Optical spin pumping of modulation doped electrons probed by a two-color   Kerr rotation technique
  We report on optical spin pumping of modulation electrons in CdTe-based quantum wells with low intrinsic electron density (by 10^10 cm^{-2}). Under continuous wave excitation, we reach a steady state accumulated spin density of about 10^8 cm^{-2}. Using a two-color Hanle-MOKE technique, we find a spin relaxation time of 34 ns for the localized electrons in the nearly unperturbed electron gas. Independent variation of the pump and probe energies demonstrates the presence of additional non-localized electrons in the quantum well, whose spin relaxation time is substantially shorter. 
H. Hoffmann, G. V. Astakhov, T. Kiessling, W. Ossau, G. Karczewski, T.   Wojtowicz, J. Kossut, L. W. Molenkamp