We present far-/near-infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The far-infrared resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots. The results show that the interband photoluminescence (PL) lines originate from conduction levels with successively increasing in-plane quantum numbers. We determine the confinement energies for both electrons and holes in the same dots. Furthermore, we show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation.