The dynamics of electrons and holes in multiquantum-well semiconductor gain media involves several different transport processes, such as diffusion and drift across the barrier region, as well as capture and escape transitions between the bound and the unbound states of the quantum wells. In addition to their fundamental interest, these processes are important because of their implications for the dynamic properties of multiquantum-well lasers and optical amplifiers. Experimentally, they have been studied with several time-domain optical techniques having (sub)picosecond resolution and, more recently, with frequency-domain techniques based on laser modulation measurements. This article gives a brief review of the work done in this area and then presents in detail a frequency-domain approach, four-wave mixing spectroscopy in semiconductor optical amplifiers, to investigate intrinsic capture and interwell equilibration. This technique allows one to extend the device modulation frequency to several hundreds of gigahertz, thus providing the required time resolution, and can be configured to isolate and directly study the transport process of interest
