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Acta Physica Sinica

The effect of biexciton, wetting layer leakage and Auger capture on Rabi oscillation damping in quantum dots

Liu Shao-Ding, Mu-Tian Cheng, Huijun Zhou, Yaoyi Li, Qu‐Quan Wang, Qi-Kun Xue, (1)武汉大学物理系，武汉 430072; (2)武汉大学物理系，武汉 430072；中国科学院物理研究所，国际量子结构中心，北京 100080; (3)中国科学院物理研究所，国际量子结构中心，北京 100080

Published Jan 1, 2006• 2 citations
Publisher

Abstract

The decoherence of Rabi oscillation with multi-level processes in semiconductor quantum dots excited by laser pulses is investigated. By using population dynamic equations of multi-level system, the effect of three kinds of multi-level processes on the damping of Rabi oscillation in quantum dots are numerically simulated and discussed. The effect of biexciton can be neglected when the pulse width is larger than 5ps; the population leakage to wetting layer results in the decreasing of the amplitude and average of the population oscillation on the exciton ground state with the increasing of the excitation intensity; the effect of the two kinds of Auger capture processes on the Rabi oscillation and the spectral width of the photoluminescence from the exciton recombination are also discussed.

DOI
10.7498/aps.55.2122

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The decoherence of Rabi oscillation with multi-level processes in semiconductor quantum dots excited by laser pulses is investigated. By using population dynamic equations of multi-level system, the effect of three kinds of multi-level processes on the damping of Rabi oscillation in quantum dots are numerically simulated and discussed. The effect of biexciton can be neglected when the pulse width is larger than 5ps; the population leakage to wetting layer results in the decreasing of the amplitude and average of the population oscillation on the exciton ground state with the increasing of the excitation intensity; the effect of the two kinds of Auger capture processes on the Rabi oscillation and the spectral width of the photoluminescence from the exciton recombination are also discussed.
The decoherence of Rabi oscillation (RO) caused by biexciton, population leakage to the wetting layer (WL), and Auger capture in semiconductor quantum dots is theoretically analyzed with multilevel optical Bloch equations. The corresponding effects on the quality factor of RO are also discussed. We have found that the biexciton effect is relatively trifling, as the pulse duration is longer than 5 ps. The population leakage to the WL leads to a decrease of the RO average even though the damping rate is similar to that observed in the experiment. Auger capture in quantum dots results in RO damping that is consistent with the experimental data, which implies that Auger capture is an important decoherence process in quantum dots.
The Rabi oscillations of the exciton of the excited state in single semiconductor quantum dots were investigated by using the photoluminesence and nano-spectroscopy technique. The population on the excited state oscillated as a function of phase angle of the qubit were recorded, in which pulse-pairs with the adjustable time delay were used as the excitation. The quality factors of free rotation is about 9.8×104, and that of the dynamic rotation is about 18 for the single quantum dot excitonic qubit based on the experimental observations, The decoherece mechanism of Rabi oscillation stem from the capture processes from wetting layer to quantum dot is also analyzed. The exciton coherent control and the statistic characteristics of single photon emission in single semiconductor quantum dot are discussed as well.
We develop a realistic model of Rabi oscillations in a quantum-dot photodiode. Based in a multiexciton density matrix formulation we show that for short pulses the two-level model fails and higher levels should be taken into account. This affects some of the experimental conclusions, such as the inferred efficiency of the state rotation (population inversion) and the deduced value of the dipole interaction. We also show that the damping observed cannot be explained using constant rates with fixed pulse duration. We demonstrate that the damping observed is in fact induced by an off-resonant excitation to or from the continuum of wetting layer states. Our model describes the nonlinear decoherence behavior observed in recent experiments.
We provide a theoretical study for the dynamical behavior of two-photon Rabi oscillations in two coupled quantum dots. Each dot is a three-level quantum system (ground, one-exciton, and biexciton states). We describe the optical dynamics within the density matrix formalism framework. The two-photon Rabi oscillations are found to be significantly affected by the strength of the interdot dipolar interaction. Using a detuned laser pulse with an exciton state of the driven quantum dot in our numerical simulations, a direct coherent excitation between the biexciton states can be established, but whether Rabi oscillations can be observed depend mainly on the strength of the dipolar interaction and bandwidth of the excitation laser. When the strength of the dipolar coupling is less than the excitation laser bandwidth, the time-dependent populations of the various two biexciton states of the system exhibit coherent oscillations with several frequencies. However, as we increase the dipolar coupling strength, the population transfer between the quantum dots levels amazingly becomes forbidden, and the excitation gets trapped at the initial state. The reduction of the influence of the dipolar interaction leads to a manipulation of two-photon Rabi oscillation in an ultrashort timescale (∼ps), opening possibilities to achieve ultrafast quantum state control and quantum logic gate generation.
Exciton-biexciton coherent coupling effects are examined in semiconductor quantum dots. The exciton absorption spectrum is measured with the microphotoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from a Lorenztian-type line shape to an unusual dip-shaped line shape with increasing excitation intensities in a higher exciton state where there is a large oscillator strength between the exciton and biexciton states. The intensity dependence of the dip energy width clearly indicates that coherent Rabi oscillation occurs between the exciton and biexciton states. The absorption properties with excitation light of different polarizations show that the dip-shaped spectra only appear when there is a large biexciton state population with linear polarization. A theoretical analysis undertaken with the density matrix method agrees well with experimental results. This agreement reveals that exciton-biexciton coherent interactions lead to unusual absorption spectra and contribute crucially to the optical properties of quantum dots. The exciton-biexciton coherent effects provide a scheme for controlling four distinguishable states, which can be applied to a demonstration of quantum gate operations.
We present measurements and calculations of optical Rabi oscillations in the excitonic ground-state transition of an InGaAs quantum dot ensemble at low temperature. Rabi oscillations which are damped versus pulse area and change period when changing pulse duration are observed. Comparisons with calculations show that the observed damping is not intrinsic to a single dot. Dephasing processes and the biexciton resonance change the amplitude and the period of the oscillations, respectively, while the damping versus pulse area is due to a distribution of transition dipole moments in the ensemble.