Pump dynamics of nuclear spins in GaAs nanostructures

Photons carry angular momentum, which must be transferred to the material when they are absorbed. This conservation law can be used to polarize the spins of charge carriers in semiconductors by irradiating the material with circularly polarized light. Part of the electrons' spin polarization is transferred via Fermi contact hyperfine interaction to the nuclear spins of the material. This can be used to increase the sensitivity of nuclear magnetic resonance by many orders of magnitude or to reduce decoherence in spin-based quantum computers working on the basis of semiconductor nanostructures. We explore the dynamics of the optical pumping process when a cw laser beam is applied to a GaAs/AlGaAs heterostructure in the presence of a magnetic field. We resonantly create electron-hole pairs in single quantum wells and measure the build-up of the nuclear spin polarization as a function of time through the effect of the average hyperfine interaction on the electron spins (the 'nuclear field'). This feedback of the nuclear spin polarization on the electron spins results in a nonlinear dynamics of the coupled electron-nuclear spin system. We model these dynamics and compare the result with experimental data.