Deep-nanometer-scale terahertz spectroscopy using a transistor geometry with metal nanogap electrodes

Terahertz (THz) spectroscopy is a powerful tool for characterizing electronic properties and vibronic excitations in various types of solids, liquids, and gases, and it has been extensively used not only for basic science but also for industrial applications. Recently, it has become necessary to understand electronic and vibronic excitations at the nanometer (nm) scale to realize state-of-the-art quantum nanodevices and the synthesis of new molecules for medicine. However, it is challenging to perform THz spectroscopy at the nm scale because the diffraction limit of electromagnetic waves hinders tight focusing of THz radiation at the nm scale. Here, we introduce a novel technique for THz spectroscopy using metal nanogap electrodes. Metal nanogap electrodes integrated with a THz antenna are employed to capture sensing targets, such as a single semiconductor quantum dot (QD) or molecule. Even extremely weak THz absorption can be detected with high sensitivity by measuring the THz-induced photocurrent through the sensing target. Taking advantage of THz-induced photocurrent spectroscopy, the electronic structures in single QDs as well as the vibrational states in single molecules are systematically investigated. The present characterization technology for nm-scale systems provides a key scientific foundation for creating nanodevices with new functions.