Net coherent optical gain in strongly pumped semiconductor microcavities

We study the subpicosecond gain dynamics in a high-finesse semiconductor mi crocavity containing quantum wells by means of degenerate pump-probe spectr oscopy. Tuning the optical excitation in resonance with both the exciton an d the cavity modes, at high pump fields the probe pulses are strongly ampli fied near the energy of the empty-cavity mode. At zero probe delay, integra tion of the probe spectrum yields a net gain of several percents. The coher ent nature of the gain is demonstrated by its oscillatory behavior versus p robe delay. A dynamical Maxwell-Bloch model for semiconductors reproduce we ll the experimental findings and gives insight into the microscopic origin of the coherent gain.