Circularly polarized light—where the electric field of light spirals in a clockwise or counterclockwise direction as it travels—has become integral to several advanced applications, including in medical imaging, optical communications, and remote sensing. It holds a crucial role because of its ability to interact with chiral molecules in ways that linear polarized light cannot, thus enhancing techniques such as circular dichroism spectroscopy. Despite its importance, generating circularly polarized light traditionally requires complex, bulky optical set-ups, restricting its use in compact systems.
Researchers in Singapore, however, are poised to change this by introducing a novel and extremely efficient metasurface design capable of generating circularly polarized light with just a single thin layer of material. This new technology, developed by a team led by Associate Professor Wu Lin of the Singapore University of Technology and Design (SUTD), uses a combination of nonlinearity, chirality, and rotational symmetry within the metasurface to generate circular polarization, pushing the boundaries of how this optical phenomenon is used in practical applications.
A Game-Changing Approach to Circularly Polarized Light
In their recent work, published in Physical Review Letters under the title “Enabling all-to-circular polarization up-conversion by nonlinear chiral metasurfaces with rotational symmetry,” the team demonstrates how to produce circularly polarized light from arbitrary optical inputs. This breakthrough challenges the traditional, bulky optical systems and opens up new possibilities for miniaturized optical technologies.
At the heart of their work is the innovative metasurface they developed. Metasurfaces are ultra-thin materials engineered to exhibit properties not found in nature. They consist of nanostructures arranged on a flat plane and are designed to control light at subwavelength scales. The metasurface developed by Prof Wu’s team exhibits chirality, a property that enables it to respond differently to left-handed and right-handed circular polarization. Chirality is essential to the unique behavior of optical devices, as it means that the object cannot be superimposed onto its mirror image, much like how our left and right hands are mirror images but not identical.
The ability to produce circularly polarized light from this type of metasurface is due to two defining characteristics: chirality and rotational symmetry. The team’s design allows for the nonlinearly converting an incoming light signal into circular polarization, regardless of the light’s initial state of polarization, in a highly efficient and compact format.
The Role of Nonlinearity in Light Conversion
What sets this new metasurface apart from traditional ones is its nonlinearity. In general, a linear optical system would filter incoming light and only allow a specific polarization to pass through. On the other hand, a nonlinear metasurface, as demonstrated by this new design, does much more than just select a polarization—it allows light to be converted to a completely different frequency while simultaneously generating a circular polarization.
For instance, this metasurface could take visible light and convert it into ultraviolet radiation while also converting that radiation into circularly polarized light. This ability to perform frequency upconversion combined with circular polarization is a significant advancement in optical technologies.
Prof Wu emphasizes the remarkable compactness of the metasurface: “All this happens within an exceptionally thin layer of just one micron.” This is a far cry from traditional optical devices that may require bulky, complicated components to achieve similar effects.
The Geometrical Magic: Twist and Stack
The secret to achieving such a remarkable result in a layer as thin as one micron lies in the geometric design of the metasurface. The researchers incorporated a twist between the individual elements of the metasurface that mimics the threads of a screw, producing the desired chirality. This configuration provides a highly efficient nonlinear response by geometrically interacting with light, enabling the surface to exhibit properties of circular polarization.
To enhance this chiral response, the metasurface is designed with multiple stacked layers—just two of which are sufficient to produce a maximally chiral response. “In our design, we incorporate a twist between the periodically arranged elements within the layers of the metasurface, creating geometries that subtly mimic the threads on screws,” Prof Wu elaborated, adding that the clever stacking strategy played a key role in creating an efficient and compact optical system.
By combining multiple stacked layers, the material’s chiral response is magnified, efficiently transforming light to circular polarization without the need for large optics or complex setups. This innovation is key in reducing the overall size and complexity typically associated with optical systems that manipulate light in sophisticated ways.
Potential Applications and Future Impacts
The potential applications of this new metasurface are far-reaching. Its ability to generate circularly polarized light within such a thin form factor makes it perfect for advanced optical and photonic devices that require compact and lightweight components. One of the immediate applications of this metasurface could be in chiral sensing technologies, which rely on the interaction of circularly polarized light with chiral molecules, as found in proteins or other biological molecules.
Furthermore, the technique could become useful in circular dichroism spectroscopy, which is employed in the study of biomolecules and other optically active materials. It would also contribute to the broader fields of quantum physics, medical diagnostics, and environmental monitoring, especially with regard to more precise, portable sensors and detectors for biochemical substances.
The team has an ambitious vision for how their work will evolve. According to Prof Wu, they aim to develop “compact sources of circularly polarized radiation emitting in hard-to-reach wavelength ranges.” This could dramatically improve the efficiency and range of technologies that rely on circularly polarized light, such as optical communication systems and microscopy, especially where space or precision is paramount.
SUTD’s Interdisciplinary Approach to Innovation
The collaboration between Prof Wu and SUTD’s Professor Joel Yang is another significant aspect of the project, highlighting SUTD’s interdisciplinary approach to research. The team did not simply develop a theory about how to convert light; they translated theoretical principles into a fully functional device. This required not only a deep understanding of optics and nanotechnology but also ingenuity in product design. The team’s ability to seamlessly merge theoretical research with real-world technology reflects the university’s core ethos of combining technology and design to solve complex problems.
Assoc Prof Wu’s next step is to conduct experiments to confirm their theoretical model. “Our primary objective is to observe the effect of all-to-circular upconversion. We aim to ‘excite’ the structure with unpolarized light and achieve a nonlinear signal characterized by a high degree of circular polarization,” she explained. The team’s optimism about the potential for these devices is grounded in their interdisciplinary expertise, and their successful demonstration of a thin, highly efficient metasurface has the scientific community eagerly awaiting the experimental confirmation.
Conclusion: Miniaturizing Optical Systems for a Better Future
In conclusion, the proposed metasurfaces by Assoc Prof Wu and her team represent a revolutionary step forward in the field of optical design. With this innovation, circularly polarized light—central to many optical and photonic applications—could be generated efficiently and on a much smaller scale than ever before. Beyond reducing the physical space required for optical systems, this work also unlocks new possibilities for future technologies, from medical diagnostics to quantum communication.
In a world where miniaturization and efficiency are increasingly valuable, the development of ultra-thin, nonlinear chiral metasurfaces offers hope for solving some of the most pressing challenges faced by industries reliant on light-based technologies. Their innovative design underscores the importance of creativity and interdisciplinary collaboration in driving progress toward more sustainable and effective solutions in optics and beyond.
Reference: Dmitrii Gromyko et al, Enabling All-to-Circular Polarization Up-Conversion by Nonlinear Chiral Metasurfaces with Rotational Symmetry, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.023804. On arXiv: DOI: 10.48550/arxiv.2407.19293