ULTRAFAST COHERENT CARRIER CONTROL IN QUANTUM-WELLS

A fast scanning ultrafast phase-dependent reflectivity technique is us ed to measure the coherent properties of multiple quantum wells at car rier densities below 10(8) cm(-2). Signal to noise ratios in the refle cted intensity >80 dB can be achieved allowing underlying coherences t o be measured at time delays beyond 30 ps. An unexpected 1.5 ps rise t ime of the coherence is resolved before a nonexponential decay and asc ribed to interference of polaritons. By time-resolving the pair of pul ses reflected off the sample, a complete history of energy flow in the quantum wells can be identified. Both spontaneous and stimulated cohe rent reemission are observed in good agreement with models based on th e Bloch equations.