Ultrafast nonlinear processes in quantum-dot optical amplifiers

Ultrafast gain dynamics in quantum-dot optical amplifiers has been studied by using the pump-probe and four-wave mixing (FWM) techniques. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier ca pture from the wetting layers into the quantum dots (QDs). The relevant tim e constants were evaluated to be similar to 90 fs, similar to 260 fs, and s imilar to2 ps, respectively, under a 50 mA bias condition. The compressed g ain recovered to 3% of its initial value in 4 ps, and no recovery component slower than 2 ps could be seen in the temporal range tested. This is quite different from the feature in quantum wells, where a very slow component ( > 50 ps) exists. This suggests a possibility of enhancing the operation spe ed of semiconductor optical amplifiers by using QDs as an active layer. The third-order optical susceptibility (chi ((3))) has been evaluated by means of both nonlinear transmission and FWM experiments. The results show that the nonlinearity expressed by chi ((3))/g(0) is quite similar to that of bu lk and quantum wells, which can be explained by the similar relaxation time s.