Yukio Kawano

Carbon Nanotube Terahertz Detector

Terahertz (THz) technologies are promising for diverse areas such as medicine, bioengineering, astronomy, environmental monitoring, and communications. However, despite decades of worldwide efforts, the THz region of the electromagnetic spectrum still continues to be elusive for solid state technology. Here, we report on the development of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive carbon nanotube THz detector that works at room temperature. The detector is sensitive throughout the entire range of the THz technology gap, with responsivities as high as ∼2.5 V/W and polarization ratios as high as ∼5:1. Complete thermoelectric and opto-thermal characterization together unambiguously reveal the photothermoelectric origin of the THz photosignal, triggered by plasmonic absorption and collective antenna effects, and suggest that judicious design of thermal management and quantum engineering of Seebeck coefficients will lead to further enhancement of device performance.

Terahertz photon-assisted tunneling in carbon nanotube quantum dots

The authors have studied the transport properties of carbon nanotube quantum dots under terahertz (THz) wave irradiation. The experimental data have shown that the satellite currents are generated with the THz irradiation, and that the peak position of the satellite currents varies linearly with the THz photon energy. These results provide experimental evidence for photon-assisted tunneling in the THz region. The present observation provides the interesting possibility of developing a highly sensitive and frequency-tunable THz detector capable of high-temperature operation.

Highly sensitive and tunable detection of far-infrared radiation by quantum Hall devices

We studied far-infrared (FIR) response due to cyclotron resonance (CR) of two-dimensional electron gas systems in GaAs/AlGaAs heterostructures by using cyclotron radiation from n-InSb devices as the illumination source. We examined the dependence of the FIR response on different experimental parameters, including the aspect ratio of Hall bars, electron mobility, bias current, illumination intensity, magnetic field, and lattice temperature. A strong photoresponse emerges only in the vicinity of integer quantum Hall effect (IQHE) regimes. Time-resolved measurements show that the recombination lifetime of excited carriers depends largely on the electron mobility, ranging from 5 μs to 1 ms at 4.2 K. The temporal decay of photoresponse is nonexponential in higher-mobility samples, whereas it is exponential with a single time constant in lower-mobility samples. This, together with the relatively large time constants, suggests that the FIR response is induced through multitrapping processes, in which excited car...

Terahertz sensing with a carbon nanotube/two-dimensional electron gas hybrid transistor

We report on a highly sensitive terahertz (THz) detector based on a carbon nanotube (CNT) transistor, which is integrated with a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The operation principle of this device is that the CNT transistor senses electrical polarization induced by terahertz-excited electron-hole pairs in the 2DEG. The magnetic field dependence of the terahertz response signal (CNT current) is shown to follow features of cyclotron resonance of the 2DEG, indicating the validity of the above mechanism. The utilization of the present device structure and mechanism has enabled the detection of a small number of terahertz photons.

Highly Sensitive Detector for On-Chip Near-Field THz Imaging

We present novel terahertz (THz) detectors, using nanoelectronic devices, such as 2-D semiconductor and carbon nanotube (CNT). Our detector is useful for high-resolution THz imaging, which requires highly sensitive detection. We have developed a frequency-tunable THz-photon detector and an on-chip near-field imaging detector. The detection mechanism of the former is that the CNT transistor detects electrical polarization generated by THz-excited electron-hole pairs in the 2-D semiconductor. In the latter, we have achieved THz imaging beyond the diffraction limit with an all-integrated device, including an aperture, a probe, and a detector. We further show applications of the THz imaging to semiconductor research. We have successfully visualized and clarified spatial properties of electrons in the 2-D semiconductors, which have not been revealed by conventional transport measurements.

Terahertz response of quantum point contacts

We measure a clear terahertz response in the low-temperature conductance of a quantum point contact at 1.4 and 2.5THz. We show that this photoresponse does not arise from a heating effect, but that it is instead excellently described by a classical model of terahertz-induced gate-voltage rectification. This effect is distinct from the rectification mechanisms that have been studied previously, being determined by the phase-dependent interference of the source drain and gate voltage modulations induced by the terahertz field.

Quantum response of carbon nanotube quantum dots to terahertz wave irradiation

Single-electron transport measurements have been carried out at 1.5 K on single-wall carbon nanotube (SWCNT) quantum dots (QDs) under terahertz (THz) irradiation with a frequency of 2.5 THz. The most important finding in this study was new side-peaks that appeared only under the THz irradiation. From the detailed analysis with energy scales associated with the QDs, we conclude that the side-peaks originate from the photon-assisted tunnelling (PAT) of an electron in the QD to the drain electrode. The present THz-PAT is the first observation in QDs because of the larger energy scales in a SWCNT QD, compared with those of standard semiconductor QDs.

Coulomb peak shifts under terahertz-wave irradiation in carbon nanotube single-electron transistors

The authors have studied the effect of terahertz irradiation on single-electron transistors (SETs) based on single-wall carbon nanotubes, and have observed that the radiation generates Coulomb peak shifts. Time-resolved measurements of the terahertz response have revealed that the peak-shift signal has very long time constants, measured in seconds, and that the time trace of the signal after the terahertz irradiation is turned off deviates from a single-exponential curve. These experimental results suggest that the terahertz irradiation causes a charging process in trap states in the close vicinity of the SET, leading to a change in its effective gate voltage.

Figure of Merit for Carbon Nanotube Photothermoelectric Detectors

Carbon nanotubes (CNTs) have emerged as promising materials for visible, infrared, and terahertz photodetectors. Further development of these photodetectors requires a fundamental understanding of the mechanisms that govern their behavior as well as the establishment of figures of merit for technology applications. Recently, a number of CNT detectors have been shown to operate based on the photothermoelectric effect. Here we present a figure of merit for these detectors, which includes the properties of the material and the device. In addition, we use a suite of experimental characterization methods for the thorough analysis of the electrical, thermoelectric, electrothermal, and photothermal properties of the CNT thin-film devices. Our measurements determine the quantities that enter the figure of merit and allow us to establish a path toward future performance improvements.

Terahertz waves: a tool for condensed matter, the life sciences and astronomy

Abstract In the wide range of the electromagnetic wave spectrum, the terahertz (THz) frequency region has for a long time been an unexplored region and its technological development has been left behind. Nowadays, however, science and technology based on THz electromagnetic waves have been increasingly progressing and are still growing. In the THz region, both features of ‘wave’ and ‘light’ appear, enabling the manipulation of the THz wave from both approaches of electronics and optics/photonics. From the viewpoint of research targets, THz technology is expected to be the key for unlocking mysteries behind quantum effects in condensed-matter physics, life activities in biology, and the birth of the celestial bodies in astronomy. In addition to such fundamental sciences, THz spectroscopy and imaging can be used as a powerful tool for nondestructive remote inspection in industrial and medical fields. In this article I review cutting-edge technologies of THz sensing, imaging and spectroscopy, and describe how effectively the THz measurements are applied to various researches.