Coherent control of exciton density and spin

We demonstrate femtosecond coherent control of excitons in quantum wells with phase-locked pairs of 100 fs infrared pulses. Copolarized pump pulses allow coherent control of exciton density and coherent destruction of excitons within a few hundred femtoseconds of their creation. This technique thus promises to avoid speed penalties in devices associated with long-lived persistent carrier populations. Cross-polarized pump pulses allow coherent control of spin dynamics and conversion of unpolarized excitons into spin polarized ones. Carrier density and spin are determined, respectively, from the differential reflection and from the Faraday rotation of a third probe pulse. The experimental results are in good agreement with calculations based on the semiconductor Bloch equations.