Resonant Spin Amplification in $\mathit{n}$-Type GaAs

Extended electron spin precession in $n$-type GaAs bulk semiconductors is directly observed by femtosecond time-resolved Faraday rotation in the Voigt geometry. Synchronous optical pumping of the spin system amplifies and sustains spin motion, exposing a regime where spin lifetimes increase tenfold at low fields and exceed 100 ns at zero field. Precise studies in field and temperature provide clues to the relevant electron relaxation mechanisms, indicating a strong dependence on doping concentration.