Left and right circularly polarized light, where the electromagnetic waves spiral in a clockwise and counterclockwise manner as they travel, plays a crucial role in a wide range of applications, from enhancing medical imaging techniques to enabling advanced communication technologies. However, generating circularly polarized light often requires complex and bulky optical setups, which hinders its use in systems with space constraints.
To address this challenge, a team of researchers from Singapore led by Associate Professor Wu Lin of Singapore University of Technology and Design (SUTD) has put forth a new type of metasurface—an ultra-thin material with properties not found in nature—that may be able to replace traditional complex and bulky optical set-ups. They have published their research in the Physical Review Letters paper " Enabling all-to-circular polarization up-conversion by nonlinear chiral metasurfaces with rotational symmetry ".
The team's proposed metasurface exhibits chirality, which makes it different from materials used in traditional set-ups. Chirality of an object means that it cannot be superimposed onto its mirror image. Like our left and right hands, chiral objects exist in two distinct forms that are mirror images of each other. The key feature of chiral optical nanostructures, such as metasurfaces, is their remarkably different response to the left and right circular polarizations of light. Assoc Prof Wu's team has shown that a combination of two peculiar geometrical properties, namely, chirality and rotational symmetry, within a nonlinear metasurface enables an interesting mechanism of generating circularly polarized light from an arbitrary optical excitation.
The nonlinearity of the metasurface is essential in this transformation of light. A linear metasurface would filter the incoming light and allow only the specific polarization of the light to pass through. On the other hand, a nonlinear metasurface not only selects and amplifies a specific circular polarization but also converts it into circularly polarized light at an entirely different frequency. For example, a nonlinear material can turn visible light into ultraviolet radiation, which is of a different frequency range. This frequency upconversion capability, combined with the inherent chirality of the metasurface, allows the metasurface to effectively produce circularly polarized light at specific frequency ranges.
"All this happens within an exceptionally thin layer of just one micron," said Assoc Prof Wu. This is a far cry from the typically bulky optical setups for creating circularly polarized light.
"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," she continued, attributing the compactness of the proposed metasurface to a unique stacking strategy devised by her team.
Through mathematical elucidations, the team demonstrated that the stacking of layers leads to the chiral response of the metasurface. "Just two stacked layers can yield a maximally chiral response," she added.
This opens doors to a wide range of exciting applications, holding immense potential for the future miniaturization of optical devices. This could also find applications in chiral sensing, circular dichroism spectroscopy of novel materials and biomolecules, which have far-reaching implications on 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," Assoc Prof Wu said.
The ingenuity of the metasurface's design is also clear evidence of SUTD's commitment to intersecting technology and design in research. In designing the metasurface, the team first had to make clear the mechanism for the upconversion of light into circularly polarized light. By then 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.
Together with fellow SUTD colleague Professor Joel Yang and his team, Assoc Prof Wu's team is now working to verify their work experimentally. "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 characterised by a high degree of circular polarization," she said. "We are optimistic that this endeavour will contribute another significant piece of research to the portfolio of SUTD scientists."
