Femtosecond near-field spectroscopy of single quantum dots

Excitonic and spin excitations of single semiconductor quantum dots currently attract attention as possible candidates for solid state based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single quantum dot on a femtosecond time scale. Transient reflectivity spectra show pronounced oscillations around the quantum dot exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the exitonic quantum dot polarization and continuum excitations. The results show that although semiconductor quantum dots resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time scales.