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To realize nm-scale THz spectroscopy, THz electromagnetic waves must be tightly focused onto nm-scale systems, and extremely weak absorption must be detected with high sensitivity. Depending on the method used to detect the extremely weak THz absorption, nm-scale THz spectroscopy techniques can be classified into the following three categories: (i) THz s-SNOM, (ii) THz-STM, and (iii) THz-SETS. Table 1 shows a comparison of the three methods, and their characteristics are described below.
THz s-SNOM THz-STM THz-SETS Imaging yes yes no Gating at the nm scale difficult difficult easy Requires powerful THz source yes yes no Signals scattered power photocurrent photocurrent Table 1. Comparison of THz s-SNOM, THz-STM, and THz-SETS.
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THz s-SNOM33−38 provided a breakthrough in the diffraction limit in free-space optics and enabled the measurement of nm-scale samples, as shown in Fig. 2a. A metal-coated atomic force microscopy (AFM) tip is used as an antenna to concentrate incident THz radiation to a nm-scale near-field spot at the tip apex. When the tip is close to the sample surface, the near-field interaction between the tip and sample modifies the tip-scattered THz field, which depends on the local dielectric properties of the sample. By collecting the scattered THz light in the far field, THz spectroscopy and imaging can be achieved at the nm scale.
Fig. 2
a Schematic of THz s-SNOM. b Model of the near-field interaction in THz s-SNOM. The AFM probe tip is assumed to be a polarizable sphere with dielectric constant εt. A point dipole can be used to replace the probe tip, which allows the prediction of the dependence of the scattered THz wave on the distance z between the tip and sample and on the complex dielectric constant εs of the sample. c Schematic of THz-STM. d Tunneling process of single molecular orbitals in THz-STM measurements. Vinst is the instantaneous voltage between the tip and substrate induced by THz pulses. e Schematic of THz-SETS. f THz-induced vibron-assisted tunneling in an SMT.The near-field interaction between the tip and sample is the key to the THz s-SNOM technique. The THz s-SNOM system can be regarded as a simple model that includes the tip and the sample, as shown in Fig. 2b. The effective polarizability of the coupled tip–sample system (αeff) can be expressed by the polarizability of the tip (α) and the dielectric surface response function of the sample (β)55.
$$ {\alpha }_{\mathrm{e}\mathrm{f}\mathrm{f}}=\frac{\alpha (1 +\beta )}{1 -\alpha \beta /\left(16\pi {\left(r+z\right)}^{3}\right)} $$ (1) $$ \alpha = 4\pi {r}^{3}\frac{{\varepsilon }_{t}- 1}{{\varepsilon }_{t}+ 2} $$ (2) $$ \beta =\frac{{\varepsilon }_{s}- 1}{{\varepsilon }_{s}+ 1} $$ (3) where r is the radius of the assumed polarizable sphere at the tip, z represents the distance between the tip and sample, and εt and εs are the dielectric constants of the tip and sample, respectively. Typically, the tip is used in tapping mode, and by detecting the scattered THz signal at the tapping frequency, the weak signal from nm-scale regions can be extracted from the overwhelming background radiation. The THz signal from the sample can be obtained by measuring only the scattered component of the THz field (Es) because Es is proportional to αeffEi (Ei is the incident THz field). Using this technique, THz intersublevel transitions in single QDs have been measured33, 36, 37. However, owing to the very weak backscattered light, a powerful THz radiation source, such as a free electron laser, is required.
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THz-STM39−48 is another method that uses a nanoprobe. The essential difference from THz s-SNOM is that the signal measured in this method is the tunneling current instead of the scattered light power. Owing to the ultrahigh spatial resolution of STM imaging, nm-scale samples can be well resolved. As shown in Fig. 2c, a nm-scale sample is placed between an atomically sharp tip and a conducting substrate, and pulsed THz radiation is focused onto the sample. To avoid hybridization between the electronic orbitals of the sample and substrate, the substrate surface is sometimes covered by a very thin insulating layer46. When a THz pulse is coupled to the STM tip, it generates an ultrafast voltage transient and forms a THz-induced tunneling photocurrent between the atomically sharp tip and sample, as shown in Fig. 2d. To overcome the large binding energies of electronic orbitals in molecules, strong incident THz fields are typically necessary. Owing to the high sensitivity and high spatial resolution of THz-induced tunneling photocurrent measurements, THz-STM can map the shapes of the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) wavefunctions46.
Extensive THz s-SNOM and THz-STM measurements have been performed to study nanostructures and molecules. For more details, please refer to an excellent recently published review article56.
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THz-SETS is a simple and powerful nm-scale THz spectroscopy method that was recently developed49−54. THz-SETS is based on a single-electron transistor (SET) geometry, as shown in Fig. 2e. The source and drain electrodes are separated by a gap on the order of sub-nanometer to a few tens of nanometers and focus THz electromagnetic waves onto a nm-scale system captured in the nanogap. Although it lacks imaging capabilities, the great advantage of the SET geometry is that, by using the gate electrode, the electrochemical potential and the charge state in the nanostructures can be precisely tuned, as shown in Fig. 2f. The ability to change the electrochemical potential is particularly important for the study of QDs and molecules, whose electron dynamics are strongly modulated by the electron number. When a QD or single molecule is illuminated by THz radiation, a very small but finite THz-induced photocurrent is generated. Furthermore, excitation spectra associated with intersublevel transitions or molecular vibrations can be obtained.
THz-SETS has an invaluable advantage over the other two methods, as summarized in Table 1. The THz-SETS geometry allows the modulation of the electrostatic potential and the number of conducting electrons in nanostructures by using a gate electrode. Because of this ability to tune the band structure of nanostructures, even very weak THz radiation from a blackbody light source can photoexcite electrons and produce photocurrent signals. Otherwise, intense THz light pulses are required to photoionize nanostructures.
Deep-nanometer-scale terahertz spectroscopy using a transistor geometry with metal nanogap electrodes
- Light: Advanced Manufacturing 2, Article number: (2021)
- Received: 10 January 2021
- Revised: 15 November 2021
- Accepted: 28 November 2021 Published online: 13 December 2021
doi: https://doi.org/10.37188/lam.2021.031
Abstract: 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.
Research Summary
Deep-nanoscale THz spectroscopy: going much beyond the diffraction limit
THz spectroscopy has become a powerful tool for characterizing electronic properties and vibronic excitations in various kinds of materials. Recently, the need to understand electronic and vibronic excitations at the nanometer (nm) scale has emerged for realization of state-of-the-art quantum nanodevices and synthesis of new molecules for medicine. However, performing THz spectroscopy at the nm scale is extremely challenging because the diffraction limit of electromagnetic waves hinders tight focusing of THz radiation down to the nm scale. A team from University of Tokyo and Tokyo University of Agriculture and Technology has introduced a novel technique of using metal nanogap electrodes for deep-nanoscale THz spectroscopy, which provides a major scientific basis for creating nanodevices with new functions.
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