Light-induced spectral diffusion in single self-assembled quantum dots

Small ensembles of ${\mathrm{In}}_{0.55}{\mathrm{Al}}_{0.45}\mathrm{As}$ self-assembled quantum dots have been studied at low temperatures using near-field scanning optical microscopy. We observe spectral diffusion in individual quantum dot luminescence lines. The phenomenon increases in magnitude with optical power density, but is not visible at low powers. We believe the spectral diffusion to be caused by long time-scale trapping and untrapping of charges created by photoionization or thermal ionization in the immediate vicinity of the quantum dots.