Single-color and two-color pump-probe measurements are used to analyze carrier dynamics in InAs/ GaAs quantum dot amplifiers. The study reveals that hole recovery and intradot electron relaxation occur on a picosecond time scale, while the electron capture time is on the order of 10 ps. A longer time scale of hundreds of picoseconds is associated with carrier recovery in the wetting layer, similar to that observed in quantum well semiconductor amplifiers. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2771374͔ Ultrafast spectroscopy of quantum dot ͑QD͒ semiconductor optical amplifiers ͑SOAs͒ provides valuable information about the potential of these devices for emerging applications such as multiwavelength regeneration while giving insight on their unique carrier dynamics. 1 Unlike bulk and quantum well materials, QDs have discrete energy levels; it is the carrier capture and relaxation dynamics between these levels that will constitute the intrinsic limiting device bandwidth. For this reason, initial SOA studies have concentrated on the gain and refractive index recovery of the QD ground state ͑GS͒, following a pump pulse at the same wavelength. These single-color studies have revealed the presence of several recovery time scales, related to electron capture and relaxation within a dot. 2 Similar SOA studies have been performed on the first excited state ͑ES͒ and revealed the presence of similar time scales. 3 To further investigate carrier transition times, two-color differential transmission spectroscopy has been applied to both quantum well 4 and QD ͑Ref. 5͒ structures. Time scales reported include dot capture and relaxation times of a few picoseconds and less than a picosecond respectively for InGaAs QDs, 5 dot to dot carrier scattering times of ϳ35 ps for InAs QDs, 6 and a Ͻ1 ps time scale for thermalization of holes together with a 15 ps time scale for electron GS to ES escape, also in InAs QDs. 7 Previously, we demonstrated that the carrier capture process for InAs QD structures was Auger mediated. 8 However, our analysis did not include the asymmetry that exists between QD electron and hole effective masses, which in turn leads to more closely spaced hole levels. This hole spacing is less than the thermal energy at room temperature, which in turn is less than the electron spacing, and consequently, the dots' hole states can be reduced to a single shared hole population. This has been shown to lead to GS gain compression in QD lasers 9 and subpicosecond hole capture time scales. 7,10 In this letter, we present two-color differential transmission measurements to confirm the presence of these fast hole redistribution processes and deduce the time scales of the remaining electron capture/escape processes. In addition, we develop a rate equation model of the electron and hole occupancies and demonstrate its agreement with the experimental results.Our experiment is based on the scheme presented in ͑Ref. 11͒ where pump and probe pulses of different wavelengths are filtered from a femtosecond pulse spect...