Experimental Measurements of Optically-pumped NMR (OPNMR) and Spin Polarization in Bulk GaAs and GaAs/AlGaAs Quantum Wells

摘要

Optically-pumped nuclear magnetic resonance (OPNMR) is a measurement scheme that utilizes optical pumping of conduction electrons within a semiconductor to polarize systems of nuclear spins to which they are coupled. The spectroscopic power of NMR techniques is brought to bear on these rare spin systems through enhancement of the nuclear spin polarization, here in direct-gap semiconductors such as bulk semi-insulating GaAs and GaAs/AlGaAs quantum wells. The nuclear spins act as reporters of the electron spins that are oriented during optical pumping with circularly polarized laser light, at specific photon energies. The effects of penetration depth of the laser in the sample can be understood when irradiating at energies less than the bandgap energy, as well as details of coupling to interband transitions originating from Landau levels at photon energies greater than the bandgap energy. We show that OPNMR is particularly sensitive to the sign of magnetization that results from light hole-to-conduction band transitions because the sign of magnetization is reversed when the light hole states in the valence band are accessed.