  Using photoluminescence spectroscopy, we have investigated the nature of Rabi
oscillation damping during active manipulation of excitonic qubits in
self-assembled quantum dots. Rabi oscillations were recorded by varying the
pulse amplitude for fixed pulse durations between 4 ps and 10 ps. Up to 5
periods are visible, making it possible to quantify the excitation dependent
damping. We find that this damping is more pronounced for shorter pulse widths
and show that its origin is the non-resonant excitation of carriers in the
wetting layer, most likely involving bound-to-continuum and continuum-to-bound
transitions.
