Self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers were grown on GaAs (1 0 0) substrates by molecular beam epitaxy. Relaxation of lattice strain in the In0.35Al0.65As nucleation layer was monitored by in situ reflection high-energy electron diffraction. Self-assembled InAs QDs were successfully formed on the strain-relaxed In0.35Ga0.35As barrier. Twenty-layer stacked InAs QDs showed optical absorption in the wavelength range of 1350-1650 nm. A fast decay of 18 ps was observed in the temporal profile of absorption saturation measurement at a wavelength of 1540 nm, which is expected to be useful for ultrafast nonlinear optical switching applications operating in the 1.55 mu m waveband. (C) 2008 Elsevier B.V. All rights reserved.