We report on the electroluminescence (EL) spectra and the differential absorption (Delta alpha) spectra for triple-layer InGaAs quantum dots (QDs) of different GaAs spacer thicknesses. Cross-sectional transmission electron microscopy directly reveals that the InGaAs QDs of 5-nm spacer are well-aligned along the growth direction. The Delta alpha spectra exhibit an increase of absorption change as the spacer layer thickness decreases from 40 to 5 nm. Meanwhile, the amount of absorption change at the photon energy of the excited transition becomes higher than that of the ground-state transition for the QDs of spacer thickness less than 20nm. The Delta alpha results are consistent with the EL spectra showing the higher emission intensity for the excited transition. From the EL and Delta alpha experiments, the higher optical gain and absorption change for the excited transition suggest that the e2-h2 transition has higher oscillator strength for the vertically coupled QDs. (C) 2008 Elsevier B.V. All rights reserved.