  Using far-infrared spectroscopy, we investigate the excitations of
self-organized InAs quantum dots as a function of the electron number per dot,
1<n<6, which is monitored in situ by capacitance spectroscopy. Whereas the
well-known two-mode spectrum is observed when the lowest s - states are filled,
we find a rich excitation spectrum for n=3, which reflects the importance of
electron-electron interaction in the present, strongly non-parabolic confining
potential. From capacitance spectroscopy we find that the electronic shell
structure in our dots gives rise to a distinct pattern in the charging energies
which strongly deviates from the monotonic behavior of the Coulomb blockade
found in mesoscopic or metallic structures.
