Valley-dependent spin polarization and long-lived electron spins in germanium

Spin orientation and relaxation of conduction band electrons in bulk Ge are addressed by studying the steady-state circular polarization of the indirect gap photoluminescence (PL) at low temperatures. This provides a direct experimental proof of recently predicted spin-dependent selection rules for phonon-mediated optical transitions in Ge. In addition, we observe valley-dependent circularly polarized emission, and map the concomitant redistribution of electron spins within the multi-valley conduction band of Ge by gaining simultaneous access to the circular dichroism of light emitted across the direct and the indirect gap transitions. Finally, the lifetime of L-valley electrons is measured by means of decay curves of the indirect gap PL emission, yielding spin relaxation times in the order of hundreds of ns