Abstract-Excited-state charging energies in quantum dots investigated by terahertz photocurrent spectroscopy

Y. Zhang, K. Shibata, N. Nagai, C. Ndebeka-Bandou, G. Bastard, and K. Hirakawa

Phys. Rev. B 93, 235313 – Published 24 June 2016

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.235313

We have investigated the excited-state (ES) charging energies in quantum dots (QDs) by measuring a terahertz (THz)-induced photocurrent in a single-electron transistor (SET) geometry that contains a single InAs QD between metal nanogap electrodes. A photocurrent is produced in the QD SETs through THz intersublevel transitions and the subsequent resonant tunneling. We have found that the photocurrent exhibits stepwise change even within one Coulomb blockaded region as the electrochemical potential in the QD is swept by the gate voltage. From the threshold for the photocurrent generation, we have determined the charging energies for adding an electron in the photoexcited state in the QD. Furthermore, the charging energies for the ESs with different electron configurations are clearly resolved. The present THz photocurrent measurements are essentially dynamical experiments and allow us to analyze electronic properties in off-equilibrium states in the QD.

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• Received 7 January 2016
• Revised 19 April 2016

DOI:http://dx.doi.org/10.1103/PhysRevB.93.235313

Abstract-Probing many-body quantum states in single InAs quantum dots: Terahertz and tunneling spectroscopy

Y. Zhang, K. Shibata, N. Nagai, C. Ndebeka-Bandou, G. Bastard, and K. Hirakawa

Phys. Rev. B 91, 241301(R) – Published 3 June 2015

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.241301

We have investigated the many-body quantum states in single InAs quantum dots (QDs) by simultaneously obtaining the terahertz (THz) intersublevel transition and single electron tunneling spectra. It is found that the intersublevel transition energies measured in the few-electron region are systematically larger than the excited state (ES) energies determined from the transport measurements. We show that tunneling and THz spectroscopy probe the same many-body excited states in the QDs, but their sensitivities depend on their selection rules. In the many-electron region, we observe THz peaks whose energies coincide with the tunneling ESs.

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Abstract-Probing many-body quantum states in single InAs quantum dots: Terahertz and tunneling spectroscopy

Y. Zhang, K. Shibata, N. Nagai, C. Ndebeka-Bandou, G. Bastard, and K. Hirakawa

http://journals.aps.org/prb/accepted/b8078Yb4Od713559716d61d251bd5c8b07d044ae4

We have investigated the manybody quantum states in single InAs quantum dots by simultaneously obtaining the terahertz (THz) intersublevel transition and single electron tunneling spectra. It is found that the intersublevel transition energies measured in the few-electron region are systematically larger than the excited state (ES) energies determined from the transport measurements. We show that the tunneling and THz spectroscopy probe the same manybody excited states in the QDs, but their sensitivities depend on their selection rules. In the many-electron region, we observe THz peaks whose energies coincide with the tunneling ESs.

Abstract-Terahertz Photon-Assisted Tunneling in InAs Quantum Dots

K. Shibata, K. Hirakawa

http://link.springer.com/article/10.1007%2Fs10762-013-0039-7#

We have investigated electron transport in a single self-assembled InAs quantum dot (QD) coupled to nanogap metal electrodes under terahertz (THz) radiation. The fabricated QD samples operated as single electron transistors in a few electron regime, exhibiting clear shell structures. Under the THz radiation, in addition to the original Coulomb oscillation peaks, new side-peaks showed up. The dependence of the new side-peak current on the THz power follows the prediction of the photon-assisted tunneling (PAT) theory. Moreover, two types of PAT processes were observed in the THz range; the ground state resonance and the photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, a very high coupling efficiency between the THz waves and the QDs was realized in our system and we observed multi-photon absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade. This high coupling efficiency between THz wave and electrons in QDs opens a way to the manipulation of single electron charge/spin states in the THz frequency range.

Tunneling with the help of Terahertz photons

http://thznetwork.net/index.php/archives/1607

Quantum dots—small semiconductor islands containing electrons confined to all three dimensions—offer many possibilities as single-electron transistors, logic elements in quantum computers, and light-emitting devices. In these dots, a phenomenon called photon-assisted tunneling can occur in which applied radiation enables electrons to tunnel through the dot. Reporting in Physical Review Letters, Kenji Shibata at the University of Tokyo, Japan, and colleagues demonstrate photon-assisted tunneling in the hard-to-reach and technologically important terahertz part of the spectrum.
Shibata et al. study quantum dots made from indium arsenide, offering an energy-level spacing well matched to terahertz frequencies. The researchers grew quantum dots with molecular-beam epitaxy and placed a single dot between electrodes separated by a nanometer-size gap. They generated the terahertz field by pumping methanol gas with an infrared laser and efficiently coupled it to the dot via an antenna structure and a hemispherical silicon lens. Under terahertz irradiation, enhancement of electron transport through the dot was observed.
Two distinct types of photon-assisted tunneling emerged from the data: electron tunneling from the dot’s ground state into the electrodes, and electron tunneling between the electrodes via the excited state. Shibata et al. also found strong coupling between the terahertz control field and electrons in the quantum dot. Their results offer encouraging signs that the behavior of single carriers could be controlled by light in a challenging frequency range suitable for applications in spintronics and nanoelectronics. – David Voss
Photon-Assisted Tunneling through Self-Assembled InAs Quantum Dots in the Terahertz Frequency Range, K. Shibata, A. Umeno, K. M. Cha, and K. Hirakawa, Phys. Rev. Lett. 109, 077401 (2012).