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Applied Physics Letters

Coulomb corrections to the slowdown factor in quantum-dot quantum coherence

S. Michael, Chi‐Wai Chow, Hans Christian Schneider

Published Oct 30, 2006• 12 citations
Publisher

Abstract

Quantum-coherence induced group-velocity slowdown in a semiconductor quantum-dot structure is investigated using a many-body theory. The predictions are found to be noticeably different from those obtained in the independent-particle treatment typically used for describing atomic quantum coherence. In particular, Hartree-Fock renormalizations can lead to over two orders of magnitude reduction in the predicted pump intensity requirement for group-velocity slowdown to occur. Results are presented for the slowdown factor and slowdown-bandwidth product in a pulsed InAs–GaAs quantum-dot Λ scheme.

DOI
10.1063/1.2364164

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Quantum-coherence induced group-velocity slowdown in a semiconductor quantum-dot structure is investigated using a many-body theory. The predictions are found to be noticeably different from those obtained in the independent-particle treatment typically used for describing atomic quantum coherence. In particular, Hartree-Fock renormalizations can lead to over two orders of magnitude reduction in the predicted pump intensity requirement for group-velocity slowdown to occur. Results are presented for the slowdown factor and slowdown-bandwidth product in a pulsed InAs–GaAs quantum-dot Λ scheme.
A many-body theory for quantum coherence in a semiconductor quantum-dot system is presented. We present the theory in detail for a Λ scheme in a InGaAs–GaAs quantum dot with a pulsed drive field. The paper shows the significant differences in the predictions of this theory and those of an independent-particle treatment, adapted from atomic quantum coherence theory. Specifically, the influence of the quantum-dot level structure on group-velocity slowdown is discussed, and the influence of different Coulomb renormalization effects is analysed.
In this paper, the many-body aspects and the differences to the independent-particle treatment of quantum coherence in QDs are discussed. For investigation the semiconductor Bloch equations, modified for a Lambda system, is used. A relatively detailed description of the electronic states of a coupled QD-QW system and the coupling of these states via collisions in the relaxation rate approximation is included. A structure consisting of InAs pyramidal dots with a base of 12 x 12 nm and height 6 nm on a wetting layer of thickness 1 nm embedded in a GaAs QW is chosen for the study. Here, the slow-down factor S, which is a measure of the refractive index changes resulting in optical group-velocity reduction and thus of interest for applications such as optical storage, is focused.
We investigate theoretically the slowdown of optical pulses due to quantum-coherence effects in InGaAs-based quantum dots and quantum dot molecules. Simple models for the electronic structure of quantum dots and, in particular, quantum-dot molecules are described and calibrated using numerical simulations. It is shown how these models can be used to design optimized quantum-dot molecules for quantum coherence applications. The wave functions and energies obtained from the optimizations are used as input for a microscopic calculation of the quantum-dot material dynamics including carrier scattering and polarization dephasing. The achievable group velocity slowdown in quantum-coherence V schemes consisting of quantum-dot molecule states is shown to be substantially higher than what is achievable from similar transitions in typical InGaAs-based single quantum dots.
The slowdown of optical pulses due to quantum-coherence effects is investigated theoretically for an ``active material'' consisting of InGaAs-based double quantum dot molecules. These are designed to exhibit a long-lived coherence between two electronic levels, which is an essential part of a quantum-coherence scheme that makes use of electromagnetically induced transparency effects to achieve group-velocity slowdown. We apply a many-particle approach based on realistic semiconductor parameters that allows us to calculate the quantum dot material dynamics including microscopic carrier scattering and polarization dephasing dynamics. The group-velocity reduction is characterized in the frequency domain by a quasiequilibrium slowdown factor and in the time domain by the probe-pulse slowdown obtained from a calculation of the spatiotemporal material dynamics coupled to the propagating optical field. The group-velocity slowdown in the quantum dot molecule is shown to be substantially higher than what is achievable from similar transitions in typical InGaAs-based single quantum dots. The dependencies of slowdown and shape of the propagating probe pulses on lattice temperature and drive intensities are investigated.
This paper explores quantum-coherence phenomena in a semiconductor quantum-dot structure. The calculations predict the occurrence of inversionless gain, electromagnetically induced transparency, and refractive-index enhancement in the transient regime for dephasing rates typical under room temperature and high excitation conditions. They also indicate deviations from atomic systems because of strong many-body effects. Specifically, Coulomb interaction involving states of the quantum dots and the continuum belonging to the surrounding quantum well leads to collision-induced population redistribution and many-body energy and field renormalizations that modify the magnitude, spectral shape, and time dependence of quantum-coherence effects.