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Physical Review Letters

Long Lived Coherence in Self-Assembled Quantum Dots

D. Birkedal, Kristján Leósson, J. M. Hvam

Published Nov 7, 2001• 225 citations
Publisher

Abstract

We report measurements of ultralong coherence in self-assembled quantum dots. Transient four-wave mixing experiments at 5 K show an average dephasing time of 372 ps, corresponding to a homogeneous linewidth of 3.5 microeV, which is significantly smaller than the linewidth observed in single-dot luminescence. Time-resolved luminescence measurements show a lifetime of the dot ground state of 800 ps, demonstrating the presence of pure dephasing at finite temperature. The homogeneous width is lifetime limited only at temperatures approaching 0 K.

DOI
10.1103/physrevlett.87.227401

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We have performed time-integrated degenerate four-wave mixing experiments in InAs/GaAs self-assembled quantum dots. The quantum dots are excited in resonance with an intersublevel transition between the valence states of the dots. The dephasing time T2{T}_{2}T2​ is deduced from the time dependence of the photon echo amplitude as a function of the delay between the two incident pulses. At low temperature, a dephasing time T2{T}_{2}T2​ of 15 ps is measured for a resonant excitation at 7.4 \ensuremath{\mu}m wavelength. This dephasing corresponds to an homogeneous broadening of the intersublevel transition of 87 \ensuremath{\mu}eV. The polarization decay time is much longer than the one observed for resonant intersubband excitations in quantum wells. The dephasing time of the intersublevel polarization exhibits only a weak temperature dependence, indicating that the dephasing is not dominated by phonon-assisted scattering mechanisms.
We investigate the dephasing mechanism of optically-induced exciton coherence in an ensemble of InAs self-assembled quantum dots (QDs) using a two-pulse photon-echo (PE) technique. The sample was fabricated on an InP (311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for enhancing the PE signal intensity. We show the measured decay time of PE signal intensity for resonant excitation of ground-state transition is extremely long, which corresponds to ultralong coherence times of excitons in our QDs. We confirmed that non-radiative population relaxation and pure dephasing are considerably smaller than the radiative relaxation at low temperature. Furthermore, we demonstrate the ultrafast coherent control of QD exciton polarization by changing the pulse areas (Rabi oscillations) in the PE experiment. We also propose a new scheme for implementing QD quantum memory by using the PE technique.