  Spin-orbit interaction in a quantum dot couples far infrared radiation to non
center of mass excitation modes, even for parabolic confinement and dipole
approximation. The intensities of the absorption peaks satisfy the optical sum
rule, giving direct information on the total number of electrons inside the
dot. In the case of a circularly polarized radiation the sum rule is
insensitive to the strength of a Rashba spin-orbit coupling due to an electric
field orthogonal to the dot plane, but not to other sources of spin-orbit
interaction, thus allowing to discriminate between the two.
