Terahertz spectroscopy on single buried InAs quantum dots by scanning near-field nano-microscopy

Quantum dots have shown to be a highly interesting material system for many application purposes such as single photon emitters in the near-infrared involving interband transitions, but also for mid- and far-infrared applications using intersublevel transitions. Studying the linewidth of these transitions offers valuable clues to the dephasing mechanisms of the trapped electrons. However, due to size fluctuations of the quantum dots, inhomogeneous broadening of the signals usually hides this information when investigating ensembles of dots. Therefore, single-dot spectroscopy has to be performed for this purpose. In contrast to studies of interband transitions (e.g. via micro-photoluminescence) this is not well established at all for intersublevel transitions. In this work, scattering type scanning near-field optical microscopy (s-SNOM) in combination with a free-electron laser is used to investigate intersublevel transitions in single self-assembled buried InAs quantum dots. Thereby, spectrally resonant optical contrast to the surrounding GaAs substrate is observed at photon energies of 83 meV and 123 meV with a linewidth of 5-8 meV. Supported by photoluminescence data these signals can clearly be assigned to the s-d and p-d transitions of single conduction band electrons.