Propagation effects on the ultrafast cross-gain modulation in semiconductor optical amplifiers

We have performed cross-gain modulation experiments on semiconductor optical amplifiers with 100 fs pump pulses. Setting the pump on the high energy side of the gain results in a complex transient for the CW probe that is situated at gain maximum. An important part of this transient is shown to be due to propagation effects. Indeed, the pump pulses, at transparency or in absorption at the entrance, may get out of the amplifier in the gain region. These effects have to be considered, particularly in the potential sub-picosecond wavelength conversion with 100 fs pulses placed at transparency. Simulations based on a microscopic theory, which goes beyond the relaxation time approximation, show that, in such conditions, electron and hole populations can have different temperatures and different evolutions. However these simulations, which neglect propagation effects and consider only one slice of the device, fail to explain some of the experimental curves. In very long amplifiers (more than 2 mm) propagation effects become even more important. We show that they can be distinguished from carrier heating due to the generation of high energy carriers that arises only above an energy per pulse of 40 pJ.