Type-II quantum-dot-in-nanowire structures with large oscillator strength for optical quantum gate applications

Abstract

Document VersionPublisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA):Taherkhani, M., Willatzen, M., Mørk, J., Gregersen, N., & McCutcheon, D. P. S. (2017). Type-II quantum-dot-innanowire structures with large oscillator strength for optical quantum gate applications. Physical Review B, 96(12) We present a numerical investigation of the exciton energy and oscillator strength in type-II nanowire quantum dots. For a single quantum dot, the poor overlap of the electron part and the weakly confined hole part of the excitonic wave function leads to a low oscillator strength compared to type-I systems. To increase the oscillator strength, we propose a double quantum dot structure featuring a strongly localized exciton wave function and a corresponding fourfold relative enhancement of the oscillator strength, paving the way towards efficient optically controlled quantum gate applications in the type-II nanowire system. The simulations are performed using a computationally efficient configuration-interaction method suitable for handling the relatively large nanowire structures.

High-fidelity optical quantum gates based on type-II double quantum dots in a nanowire

“…1(b) shows the lowest energy hole state confined inside the barrier in the valence band. We are particularly interested in this hole state inside the barrier since the corresponding exciton [19] has a considerable dipole coupling to both electronic ground-state orbitals. Furthermore, due to the small real-space overlap of the hole wave functions, phonon decay processes from the ground-state exciton with hole part inside the barrier to the lower energy excitons with the hole part outside the barrier can be neglected [28].…”

“…This results in a larger overlap with the hole inside the barrier which in turn increases the transition dipole moment. On the other hand, as the height of the QDs decreases below ∼2 nm, the electron states will have a significant overlap with each other and the tunneling probability of the electron states which is not desirable will increase [19]. The same argument also applies to the distance between the QDs.…”

“…The same argument also applies to the distance between the QDs. The diameter of the QD should also be chosen as a trade-off between a large dipole moment and sufficient energy level difference between the ground-state charged exciton and the first higher order charged exciton [19,28].…”

“…On the other hand, the type II band structure allows for the possibility of good overlap between electrons of neighboring QDs with a common hole state. It was recently shown that the otherwise weak transition oscillator strength of the exciton of a crystal-phase type II QD in an InP nanowire can be increased [19] by implementing a DQD structure leading to an improved overlap between the electron and hole parts confined inside the QD and the barrier, respectively. Additional advantages of DQDs in nanowires include the possibility for implementing multiple local metallic contacts and electrostatic gates on top, beneath, and next to the wire, which can be coupled to measuring apparatus or to adjacent DQD nanowires [20,21].…”

“…However, to obtain a numerically fast model suitable for design optimization, we perform our dipole moment calculations using a single-band model based on the envelope function and effective mass approximations. Additionally, in the type II DQD structure considered in this work, the electron and the hole are confined respectively by the band-edge potential profile inside the QDs and inside the barrier [19] and the Coulomb interaction is negligible in comparison to the potential profile confinement. For this reason, we have not considered the Coulomb interaction when calculating the charged exciton state.…”