The figure below shows data from a modulation-doped II-VI semiconductor which contains a strained 10.5 nm ZnCdSe quantum well with Cl-doped ZnSe barriers, and is grown by a molecular beam epitaxy on a semi-insulating (100) GaAs substrate. The sample is not symetrically doped, as the quantum well is grown directly atop a 2.5 micron buffer layer and is followed by a 12 nm spacer and a 20 nmdoping layer (n~10^17 per cubic centimerter). The upper portion of the figure shows Faraday rotation data with photoluminescence data overlaid in red. Used as a measure of carrier recombination, the photoluminescence data highlights an important aspect of the system: the spin lifetime is longer than the carrier lifetime. This fact is due to the transfer of spin precession from the injected carriers to the electron gas of the semiconductor. The bottom part of the figure shows how this transition occurs.
The evolution of the electronic spin up and spin down states is shown along the pump/probe direction . Before the pump pulse arrives the states are in equilibrium. At t = 0 the pump excites carriers into one of the two spin states (dependent upon the direction of circular polarization of the pump) and leaves holes in the valence band. As these electron hole pairs recombine momentum is transferred between the electrons, ultimately leaving precessing spins in the states previously occupied by the electron gas. By the time the holes are gone the carrier population has acchieved equilibrium, but the spin population has not, allowing the spin precession to continue for nanoseconds within the electron gas.