The Magnetic Field Dependence of the Light hole Transition and Mixing in a GaAs/AlGaAs Quantum Well from Optically Pumped NMR

We present a theoretical interpretation of OPNMR (Optically Pumped NMR) measurements in undoped, lattice-matched Al_0.31Ga_0.69As/GaAs MQWs grown on (001) GaAs substrate subjected to transverse static magnetic field (B) varying between 1 to 12 T. An 8-band k.p band structure calculation is performed for the energy eigenvalues and band mixing probabilities using a finite-difference method. Using Fermi’s golden rule, inter-band absorption coefficients and conduction band spin-polarization are calculated as a function of photon energy for both σ+ and σ- optical helicities. The calculated band structure and transition probabilities aid in the understanding the origin of the OPNMR profile. We show that the strongest light hole transition for σ- absorption occurs at energies higher than the energies for transitions for σ+ absorption. The curvature seen in the B-dependence for the σ+ OPNMR profile is attributed to the strong Landau level mixing of the LH↓ (light hole down) with the HH↓ (heavy hole down) states. For the σ- absorption, a strong mixing of the LH↑ (light hole up) with the HH↓ states is seen in the B<6T regime. Finally, we discuss additional effects including excitons and self-consistent electric field that can improve the accuracy of the calculations with the experiments.