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 ${10}^{10}{\mathrm{cm}}^{-2}$). Under continuous-wave excitation, we reach a steady-state accumulated spin density of about ${10}^8{\mathrm{cm}}^{-2}$. Using a two-color Hanle-MOKE technique, we find a spin-relaxation time of $34\mathrm{ns}$ in the nearly unperturbed electron gas. Independent variation of the pump and probe energies demonstrates the presence of additional electrons in the quantum well, whose spin-relaxation time is substantially shorter.