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physica status solidi (b)

Theory of Exciton Coherence and Decoherence in Semiconductor Quantum Dots

T. Takagahara

Published Nov 1, 2002• 7 citations
Publisher

Abstract

We discuss the Rabi splitting and Rabi oscillation of excitons in semiconductor quantum dots and clarify the fundamental physics and new characteristic features. These coherent optical phenomena are not restricted to semiconductor quantum dots and the predicted features would be observed also in atoms and molecules. The universality of the phenomena ensures the possibility for application to quantum state control and quantum information processing by optical means. We discuss briefly the mechanisms of exciton dephasing in semiconductor quantum dots in conjunction with recent observations of the lifetime-limited exciton dephasing time and show that the pure dephasing rate is very small but is not vanishing even at zero Kelvin.

DOI
10.1002/1521-3951(200211)234:1<115::aid-pssb115>3.0.co;2-6

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Summary form only given. Semiconductor quantum dots are attracting much interest as a candidate to implement the quantum information processing and the quantum computation. In order to examine the possibility of the quantum state manipulation, we study the excitonic Rabi flopping and a new type of quantum interference phenomena in semiconductor quantum dots. One of the most striking manifestation of the quantum coherence is the Rabi flopping. The observation of the excitonic Rabi splitting in an InGaAs quantum dot was reported recently. Under the strong excitation of an excited exciton state in a single quantum dot, the luminescence from the exciton ground state shows a doublet splitting. This splitting can be interpreted in the dressed exciton picture. Namely, under the strong and resonant excitation of a particular exciton state, the ground state of the system splits into two levels. Then the transition from the exciton ground state to the split ground state results in the doublet structure of the emission spectrum.
This Chapter contains a review of the recent results, both experimental and theoretical, related to optical control of carriers confined in semiconductor quantum dots. The physics of Rabi oscillations of exciton and biexciton occupations, as well as time-domain interference experiments are discussed. Next, the impact of carrier--phonon interaction in a semiconductor structure is described and modern methods of theoretical description of the carrier--phonon kinetics and of the resulting dephasing are presented.
Semiconductor quantum dots tightly confine excited electron-hole pairs, called excitons, resulting in discrete energy levels similar to those of single atoms. Transition energies in the visible or near-infrared make quantum dots suitable for many applications in quantum optics and quantum information science, but to take advantage of all the properties of quantum dot emission, it is necessary to excite them coherently which has been a great challenge due to background scattering of the excitation laser. This dissertation presents the first coherent control of a single quantum dot with observation of its resonance fluorescence and decoherence phenomena. Strong continuous-wave excitation causes the dot to undergo several Rabi oscillations before emitting. These are visible as oscillations in the first- and second-order correlation functions of the emission, and the quantum dot states are "dressed", resulting in a Mollow triplet in the emission spectrum. Some resonantly excited dots, in addition to resonance fluorescence, also emit light from excited states several meV higher in energy. Such up-conversion fits existing theories of decoherence but has never been directly observed before. The up-conversion intensity is shown to be described well by a fairly simple three-level model with single-phonon absorption. The coherent phenomena of resonance fluorescence and the decoherence due to up-conversion paint a dual picture of single quantum dots wherein they can sometimes be treated as an ideal two-level system, but their interactions with the host crystal can lead to many complex behaviors.
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 report on coherent control of the fundamental exciton state in a single quantum dot by means of two phase-locked picosecond laser pulses. By analyzing the experiments performed in the Rabi regime and comparing them to numerical simulations, we show that we can address the exciton coherence and get an insight into the involved dephasing processes. The total decoherence time T2{T}_{2}T2​ (170 ps) is comparable to the effective exciton lifetime T1{T}_{1}T1​ (200 ps), although not reaching the upper theoretical limit of 2T12{T}_{1}2T1​. We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons both contribute on an equal footing to the loss of coherence in this kind of monolayer-step-induced GaAs quantum dots.