"In nonlinear optics, light can be reshaped to create new colors, faster pulses or optical switches that turn signals on and off," said Kunyan Zhang, a Rice doctoral alumna and first author of the study. "Two-dimensional materials, which are only a few atoms thick, make it possible to build these optical tools on a very small scale."
TMDs are layered crystals formed of a transition metal such as molybdenum and chalcogen elements like sulfur or selenium. Their combination of electrical conductivity, light absorption, and mechanical flexibility makes them versatile for next-generation electronics and optoelectronics.
Janus materials stand out for their asymmetry, with differing chemical species on the top and bottom atomic layers. This creates internal imbalance and a built-in electrical polarity that increases sensitivity to light and external forces.
"Our work explores how the structure of Janus materials affects optical behavior and how light itself can generate a force in the materials," Zhang explained.
The team used laser light of multiple colors to investigate how a two-layer Janus TMD - molybdenum sulfur selenide stacked on molybdenum disulfide - converts light via second harmonic generation (SHG), emitting light at double the incoming frequency. When the incoming beam matched the material's resonances, the doubled-frequency light pattern distorted, indicating displacement of atoms inside the lattice.
"We discovered that shining light on Janus molybdenum sulfur selenide and molybdenum disulfide creates tiny, directional forces in the material, which show up as changes in its SHG pattern," Zhang said. "Normally, the SHG signal forms a six-pointed 'flower' shape mirroring the crystal's symmetry. But when light pushes on the atoms, this symmetry breaks - the petals shrink unevenly."
The team traced the distortion to optostriction, the mechanical push generated directly by the electromagnetic field of light. In Janus materials, strong interlayer coupling amplifies this optostriction, enabling even minimal forces to create measurable strain.
"Janus materials are ideal for this because their uneven composition creates enhanced coupling between layers, making them more sensitive to light's tiny forces - forces so small that we detect them through changes in the SHG signal pattern," Zhang said.
This sensitivity could make such materials valuable beyond laboratory research. Switches or routing components using this principle may allow for faster and more energy-efficient optical chips, while the same responsiveness can serve in precise sensors or tunable light sources for advanced displays and imaging devices.
"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," said Shengxi Huang, associate professor and corresponding author of the study.
By showing how the built-in imbalance of Janus TMDs opens new ways to guide the flow of light, the research highlights how small structural features can unlock significant technological possibilities.
Research Report:Optomechanical Tuning of Second Harmonic Generation Anisotropy in Janus MoSSe/MoS2 Heterostructures
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