Imagine a chip so thin that you can barely see anything—until you shine distorted light at it. Scientists have developed a metasurface that can hide and reveal two distinct images based on the polarization of light.

This technology uses carefully arranged nanostructures to not only fool the eye, but also opens the door to the next generation of encryption, biosensing and quantum technologies. From invisible watermarks to testing drug purity, this flat device exploits the fundamental physics of handedness to uncover long-hidden secrets from nature and science.

Trying to put a glove on your left hand on your right hand won't work because the two are mirror images - they look similar but are not aligned in the same direction. This concept, known as "chirality," is a key principle in biology, chemistry and materials science. In nature, most DNA strands and sugars are right-handed, while most amino acids are left-handed. If a molecule's handedness is reversed, it stops functioning properly. Nutrients can become ineffective and medications can lose their benefits or even become dangerous.

Light also has some handedness. When light is circularly polarized, its electric field rotates as it propagates forward, forming a left- or right-handed spiral. Chiral materials respond differently to each polarization of light. By shining circularly polarized light onto a substance and measuring the absorption, reflection, or retardation of each helix, scientists can learn about the substance's own chirality. But because these interactions are extremely subtle, precisely controlling chirality has been a long-standing challenge.

Differently oriented superatoms on chiral metasurfaces. Image source: 2025 EPFL Bio-Nanophotonic Systems Laboratory CC BY SA 4.0

Now, scientists from the Laboratory of BioNanophotonic Systems at EPFL's School of Engineering, in collaboration with scientists in Australia, have created an artificial optical structure called a "metasurface": a two-dimensional lattice of tiny elements (superatoms) whose chiral properties can be easily tuned. By changing the orientation of metaatoms within the crystal lattice, scientists can control how the resulting metasurface interacts with polarized light.

"Our 'chiral design toolkit' is elegant in its simplicity, yet more powerful than previous methods of controlling light through very complex superatomic geometries. Instead, we exploit the interplay between superatomic shapes and metasurface lattice symmetries," explains Hatice Altug, head of the BioNanophotonics Laboratory.

The innovation, published in the journal Nature Communications, has potential applications in data encryption, biosensing and quantum technology.

Two different images are simultaneously encoded on a metasurface optimized for the invisible mid-infrared range of the electromagnetic spectrum. Image source: 2025 EPFL Bio-Nanophotonic Systems Laboratory CC BY SA 4.0

The team's metasurface, made from germanium and calcium difluoride, exhibits a gradient arrangement of superatoms whose orientation changes continuously along the chip. The shape and angle of these metaatoms, as well as the lattice symmetry, work together to modulate the metasurface's response to polarized light.

In a proof-of-concept experiment, the scientists simultaneously encoded two different images on a metasurface optimized for the invisible mid-infrared band of the electromagnetic spectrum. In the first image of an Australian cockatoo, the image data was encoded in superatomic sizes (representing pixels) and decoded using unpolarized light. The second image encodes the orientation of the metaatoms so that when exposed to circularly polarized light, the metasurface appears to resemble the iconic Swiss Matterhorn.

"This experiment demonstrates the ability of our technology to generate a double-layer 'watermark' invisible to the human eye, paving the way for advanced anti-counterfeiting, camouflage and security applications," said Ivan Sinev, a researcher at the BioNanophotonic Systems Laboratory.

In addition to encryption, the team's method has potential applications in quantum technologies, many of which rely on polarized light for calculations. The ability to map chiral responses over large areas of surface could also simplify biosensing.

"We can use chiral superstructures like ours to sense information such as drug content or purity in small-volume samples. Nature is chiral, and the ability to distinguish left-handed and right-handed molecules is critical because it can differentiate between drugs and toxins," said Felix Richter, a researcher in the BioNanophotonic Systems Laboratory.

Compiled from /scitechdaily