Researchers at EPFL, collaborating with Australian counterparts, have engineered an optical metasurface utilising chirality – the property of asymmetry – to control light polarisation. Published in Nature Communications in 2025, the innovation encodes information via both the size and orientation of nanoscale elements within the metasurface, allowing two distinct images – a cockatoo and the Matterhorn – to be simultaneously embedded and revealed using different light conditions. This technique, utilising germanium and calcium difluoride, demonstrates potential applications in data encryption, anticounterfeiting measures, and advanced biosensing, including the assessment of drug composition and purity.
Chirality-Enabled Metasurfaces
The engineered metasurface utilizes chirality, the property of non-superimposability on a mirror image, to control light and enable versatile techniques for data encryption, sensing, and computing. This principle is fundamental across multiple scientific disciplines, including biology and chemistry, where molecular handedness significantly impacts efficacy and function; for instance, DNA and sugars typically exhibit right-handedness, while amino acids are generally left-handed. Light itself possesses chirality through circular polarization, with electric fields spiralling in either a left- or right-handed fashion, and chiral structures demonstrate differing interactions with these polarizations.
The metasurface is constructed from germanium and calcium difluoride and comprises a two-dimensional lattice of tiny elements, termed meta-atoms, whose chiral properties can be adjusted. By varying the orientation of these meta-atoms within the lattice, scientists can control the metasurface’s interaction with polarized light, representing an advancement over previous methods reliant on complex meta-atom geometries. The interplay between meta-atom shape and lattice symmetry governs the response of the metasurface to polarized light.
A proof-of-concept experiment demonstrated the encoding of two distinct images simultaneously on a metasurface optimized for the mid-infrared range. The image of an Australian cockatoo was encoded in the size of the meta-atoms and decoded using unpolarized light, while the image of the Swiss Matterhorn was encoded in the orientation of the meta-atoms and revealed when exposed to circularly polarized light. This dual-layer watermark is imperceptible to the human eye, suggesting potential applications in anticounterfeiting measures, camouflage technologies, and enhanced security protocols.
Beyond encryption, the team’s approach offers potential benefits for quantum technologies, which frequently utilize polarized light for computational processes. The ability to map chiral responses across large surfaces could also facilitate advancements in biosensing, as chiral metastructures can be employed to assess drug composition or purity from limited samples. Distinguishing between left- and right-handed molecules is crucial in determining the efficacy and safety of pharmaceutical substances.
Multifaceted Encoding and Decoding
The innovation has been published in Nature Communications, detailing the construction and properties of the metasurface. The team’s metasurface presents a gradient of meta-atoms with continuously varying orientations, allowing for precise control over light interaction. The shape and angles of these meta-atoms, combined with the lattice symmetry, work in concert to tune the response of the metasurface to polarized light.
Beyond its application in dual-image encoding, the approach has potential applications for quantum technologies, as these often rely on polarized light for computation. Furthermore, the ability to map chiral responses across large surfaces could streamline biosensing; chiral metastructures can be used to sense drug composition or purity from small samples, as distinguishing between left- and right-handed molecules is crucial in determining whether a substance is a medicine or a toxin.
Potential Applications and Future Research
The team’s research indicates potential applications for chiral metasurfaces applications beyond encryption and biosensing, extending into quantum technologies where polarized light is frequently employed for computation. The ability to map chiral responses across large surface areas could also streamline biosensing processes, allowing for the assessment of drug composition or purity from small samples. Distinguishing between left- and right-handed molecules is a critical factor in determining whether a substance functions as a medicine or a toxin.
The research, published in Nature Communications, details the construction of the metasurface from germanium and calcium difluoride, presenting a gradient of meta-atoms with continuously varying orientations. This arrangement allows for precise control over light interaction, as the shape and angles of the meta-atoms, combined with the lattice symmetry, work in concert to tune the response of the metasurface to polarized light.
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