Near-field coherent excitation spectroscopy of InGaAs/GaAs self-assembled quantum dots

Using near-field optical microscopy, we have performed coherent excitation spectroscopy of self-assembled quantum dots (SAQDs). A pair of coherent pul ses with a time delay between them allows measurement of the temporal coher ence of the carrier wave function in single quantum dots. The observed deco herence time is about 15 ps and is well explained by resonant Raman scatter ing of phonons. Furthermore, quantum beats originating from the superpositi on of two closely spaced coherent states have been observed. This opens up possibilities of quantum mechanical control of the carrier wave function in SAQDs. (C) 2000 American Institute of Physics. [S0003-6951(00)01426-1].