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| Concept and experimental demonstration of terahertz circular dichroism imaging. Circularly polarized terahertz radiation (left: blue, right: red) interacts with a moiré metasurface, producing distinct spectral responses and spatially resolved circular dichroism distributions (top). The chirality-dependent response reverses for mirror-imaged structures, demonstrating the ability to visualize the spatial distributions of chirality. Image Credit: © Katsuhiko Miyamoto |
Scientific Frontline: Extended "At a Glance" Summary: Visualizing Spatial Chirality with Terahertz Imaging
The Core Concept: A novel imaging technique utilizing spiral-shaped terahertz light to directly visualize and map the two-dimensional spatial distribution of right- and left-handed chirality across a material.
Key Distinction/Mechanism: Unlike conventional terahertz measurements that average chiral signals across an entire sample, this method employs circularly polarized terahertz radiation to generate spatially resolved circular dichroism distributions, achieving a precise resolution of approximately 100 μm.
Major Frameworks/Components:
- Terahertz (THz) Radiation: The use of circularly polarized waves situated between microwaves and infrared light to interact with subtle structural twists.
- Moiré-Type Metasurfaces: Microscopic silver disk patterns stacked with slight offsets or rotations to generate engineered artificial chiral structures.
- Circular Dichroism Spectroscopic Imaging: Measuring the differential absorption of right- and left-circularly polarized light to create a high-resolution chirality map.
Branch of Science: Photonics, Optics, Materials Science, and Nanotechnology.
Future Application: Evaluating next-generation chiral materials, inspecting advanced signal-control devices for Beyond 5G/6G communication systems, and analyzing complex biomolecular structures (such as abnormal disease-linked protein aggregates).
Why It Matters: This technique marks the world's first direct observation of spatial chirality distributions within a material, establishing a non-destructive structural analysis pipeline critical for medical diagnostics, drug design, and quantum material development.
Chiral objects can behave differently depending on their handedness. However, existing methods cannot reveal how chirality varies across a material. A research team from Chiba University and Tohoku University developed a terahertz imaging technique that maps right- and left-handed chirality using spiral-shaped light. The researchers visualized different chiral regions on a moiré-type metasurface with a resolution of about 100 μm, marking the first direct observation of spatial chirality distributions within a material.
In nature, structures exist that are mirror images of each other but cannot be perfectly superimposed. These are known as chiral objects, a term derived from the Greek word for "hand," since left and right hands share the same relationship. Although similar in structure, chiral molecules exhibit different behaviors, and chirality is central to life itself. DNA has a twisted chiral structure, and living organisms prefer one handedness over the other. This distinction is equally important in drug design, materials science, and nanotechnology.
One way to distinguish chiral molecules is by measuring their response to circularly polarized light in the terahertz (THz) region. THz waves lie between microwaves and infrared light and are especially sensitive to subtle collective motions and twisting structures in materials. However, conventional THz measurements average the signal across an entire sample, making it impossible to determine how chirality varies across different locations.
Now, researchers in Japan from Chiba University and Tohoku University have overcome this limitation, allowing chirality to be visualized as two-dimensional images, much like creating a map of chirality across a material.
"This work was inspired by a simple question. Conventional measurements only reveal averaged chirality, but what does the actual spatial distribution look like? We wondered whether directly visualizing chirality as an image could provide deeper insights, which motivated us to pursue this research," said Professor Katsuhiko Miyamoto of Chiba University.
To generate regions with different chiralities in the same material, the researchers built a moiré-type metasurface by stacking microscopic silver disk patterns with a slight offset or rotation. These structures were fabricated at the micrometer scale so that they could strongly interact with THz light. By carefully designing the overlapping patterns, the researchers created an artificial surface containing both right- and left-handed twisting regions, allowing them to create and control different chiral configurations in a designed system.
When circularly polarized THz waves were directed onto the metasurface, different regions responded differently depending on their local chirality. The new approach could spatially resolve chirality distributions with a resolution of approximately 100 μm, roughly the thickness of a human hair.
"We succeeded in visualizing the coexistence of different chiralities within a single sheet for the first time in the world. These findings are expected to find applications in the quality evaluation of next-generation materials, the analysis of biomolecular structures, and the development of new THz devices," said Miyamoto.
As advances in nanofabrication make increasingly sophisticated chiral materials possible, the proposed method could provide a reliable way to examine whether these structures function as intended without damaging the material.
Looking ahead, the researchers expect to expand the technology to a broader frequency range from 2 to 15 THz, enabling more detailed structural analyses. The approach could eventually support new diagnostic techniques for visualizing abnormal protein aggregates linked to disease, help inspect advanced signal-control devices for next-generation communication systems such as Beyond 5G and 6G, and detect subtle distortions inside quantum and soft materials.
Published in journal: ACS Photonics
Authors: Uina Chiba, Shota Tsuji, Gaku Oritani, Takumi Yoichi, Rinpei Sasaki, Takeo Minari, Seigo Ohno, and Katsuhiko Miyamoto
Source/Credit: Tohoku University
Edited by: Scientific Frontline
Reference Number: ms060326_01