Low temperature near-field optical mapping and control of electron wavefunctions confined in nanostructures

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We have established a near-field scanning optical microscope (NSOM) system with a high spatial resolution under a magnetic field up to 5T at low temperature. The spatial resolution as high as 30-50 nm enabled us to visualize exciton wave functions confined in single quantum dots.In GaAs/AlGaAs interface quantum dots, the wavefunctions of ground-state exciton, excited-state exciton, and biexciton states are clearly mapped out. A numerical calculation reproduced the experimental results well in their quantitative nature. We also applied this technique to visualize the local density of states of excitons confined in an electric-field induced quantum dot system. We observed that electrons confined in lower energy states localized more strongly.Dilute nitride alloy systems such as GaNAs and GaInNAs were investigated to clarify nonuniform distribution of N atoms through the spatially resolved spectroscopy of localized and delocalized electron states. The depths and spatial profiles of exciton confinement potential were visualized through the measurement of power-dependent PL spectra and high-resolution PL images. For two GaInNAs quantum wells with different N contents, we observed strongly localized excitons confined in N clusters and rather delocalized excitons, which indicate the onset of alloy formation.From magneto-PL spectroscopy of a single QD, a diamagnetic shift of the exciton emission was observed. Referring to the theoretical value we estimate the size of the QD from a diamagnetic coefficient, which is determined by the Bohr radius of excitons confined in QDs. The result was in good agreement with that obtained by the real-space NSOM mapping.