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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Reduced pump-requirement for group-velocity slowdown in quantum-dot quantum-coherence

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

Published Jun 1, 2007• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-iqec.2007.4386116

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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.
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.
A variable semiconductor optical buffer based on the electromagnetically induced transparency in a quantum dot waveguide is theoretically investigated with feasible parameters for applications to a 40 Gbps optical network. We show the refractive index and absorption spectra of the quantum dot waveguide at various pump levels, which exhibit an optimal pump power for maximum slow-down factor, in agreement with the previous experimental observation using a Pr-doped solid. The group velocity slow-down factor is theoretically analysed as a function of the pump intensity at different broadened linewidths. Inhomogeneous broadening in self-assembled quantum dots degrades the slow-down factor. In order to reduce the inhomogeneous broadening effects, we propose to use a resonant microcavity structure with quantum dots embedded in the active layer to enhance the slow-down factor.
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.
The gain and phase recovery dynamics of quantum-dot (QD) semiconductor optical amplifiers are calculated, including all the optical transitions involved in successive carrier recovery processes. The carrier recovery dynamics of inhomogeneously broadened QDs is simulated by solving 1088 coupled rate equations. The respective contributions of QD states and quantum-well carrier reservoirs to the gain and phase changes are identified both temporally and spectrally. We show that the slow phase recovery component of the QD ground state is induced by the slow carrier dynamics of the carrier reservoir due to a slowly varying line shape function of the refractive index change.
This is a detailed study of slow light based on quantum effects in semiconductor nanostructures. We propose to implement an optical buffer using semiconductor quantum dots and quantum wells based on coherent population oscillation and spin related electromagnetically induced transparency. We identify different pump-and-probe schemes and develop comprehensive theoretical models which include important physical mechanisms in semiconductors. Semiconductors have unique properties which are inaccessible to other slow light media, and these enable the possibilities of controlling the semiconductor-based optical buffer in addition to the optical method. We investigate how to electrically control group index (slowdown factor) in quantum dots by using reverse bias voltage and forward injection current. We also develop a model to use the anisotropy of the light-hole exciton to vary the slowdown factor via two different mechanisms (coherent population oscillation and spin related electromagnetically induced transparency). Finally, we propose an idea to use the strain to control the spin relaxation time in semiconductors, which can help spin-dependent slow light in semiconductors. This research shows that the semiconductor is a promising candidate to demonstrate an optical buffer and can provide more flexible control than other slow light materials.