Optically-pumped nuclear magnetic resonance (OPNMR) spectroscopy is an emerging technique to probe electronic
and nuclear spin properties in bulk and quantum well semiconductors. In OPNMR, one uses optical
pumping with light to create spin-polarized electrons in a semiconductor. The electron spin can be transferred
to the nuclear spin bath through the Fermi contact hyperfine interaction which can then be detected by conventional
NMR. The resulting NMR signal can be enhanced four to five orders of magnitude or more over the
thermal equilibrium signal. In previous work, we studied OPNMR in bulk GaAs where we investigated the
strength of the OPNMR signal as a function of the pump laser frequency. This allowed us to study the spin-split
valence band. Here we report on OPNMR studies in GaAs/AlGaAs quantum wells. We focus on theoretical
calculations for the average electron spin polarization at different photon energies for different values of external
magnetic field in both unstrained and strained quantum wells. Our calculations allow us to identify the Landau
level transitions which are responsible for the peaks in the photon energy dependence of the OPNMR signal
intensity. The calculations are based on the 8- band Pidgeon-Brown model generalized to include the effects
of the quantum confinement potential as well as pseudomorphic strain at the interfaces. Optical properties are
calculated within the golden rule approximation. Detailed comparison to experiment allows one to accurately
determine valence band spin splitting in the quantum wells including the effects of strain.
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.
We report on combined theoretical and experimental studies of spin-split bands in semiconductors in magnetic fields. We have studied a wide range of systems including: 1) electron and valence band splitting in dilute magnetically doped semiconductors (DMS) systems like InMnAs, 2) electron and valence band splitting in strained InSb/AlInSb heterostructures and 3) valence band splitting in GaAs. The systems have been studied with a variety of experimental techniques including: i) ultra-high magnetic field cyclotron resonance ii) magnetoabsorption and iii) optically pumped NMR (OPNMR). Calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the quantum confinement potential as well as pseudomorphic strain at the interfaces and sp-d coupling between magnetic impurities and conduction band electrons and valence band holes. Optical properties are calculated within the golden rule approximation and compared with experiments. Detailed comparison to experiment allows one to accurately determine conduction and valence band parameters including effective masses and g-factors. Results for InMnAs show shifts in the cyclotron resonance peaks with Mn doping. For InSb, we find a sensitive dependence of the elecronic structure on the strain at the pseudomorphic interfaces. For GaAs, we show that OPNMR allows us to spin-resolve the valence bands and that structure in the OPNMR signal is dominated by the weaker light hole to conduction band Landau level transitions.
In this work we investigate carrier dynamics of narrow gap ferromagnetic alloys grown by MOVPE. We determine the intraband and interband relaxation times in these material systems where the samples are excited with photon energies above the band gap of InMnAs and InMnSb films. Our results are important for understanding the electronic states and the relaxation mechanisms in these ferromagnetic materials.