Flat chip uses twisted light to reveal hidden images and detect molecular chirality

A left-handed glove will never fit a right hand. The mismatch is absolute—the two are mirror images that cannot be forced into alignment. This principle of handedness, which scientists call chirality, governs far more than clothing. It shapes the structure of DNA, determines whether a sugar molecule can nourish a body or pass through it unused, and decides whether a pharmaceutical compound will heal or harm. Most naturally occurring DNA and sugars spiral one way; most amino acids spiral the other. Flip the handedness of a drug molecule, and it may become inert or toxic. The stakes are high, and the ability to detect and control chirality has long been a frontier of molecular science.

Light itself can be handed. When a beam of light becomes circularly polarized, its electric field twists through space like a corkscrew—either left-handed or right-handed. Because chiral molecules interact differently with these two types of twisted light, researchers can shine polarized light onto a sample and measure how much of each twist gets absorbed, reflected, or delayed. The result reveals the sample's own handedness. But this interaction is faint, making precise control of the effect extraordinarily difficult. Until now.

A team at EPFL's Bionanophotonic Systems Laboratory, working with collaborators in Australia, has engineered artificial optical structures called metasurfaces—flat, two-dimensional lattices built from tiny elements called meta-atoms. By varying how these meta-atoms are oriented within the lattice, the researchers can tune how the metasurface responds to polarized light. The breakthrough lies not in making the meta-atoms themselves more complex, but in orchestrating their arrangement. As Hatice Altug, who leads the lab, explains it: the interplay between the shape of each meta-atom and the symmetry of the overall lattice creates the control. It is elegant simplicity outperforming brute-force complexity. The work was published in Nature Communications.

To demonstrate what this means in practice, the team built a metasurface from germanium and calcium difluoride, with meta-atoms arranged in a gradient—their orientations shifting continuously across the chip. They then encoded two separate images on the same surface. The first image, a cockatoo, was written into the size of the meta-atoms themselves, which act as pixels. Shine unpolarized light on the chip, and the cockatoo appears. The second image, the Matterhorn, was encoded in the orientation of those same meta-atoms. Only when the chip is exposed to circularly polarized light does the mountain reveal itself. Two images, one surface, invisible to the naked eye until the right light illuminates it. The researchers call this a dual-layer watermark, and it opens immediate doors to anticounterfeiting, camouflage, and security applications where hidden information must remain undetectable until authenticated.

Beyond encryption, the implications branch in multiple directions. Quantum technologies often rely on polarized light to perform calculations; the ability to map chiral responses across large surfaces could unlock new computational possibilities. But perhaps the most consequential application lies in biosensing and pharmaceutical safety. Felix Richter, a researcher in the lab, frames it plainly: chiral metastructures like theirs could detect drug composition and purity from tiny samples. Nature is built on chirality. The difference between a medicine and a poison often comes down to which way a molecule spirals. A technology that can reliably distinguish left-handed from right-handed molecules at scale could transform how drugs are verified before they reach patients. The flat chip, no thicker than a wafer, sits at the intersection of optical physics and molecular biology—a place where the shape of light and the shape of matter finally speak the same language.