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Catching excitons in motion—ultrafast dynamics in carbon nanotubes revealed by nano-infrared spectroscopy

A research team has successfully visualized the ultrafast dynamics of quasi-particles known as excitons, which are generated in carbon nanotubes (CNTs) upon light excitation.

This was achieved with spatial and temporal resolution beyond the capabilities of conventional techniques, thanks to a cutting-edge instrument called an ultrafast infrared near-field optical microscope. This advanced technique focuses femtosecond infrared pulses into nanoscale regions, enabling the sensitive detection of local light-matter interactions in real space and time.

The work is in the journal Science Advances.

CNTs are nanometer-scale semiconductor wires with exceptional electrical and optical properties, making them promising candidates for future nanoelectronic and nanophotonic applications.

When exposed to light, CNTs generate excitons—bound pairs of electrons and holes—that govern key processes such as light absorption, emission, and charge transport. However, since excitons are confined to just a few nanometers and exist for only femtoseconds to picoseconds, capturing their behavior directly has remained a significant experimental challenge.

In this study, the team, led by Dr. Jun Nishida (Assistant Professor), and Dr. Takashi Kumagai (Associate Professor) at the Institute for Molecular Science (IMS)/SOKENDAI, in collaboration with Dr. Taketoshi Minato (Senior Researcher at IMS), Dr. Keigo Otsuka (Assistant Professor at The University of Tokyo) and Dr. Yuichiro K. Kato (Chief Researcher at RIKEN) overcame that challenge by first generating excitons in CNTs using visible light pulses, and then probing their dynamics with ultrafast infrared near-field pulses.

This approach enabled direct observation of how excitons evolve in both space and time within individual CNTs. The measurements revealed that subtle structural distortions and interactions with neighboring CNTs—particularly in complex bundled configurations—can largely influence exciton relaxation dynamics.

These findings offer new insights into the role of the local nanoscale environment in shaping exciton behavior.

To interpret the experimental data, the researchers also developed a theoretical model that describes the interaction between excitons and the infrared near-field, taking into account dielectric responses from intra-excitonic transitions. Simulations based on a point-dipole model successfully reproduced the experimental results, offering a strong theoretical foundation for future studies using this technique.

Dr. Nishida says, "The capability to directly observe quantum particles such as excitons in one-dimensional systems like CNTs marks a major advancement in measurement technology."

Prof. Kumagai says, "This achievement paves the way for designing next-generation high-speed nano-optoelectronic devices and quantum photonic technologies based on CNTs."