  We study optically driven Rabi rotations of a quantum dot exciton transition
between 5 and 50 K, and for pulse-areas of up to $14\pi$. In a high driving
field regime, the decay of the Rabi rotations is nonmonotonic, and the period
decreases with pulse-area and increases with temperature. By comparing the
experiments to a weak-coupling model of the exciton-phonon interaction, we
demonstrate that the observed renormalization of the Rabi frequency is induced
by fluctuations in the bath of longitudinal acoustic phonons, an effect that is
a phonon analogy of the Lamb-shift.
