Resonant spin amplification in intrinsic bulk germanium: Evidence for electron spin lifetimes exceeding 50 ns

Abstract

Recent experiments have revealed the possibility of optically orienting electron spins in bulk germanium via indirect optical transitions. However, the temporal limitations to both the spin lifetime and the coherence of photogenerated electrons have remained unexplored so far. Here we demonstrate resonant spin amplification in intrinsic bulk germanium using a 90 MHz femtosecond pulse train at 0.8 eV central photon energy. Most importantly, we find remarkably long spin lifetimes exceeding 50 ns at temperatures of up to 60 K, limited by Elliott-Yafet type processes. Consistent with model simulations we also find pronounced signatures of the $g$-factor anisotropy in germanium in the resonant spin amplification data.