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Physical review. B./Physical review. B

Effect of the depolarization field on coherent optical properties in semiconductor quantum dots

Yasuyoshi Mitsumori, Shunta Watanabe, Kenta Asakura, Keisuke Seki, Keiichi Edamatsu, Kouichi Akahane, Naokatsu Yamamoto

Published Jun 6, 2018• 2 citations
Publisher

Abstract

We study the photon echo spectrum of self-assembled semiconductor quantum dots using femtosecond light pulses. The spectrum shape changes from a single-peaked to a double-peaked structure as the time delay between the two excitation pulses is increased. The spectrum change is reproduced by numerical calculations, which include the depolarization field induced by the biexciton-exciton transition as well as the conventional local-field effect for the exciton--ground-state transition in a quantum dot. Our findings suggest that various optical transitions in tightly localized systems generate a depolarization field, which renormalizes the resonant frequency with a change in the polarization itself, leading to unique optical properties.

DOI
10.1103/physrevb.97.235305

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We study the photon echo spectrum of self-assembled semiconductor quantum dots using femtosecond light pulses. The spectrum shape changes from a single-peaked to a double-peaked structure as the time delay between the two excitation pulses is increased. The spectrum change is reproduced by numerical calculations, which include the depolarization field induced by the biexciton-exciton transition as well as the conventional local-field effect for the exciton--ground-state transition in a quantum dot. Our findings suggest that various optical transitions in tightly localized systems generate a depolarization field, which renormalizes the resonant frequency with a change in the polarization itself, leading to unique optical properties.
We experimentally studied the excitonic Rabi oscillations in self-assembled semiconductor quantum dots using two-pulse photon echo spectroscopy. We observed the change in the shape of the Rabi oscillations by the second pulse intensity. Our experimental results were systematically reproduced by numerical calculations taking into account the local field effect arising from the Coulomb interaction between photoinduced charge densities in a quantum dot. Our experimental findings indicated that the local field effect contributes considerably to the coherent optical responses of the excitons in quantum dots.
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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.
Simulations of two dimensional coherent photon echo (2D‐PE) spectra of self‐assembled InAs/GaAs quantum dots (QD) in different charged states are presented revealing the coupling between the individual mono‐exciton transitions and contributions of bi‐excitons . The information about the states is crucial for various application scenarios of QDs, like e.g. highly efficient solar cells. The simulations rely on a microscopic description of the electronic structure by high‐level atomistic many‐body pseudopotential calculations. It is shown that asymmetric diagonal peak shapes and double cross‐peaks are the result of state contributions to the PE signal by analyzing the contributions of the individual pathways excited state emission, ground state bleach and excited state absorption. The results show that from the detuned and contributions the bi‐exciton binding energies of the manifold are revealed in 2D‐PE signals.
An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in the form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot subensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for $\ensuremath{\pi}/2$ pulses, while for $3\ensuremath{\pi}/2$ the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations.
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