COHERENT CONTROL OF EXCITON DENSITY AND SPIN

We demonstrate femtosecond coherent control of excitons in quantum wel ls with phase-locked pairs of 100 fs infrared pulses. Copolarized pump pulses allow coherent control of exciton density and coherent destruc tion of excitons within a few hundred femtoseconds of their creation. This technique thus promises to avoid speed penalties in devices assoc iated 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 spi n are determined, respectively, from the differential reflection and f rom the Faraday rotation of a third probe pulse. The experimental resu lts are in good agreement with calculations based on the semiconductor Bloch equations.