Light isn\u2019t just illuminating materials anymore, it\u2019s moving them.<\/p>\n
Scientists at Rice University have discovered that beams of light can physically shift atoms in a class of ultra-thin semiconductors known as Janus transition metal dichalcogenides (TMDs), unlocking a new way to tune materials for next-generation optical and quantum technologies.<\/p>\n
The finding offers a rare glimpse into how light and matter interact at the atomic level.<\/p>\n
When laser light hits these Janus TMDs, it doesn\u2019t just pass through, it exerts a mechanical push on atoms inside the crystal, changing its symmetry and optical behaveior.<\/p>\n
The phenomenon, called optostriction, could help build faster and cooler computer chips that use light instead of electricity.<\/p>\n
\u201cIn nonlinear optics, light can be reshaped to create new colors, faster pulses, or optical switches that turn signals on and off,\u201d said Kunyan Zhang, a Rice doctoral alumna and first author of the study.<\/p>\n
\u201cTwo-dimensional materials, which are only a few atoms thick, make it possible to build these optical tools on a very small scale.\u201d<\/p>\n
Janus materials are a special subtype of TMDs, named after the two-faced Roman god of transitions. <\/p>\n
Their top and bottom atomic layers are made of different chemical species, creating an internal imbalance that gives the crystal built-in polarity. <\/p>\n
This asymmetry makes them particularly sensitive to light, electric fields, and mechanical strain, allowing researchers to \u201ctune\u201d their behavior more precisely than ordinary semiconductors.<\/p>\n
When light pushes back<\/p>\n
Using laser light of different colors, the Rice team studied how a two-layer Janus TMD\u2014molybdenum sulfur selenide stacked on molybdenum disulfide\u2014responded to illumination through a process called second harmonic generation (SHG). In SHG, a material emits light at twice the frequency of the incoming beam.<\/p>\n
They found that when the incoming light matched the material\u2019s resonant frequencies, the emitted SHG pattern distorted\u2014a sign that atoms were being displaced.<\/p>\n
\u201cWe 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,\u201d Zhang said.<\/p>\n
Under normal conditions, the SHG pattern looks like a six-pointed flower, mirroring the crystal\u2019s symmetry.<\/p>\n
But as light pushed the atoms, \u201cthis symmetry breaks\u2014the petals of the pattern shrink unevenly,\u201d Zhang explained.<\/p>\n
The team traced this distortion to optostriction, where the electromagnetic field of light exerts a small but measurable mechanical force on the atoms.<\/p>\n
Because Janus materials have uneven compositions, this push is amplified by strong interlayer coupling, making them ideal for studying and harnessing light-driven atomic motion.<\/p>\n
Lighting the future<\/p>\n
\u201cJanus materials are ideal for this because their uneven composition creates an enhanced coupling between layers, which makes them more sensitive to light\u2019s tiny forces\u2014forces so small that it is difficult to measure directly, but we can detect them through changes in the SHG signal pattern,\u201d Zhang said.<\/p>\n
Such sensitivity could help create optical chips that route and process light instead of electricity, drastically improving energy efficiency.<\/p>\n
\u201cSuch active control could help design next-generation photonic chips, ultra-sensitive detectors, or quantum<\/a> light sources\u2014technologies that use light to carry and process information instead of relying on electricity,\u201d said Shengxi Huang, associate professor of electrical and computer engineering at Rice.<\/p>\n By showing that light can quite literally nudge atoms in two-dimensional semiconductors<\/a>, the research opens a path toward tunable, light-responsive materials that could reshape the future of computing and sensing, one photon push at a time.<\/p>\n