Scanning near-field infrared nano-spectroscopy on buried InAs quantum dots

Since providing optical resolution on the nanometer length scale, scanning near-field optical microscopy (SNOM) has turned out to be a powerful technique to investigate the optical properties of perovskites and nanostructured semiconductors, e.g. having a buried doping profile. Using a scattering-type-SNOM (s-SNOM) combined with a tunable free-electron laser (FEL) light source operating in the mid- and far-infrared regime we investigated the electronic structure of single InAs quantum dots (Q-dots) that were capped with a 70 nm thick GaAs layer. Spectroscopic near-field scans on individual Q-dots clearly identified two inter-sublevel transitions at 85 meV and 120 meV, providing optical contrast to the surrounding GaAs substrate. As a consequence the room temperature linewidth of these transitions measure 5 - 8 meV only, hence being significantly smaller as compared to the inhomogeneously broadened peaks resulting from integral spectroscopic analysis. Moreover, spatially scanning the s-SNOM tip at fixed excitation energies allowed mapping the spatial distribution of such buried quantum dots.