Femtosecond Dynamics in Semiconductor Lasers: "Dark Pulse" Formation

We track the dynamic response of GaAs quantum-well semiconductor diode lasers after injection of a femtosecond pulse, using ultrafast upconversion. The continuous-wave emission shows gain dynamics and relaxation oscillations on timescales of 1–100 ps. The recirculating femtosecond pulse evolves into an ultrafast ‘‘dark pulse’’ in the wake of subpicosecond oscillations.