Abstract-Negative free carrier absorption in terahertz quantum cascade lasers

C. Ndebeka-Bandou, M. Rösch, K. Ohtani, M. Beck, J. Faist

(Submitted on 28 Nov 2016)

https://arxiv.org/abs/1611.09269

We analyze the peculiar case where the free carrier absorption arising from LO phonon absorption-assisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm−1's at 200K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime.

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.