A highly sensitive detector in the terahertz (THz) range is in strong demand in various research fields, such as radio astronomy, biochemical spectroscopy, and medicine as well as solid-state physics. Nevertheless, the photon energy of the THz wave, typically ~10 meV, is two to three magnitudes lower than that of the visible light, thus making the development of a high-performance THz detector a difficult task. Recent progress in the fabrication of nanoscale devices, however, opens up a new possibility of significantly enhancing detector performance. Here, we experimentally present a novel THz detector using photon-assisted tunneling in carbon nanotube quantum dots (CNT-QDs). This sensing mechanism has allowed us to achieve highly sensitive and frequency-tunable detection of THz photons.
In this experiment, CNT-QD devices were immersed into a 4He cryostat and single-electron transport measurements were performed at 1.5 K without and with THz irradiation. We have found that THz irradiation causes an appearance of new-side peaks in the Coulomb blockade regime, and that the energy spacing between the new-side peaks and the original peaks is proportional to the photon energy of the incident THz wave in the range of 1.4-4.2 THz (photon energy: 5.7-17.2 meV). These observations provide direct evidence of quantum response to the THz irradiation, i.e. THz photon-assisted tunneling. Furthermore, we have observed a multi-photon process under a strong THz electric field.
An interesting application using the above features is as a highly sensitive and tunable detector for THz microscopy and spectroscopy, especially for a near-field THz imaging that remains challenging. We are now developing a new type of near-field THz microscope integrated with a CNT-QD THz detector. This technique is expected to make it possible to obtain far higher spatial resolution beyond the diffraction limit, compared to our previous THz microscope based on a solid immersion lens system [1]. Moreover, the present results demonstrate the promising feasibility of a two-qubit controlled-NOT operation on a two-coupled CNT-QD.
[1] Y. Kawano et al., Phys. Rev. B 68, 085328 (2003); Phys. Rev. B 70, 081308(R) (2004); Phys. Rev. Lett. 95, 166801 (2005).
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