Researchers at Rice University have discovered that light can physically alter the atomic structure of a special class of atom-thin semiconductors, a finding that could pave the way for a new generation of optical computing devices. The breakthrough involves a unique material that responds to light by subtly shifting its internal lattice, providing a novel mechanism for controlling and manipulating its electronic and optical properties. This discovery opens the door to developing faster, smaller, and more energy-efficient components for technologies that use light instead of electricity to process information.
The study focuses on a type of two-dimensional material known as a Janus transition metal dichalcogenide (TMD), named after the two-faced Roman god. This material’s unique, asymmetric structure is key to its remarkable responsiveness to light. By demonstrating that light can induce a mechanical force within the semiconductor, the research provides a new toolkit for designing advanced optical switches, sensors, and flexible optoelectronic devices, potentially overcoming the heat-related limitations of conventional electronics.
Asymmetric Materials at the Core
The foundation of this research lies in a specific subclass of TMDs called Janus materials. Unlike typical TMDs, which have a symmetric sandwich structure, Janus TMDs are asymmetric. They consist of a central layer of transition metal atoms, such as molybdenum, bonded to two different types of chalcogen atoms, like sulfur on one side and selenium on the other. This structural imbalance creates a built-in electric field, or polarity, within the material, making it exceptionally sensitive to external stimuli like light.
This inherent asymmetry is what makes Janus TMDs particularly promising for optoelectronic applications. Their two-faced nature, with distinct chemical compositions on the top and bottom surfaces, leads to unique physical and electronic behaviors not found in their symmetric counterparts. The internal electric dipole moment makes them highly responsive to the electromagnetic fields of light, enabling the direct conversion of light energy into mechanical strain within the crystal lattice.
Observing Light-Induced Forces
Probing with Nonlinear Optics
To observe the material’s response to light, the Rice University team employed a technique called second harmonic generation (SHG) spectroscopy. SHG is a nonlinear optical process where the material converts two photons of an incoming light source into a single photon with twice the energy and frequency. This process is highly sensitive to the material’s crystal symmetry. In its normal state, the Janus TMD material, specifically molybdenum sulfur selenide (MoSSe) layered on molybdenum disulfide (MoSâ‚‚), produces a distinct, six-pointed “flower” pattern in the SHG signal that mirrors its hexagonal atomic structure.
Symmetry Breaking as Evidence
The researchers made their key discovery when they tuned the frequency of the incoming laser to match the material’s natural resonance. They observed that the SHG pattern became distorted, losing its perfect six-fold symmetry. The petals of the pattern would shrink unevenly, providing direct evidence that the light was exerting a tiny, directional force on the atoms. This effect, known as optostriction, occurs when the light’s electromagnetic field mechanically displaces the atoms in the lattice, thereby breaking the crystal’s inherent symmetry and altering its optical output.
Implications for Optical Computing
This discovery has significant implications for the future of computing and sensor technology. By demonstrating a method to control a material’s physical structure with light, the research opens a pathway to creating all-optical switches and modulators. These components could operate at much higher speeds and with greater energy efficiency than current electronic transistors, which are limited by heat generation from electrical resistance. Light-based circuits would generate significantly less heat, allowing for more compact and powerful devices.
The ability to minutely deform the material’s lattice using light could also be harnessed to develop ultrasensitive sensors. Such devices could be engineered to detect tiny vibrations or pressure changes, with applications ranging from advanced imaging to environmental monitoring. The principle of using light to reconfigure a material’s properties on demand provides a foundation for developing a wide range of tunable, next-generation photonic technologies.
A New Frontier in Materials Science
This work stands at the intersection of condensed matter physics, materials science, and photonics, establishing a new avenue of research into the optomechanical properties of 2D materials. The findings highlight how subtle asymmetries at the atomic scale can be leveraged for significant technological gains. According to Kunyan Zhang, a first author of the study, this research explores how the structure of Janus materials influences their optical behavior and, in turn, how light itself can generate a force within them.
By providing a framework for manipulating matter with light, the research paves the way for designing smarter materials with programmable properties. Future work will likely explore the full potential of these light-induced forces in other types of Janus materials and complex heterostructures. This could lead to the development of highly efficient light sources, advanced display technologies, and other devices that form the backbone of a new era in light-based technology.