Coherent control of photo-excited carriers in semiconductor quantum dots using near-field scanning optical microscope

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The strain-driven self-assembly of semiconductor quantum dots (SAQDs) provides nanometer scale carrier confinement in all three dimensions. One of the fascinating characteristics of SAQDs is their discrete density of states (DOS) with large energy-level spacing, in which acoustic phonon-mediated scattering should be suppressed compared with higher-dimensional structures. As a consequence, excitons in SAQDs are expected to exhibit long coherence time, which is advantageous for application of quantum information processing. In terms of inter-level relaxation, interaction with longitudinal optical (LO) phonons is an important aspect of SAQDs. These phonon-mediated transitions can be confirmed in the single-dot photoluminescence excitation (PLE) spectra, in which the reduction of relaxation paths renders individual LO phonons visible in the ground state emissions. To date several LO phonon-related transitions including bulk GaAs have been clearly observed. In the energy region higher than … More 1LO, single-dot PLE spectra reveal a number of intense PLE features, recognized as 2LO phonon resonances. As excitonic excited states are also present, several relaxation pathways with qualitatively different processes exist in the 2LO region.In this research, we investigate in detail the 2LO resonances in the single-dot PLE. Several 2LO resonances emerge as dip structures in the peaks under various conditions of excitation power. The observed dip structures suggest line broadening, even in the individual resonace lines. In contrast, the corresponding biexciton resonances still have sharp peaks, indicating that the 2LO resonances consist of several relaxation channels with differing scattering processes. These observations can explain why PLE resonances exhibit short dephasing times even in single SAQDs. Moreover, we found that phonon mediated transitions can be largely removed by two-color excited PLE spectroscopy. The reduction of line broadening indicates that the PLE includes a number of phonon-related transitions with large relaxation windows. Less