG pattern exhibited a distinct distortion. This deviation from the norm served as a clear indicator that the atoms within the material were undergoing physical displacement.

"We made a fascinating discovery: when light interacts with Janus molybdenum sulfur selenide and molybdenum disulfide, it generates subtle, yet directional, forces within the material," Zhang recounted. "These internal forces manifest as discernible changes in the material’s SHG pattern. Normally, the SHG signal forms a symmetrical, six-pointed ‘flower’ shape that faithfully reflects the inherent symmetry of the crystal lattice. However, when light exerts its influence, pushing on the atoms, this symmetry is disrupted. We observed that the petals of the ‘flower’ pattern would shrink unevenly, a direct consequence of the atomic motion."

The Role of Optostriction and Layer Coupling

Through their detailed analysis, the researchers traced the origin of these SHG pattern distortions to a phenomenon known as optostriction. Optostriction describes the process by which the oscillating electromagnetic field of light exerts a mechanical force on the atoms within a material. In the case of Janus materials, a key factor amplifying this effect is the exceptionally strong coupling that exists between their constituent layers. This enhanced interlayer coupling means that even incredibly minuscule forces generated by light can result in a measurable and significant strain within the material.

"Janus materials are ideally suited for observing this phenomenon precisely because of their asymmetric composition," Zhang emphasized. "This asymmetry fosters a heightened level of coupling between the layers, rendering them extraordinarily sensitive to the subtle forces exerted by light. These forces are so minute that they would be exceedingly difficult to measure directly. However, we can effectively detect them through the observable changes in the SHG signal pattern, which acts as a sensitive indicator of atomic displacement."

Profound Implications for Future Optical Technologies

The remarkable sensitivity exhibited by Janus materials to light-induced forces holds immense promise for their integration into a wide array of future optical technologies. Devices designed to actively guide or modulate light through this optostrictive mechanism could lead to the development of photonic chips that operate at significantly higher speeds and consume far less energy than conventional electronic circuits. This is primarily because light-based communication and processing generate considerably less heat.

Furthermore, these unique properties could be harnessed to create highly precise sensors capable of detecting extremely subtle vibrations or minute pressure fluctuations. The ability to finely tune light sources for advanced display systems and sophisticated imaging technologies also represents another exciting avenue of application.

"This capacity for active, light-induced control is instrumental in the design of next-generation photonic chips, ultrasensitive detectors, and even quantum light sources," stated Shengxi Huang, an associate professor of electrical and computer engineering, materials science and nanoengineering at Rice, and a corresponding author of the study. Huang’s extensive affiliations include the Smalley-Curl Institute, the Rice Advanced Materials Institute, and the Ken Kennedy Institute, underscoring the interdisciplinary nature of this research. "These are technologies that leverage the power of light to carry and process information, offering a compelling alternative to the limitations of traditional electricity-based systems."

The Transformative Power of Subtle Structural Differences

By unequivocally demonstrating how the inherent internal asymmetry of Janus TMDs unlocks novel pathways to influence the behavior and flow of light, this study underscores a profound principle: even seemingly minor structural variations at the atomic level can unlock significant and transformative technological opportunities. The research was generously supported by funding from the National Science Foundation (grants 2246564 and 1943895), the Air Force Office of Scientific Research (grant FA9550-22-1-0408 and FA2386-24-1-4049), the Welch Foundation (grant C-2144), the U.S. Department of Energy (grants DE-SC0020042 and DE-AC02-05CH11231), and the Taiwan Ministry of Education. The content presented in this article is the sole responsibility of the authors and does not necessarily reflect the official views or positions of the funding organizations and institutions involved.