We track the dynamic response of GaAs quantum-well semiconductor diode
lasers after injection of a femtosecond pulse, using ultrafast upconv
ersion. The continuous-wave emission shows gain dynamics and relaxatio
n oscillations on timescales of 1-100 ps. The recirculating femtosecon
d pulse evolves into an ultrafast ''dark pulse'' in the wake of subpic
osecond oscillations. (C) 1998 American Institute of Physics.