Ultrafast coherent carrier control in quantum wells

A fast scanning ultrafast phase-dependent reflectivity technique is used to measure the coherent properties of multiple quantum wells at carrier densities below 108 cm (exp -2). Signal to noise ratios in the reflected intensity >80 dB can be achieved allowing underlying coherences to be measured at time delays beyond 30 ps. An unexpected 1.5 ps rise time of the coherence is resolved before a non-exponential decay and ascribed to interference of polaritons. By time-resolving the pair of pulses reflected off the sample, a complete history of energy flow in the quantum wells can be identified. Both spontaneous and stimulated coherent reemission are observed in good agreement with models based on the Bloch equations.