Photoluminescence (PL) spectroscopy with subwavelength lateral resolution has been employed to probe individual localization centers in a thin InGaN/GaN quantum well. Spectrally narrow emission lines with a linewidth as small as 0.8 meV can be resolved, originating from the recombination of an electron-hole pair occupying a single localized state. Surprisingly, the individual emission lines show a pronounced blueshift when raising the temperature, while virtually no energy shift occurs for increasing excitation density. These findings are in remarkable contrast to the behavior usually found in macro-PL measurements and give a fundamental new insight into the recombination process in semiconductor nanostructures in the presence of localization and strong internal electric fields. We find clear indications for a biexciton state with a negative binding energy of about -5+/-0.7 meV.

Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only approximately equal 100 micro(B) and to resolve statistical changes on the order of a few micro(B).

Optical single dot studies in wide-band gap diluted magnetic CdSe/ZnMnSe quantum dots have been performed. Due to the sample design, the photoluminescence energy of the quantum dot signal is energetically below the internal Mn2+ transition, resulting in high quantum efficiencies comparable to nonmagnetic CdSe/ZnSe quantum dots. Magnetic-field- and temperature-dependent measurements on individual dots clearly demonstrate the exchange interaction between single excitons and individual Mn2+ ions, resulting in a giant Zeeman effect and a formation of quasi-zero-dimensional magnetic polarons.

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