  The energy transport of acoustic phonons generated by the optical excitation
of a quantum dot as well as the coherence properties of these phonons are
studied theoretically both for the case of a pulsed excitation and for a
continuous wave (cw) excitation switched on instantaneously. For a pulsed
excitation, depending on pulse area and pulse duration, a finite number of
phonon wave packets is emitted, while for the case of a cw excitation a
sequence of wave packets with decreasing amplitude is generated after the
excitation has been switched on. We show that the energy flow associated with
the generated phonons is partly related to coherent phonon oscillations and
partly to incoherent phonon emission. The efficiency of the energy transfer to
the phonons and the details of the energy flow depend strongly and in a
non-monotonic way on the Rabi frequency exhibiting a resonance behavior.
However, in the case of cw excitation it turns out that the total energy
transferred to the phonons is directly linked in a monotonic way to the Rabi
frequency.
