Twisted Light: A new metasurface offers a compact solution for circularly polarized light

Generation of circularly polarized nonlinear signals from rotationally symmetric chiral metasurfaces under arbitrary polarization excitation. Credit: SUTD
Left and right circularly polarized light, in which electromagnetic waves spiral clockwise and counterclockwise, plays an important role in a wide range of applications, from enhancing medical imaging technology to enabling advanced communication technologies. However, generating circularly polarized light often requires complex and bulky optical setups, which precludes its use in space-constrained systems.
To address this challenge, a team of Singaporean researchers led by Associate Professor Wu Lin from the Singapore University of Technology (SUTD) has developed a new type of metasurface – an ultra-thin material with properties not found in nature. I devised it. It can replace traditional complex and bulky optical setups.
They published their work in the paper “Achieving upconversion from fully polarized light to circularly polarized light with a nonlinear chiral metasurface with rotational symmetry” in Physical Review Letters.
The research team’s proposed metasurface exhibits chirality, which distinguishes it from materials used in traditional setups. The chirality of an object means that it cannot be superimposed on its mirror image. Like our left and right hands, chiral objects exist in two different forms that are mirror images of each other. An important feature of chiral optical nanostructures, such as metasurfaces, is that they respond markedly differently to left-handed and right-handed circularly polarized light.
Associate Professor Wu’s team shows that the combination of two unique geometric properties within nonlinear metasurfaces, chirality and rotational symmetry, enables an interesting mechanism for generating circularly polarized light from arbitrary optical excitation. I showed it.
Metasurface nonlinearity is essential for this light transformation. Linear metasurfaces filter incoming light, allowing only certain polarizations of light to pass through. Nonlinear metasurfaces, on the other hand, not only select and amplify a particular circularly polarized light, but also convert it into circularly polarized light of a completely different frequency.
For example, nonlinear materials can transform visible light into ultraviolet light at different frequency ranges. By combining this frequency upconversion capability with the metasurface’s inherent chirality, the metasurface can effectively generate circularly polarized light in a specific frequency range.
“All of this happens in a very thin layer, just one micron,” said Wu. This is very different from the usual bulky optical setups for creating circularly polarized light.
“Our design incorporates twists between periodically placed elements within the layers of the metasurface to create a geometry that subtly mimics the threads of a screw,” she continues, suggesting We attributed the compactness of the resulting metasurface to the unique stacking strategy that she devised. team.
Through mathematical elucidation, the research team demonstrated that layer stacking leads to a chiral response of the metasurface. “We can create the maximum chiral response by stacking just two layers,” she added.
This opens the door to a wide range of exciting applications and holds immense potential for the miniaturization of future optical devices. This also has potential applications in chiral sensing, circular dichroism spectroscopy of novel materials and biomolecules, and has far-reaching implications for fields as diverse as medicine and quantum physics.
“We envision that such metasurfaces can be used as compact sources of circularly polarized radiation emitting in hard-to-reach wavelength ranges,” said Wu.
The ingenuity of the metasurface design is also clear evidence of SUTD’s commitment to intersecting technology and design in its research. When designing the metasurface, the team first needed to figure out the mechanism that upconverts light to circularly polarized light.
By incorporating this technology into the design of the metasurface, the team effectively translated their theoretical understanding into a functional and compact device. This seamless use of design and technology is a hallmark of SUTD’s interdisciplinary approach to research.
Along with SUTD colleague Professor Joel Yang and his team, Associate Professor Wu’s team is now working to experimentally validate the research. “Our main objective is to observe the effect of full-circle 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.” “I’m aiming for that,” she said. “We are optimistic that this effort will contribute another important piece of research to the portfolio of SUTD scientists.”
Further information: 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. For arXiv: DOI: 10.48550/arxiv.2407.19293
Provided by Singapore University of Technology and Design
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