Coherent spin dynamics in diluted-magnetic quantum wells

Spins of charge carriers, paramagnetic centers, and nuclei in semiconductor structures are known to be oriented if circular-polarized light is shined upon the structure. This is due to transfer of angular momentum from photons to various excitations in the semiconductor. What would happen if the structure is anisotropic, so that the angular momentum is not conserved? Experiments performed on diluted-magnetic II-VI quantum wells (such as CdMnTe/CdMgTe) show that at some conditions the effect of light upon the spin state of the semiconductor structure can be greatly amplified. Exchange field of photoexcited heavy holes initiates a coherent evolution of a great number of localized spins of Mn-ions. In the simplest case, it is just a precession in an effective field, but generally the spin dynamics is more intricate. It shows up, for instance, in long sequences of spin-flip peaks in Raman spectra, in enhanced magnetic-polaron effect, and in modulation of optical response within the picosecond time scale.