123rd CEMS Colloquium

Title

Terahertz dynamics of quantum nanostructures sensed by nanogap electrode

Abstract

Terahertz (THz) spectroscopy is a powerful tool for characterizing electronic properties and vibronic excitations in various types of solids, liquids, and gases.  However, it is extremely challenging to perform THz spectroscopy at nm-scale because the diffraction limit of electromagnetic waves hinders tight focusing of THz radiation.

We have introduced a novel technique for THz spectroscopy that utilizes metal nanogap electrodes [1]. Metal nanogap electrodes integrated with a THz antenna are employed to capture a single molecule. Even extremely weak THz absorption can be detected by measuring the THz-induced photocurrent through the target molecule by employing a single molecule transistor (SMT) geometry. Taking advantage of THz-induced photocurrent spectroscopy, vibrational states [2] have been investigated.

Furthermore, we have investigated electron transport through H₂O@C60 SMTs [3] and observed that H₂O@C60 SMTs exhibit three excited states below 10 meV.  From comparison between experiment and theory, the excitations observed below 10 meV are identified to be the quantum rotational excitations of the water molecule encapsulated in the C60 cage.  Surprisingly, the quantum rotational excitations of both para- and ortho-water molecule are observed simultaneously even when a single water molecule is measured, indicating that the fluctuation between the two nuclear isomer states takes place in a time scale much shorter than our measurement time (~1 min), probably due to interaction between the encapsulated water molecule and conducting electrons.

[1] K. Yoshida, K. Shibata, and K. Hirakawa, Phys. Rev. Lett. 115, 138302 (2015).
[2] S. Du, K. Yoshida, Y. Zhang, I. Hamada, and K. Hirakawa, Nature Photon. 12, 608 (2018).
[3] S. Du, Y. Hashikawa, H. Ito, K. Hashimoto, Y. Murata, Y. Hirayama, and K. Hirakawa, Nano Lett. 21, 10346 (2021).