Scientists study single molecules with terahertz spectroscopy for the first time

For the first time, scientists have used terahertz spectroscopy to study a single molecule.

Spectroscopy is the study of the interactions between light and matter. Most frequently, scientists use infrared light or X-rays to investigate atomic and molecular worlds.

Terahertz light lies between infrared and microwaves on the electromagnetic spectrum. Its frequency can excite molecules, causing them to vibrate, but its especially long wavelength makes it near-impossible to be focused onto single molecules.

Scientists at the University of Tokyo's Institute of Industrial Science developed a new method for focusing terahertz light beams. Physicists were able to measure the tunneling of a single electron using terahertz radiation.

To overcome the classical diffraction limit for focusing light beams, scientists deployed a single-molecule transistor, composed of two metal electrodes juxtaposed on a bowtie-shaped silicon wafer. The two electrodes form the transistor's source and drain.

Scientists deposited fullerene molecules in the nanoscale gaps between the two electrodes. The source and drain focus the incoming THz beam onto the fullerene molecules.

"The fullerenes absorb the focused THz radiation, making them oscillate around their center-of-mass," researcher Shaoqing Du said in a news release. "The ultrafast molecular oscillation raises the electric current in the transistor, on top of its inherent conductivity."

The change in current is tiny, but it is measurable. The same electrodes used to trap the THz radiation can be used to measure the slight current shift. The two electrodes picked up the change in fullerene absorption peaks when a single electron was added or subtracted from the transistor.

Scientists think their research -- detailed this week in the journal Nature Photonics -- could be used to develop terahertz technologies that complement visible-light and X-ray spectroscopy.

"This scheme provides an opportunity to investigate the ultrafast THz dynamics of subnanometer-scale systems," researchers wrote.

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