Abstract

Cohernt phenomena in semiconductors play a crucial role on time scales faster than the dephasing time. The relative phase in a sequence of laser pulses then strongly influences the dynamics. For example, experiments have demonstrated that excitons in quantum wells created by a first laser pulse can be destroyed again with a second phase-locked pulse when it interferes destructively with the coherent excitons from the first pulse. ¹ This interference requires the same polarization orientation of both pulses. Here we consider the case that quantum wells are excited with a phase-locked Pair of perpendicularly polarized pulses as shown in Fig. 1. Such pulses do not interfere directly, even if they overlap in time. Their relative phase therefore does not influence the total exciton density. However, the pulses can produce phase dependent circularly polarized light, inducing interference effects in the angular momentum l_Z and in the spin polarization of the excitons. This net angular momentum results in different reflectivities of positive and negative circularly polarized light, causing Faraday rotation of a linearly polarized reflected probe pulse.² Coherent control of the excitonic angular momentum is also possible with non overlapping pulses when the second pulse interacts with the coherent excitons from the first pulse.

© 1996 Optical Society of America