Spin noise of conduction electrons in n-type bulk GaAs

We report a comprehensive study of stochastic electron spin fluctuations—spin noise—in lightly doped (n-type) bulk GaAs, which are measured using sensitive optical magnetometry based on off-resonant Faraday rotation. Frequency spectra of electron spin noise are studied as a function of electron density, magnetic field, temperature, probe-laser wavelength and intensity, and interaction volume. Electron spin lifetimes τ_s are inferred from the width of the spin noise spectra and are compared to direct measurements of τ_s using conventional Hanle-effect methods. Both methods reveal a strong and similar dependence of τ_s on the wavelength and intensity of the probe laser, highlighting the undesired influence of sub-bandgap absorption effects on the nominally “nonperturbative” spin noise measurements. As a function of temperature, the spin noise power increases approximately linearly from 1.5 to 30 K, as expected for degenerate electrons obeying Fermi-Dirac statistics, but with an additional zero-temperature offset. Finally, as the cross-sectional area of the probe laser shrinks and fewer electrons are probed, the measured Faraday rotation fluctuations due to electron spin noise are shown to increase, as expected.