Probing Quantum Confinement at the Atomic Scale with Optically Detected Nuclear Magnetic Resonance

Abstract

Near-band-gap circularly polarized excitation in III-V semiconductors provides spin-polarized electrons that transfer spin order to lattice nuclei via fluctuations in the contact hyperfine interaction. This process of optical nuclear polarization and the complementary technique of optical detection of nuclear magnetic resonance (NMR) provide extreme sensitivity enhancement and spatial selectivity in structured samples, enabling collection of NMR spectra from samples such as single quantum wells or dots containing as few as ~10^5 nuclei. Combining these advances with novel techniques for high spectral resolution, we have probed quantum-confined electronic states near the interface of a single epitaxially grown Al(1-x)Ga(x)As/GaAs (x = 0.36) heterojunction. Using a novel strategy that we refer to as POWER (perturbations observed with enhanced resolution) NMR, multiple-pulse time suspension is synchronized with bandgap optical irradiation to reveal spectra of effective spin Hamiltonians that are differences between those of the occupied and unoccupied photoexcited electronic state. The underlying NMR linewidth is reduced by three orders of magnitude in these experiments, enabling resolution of an asymmetric line shape due to light-induced hyperfine interactions. The results are successfully fit with the coherent nuclear spin evolution and relaxation theoretically expected for sites distributed over the volume of an electronic excitation weakly localized at a point defect. This analysis establishes a one-to-one relationship, which can be used to follow nuclear spin diffusion, between optical Knight shift and the radial position of lattice nuclei. We have also introduced POWER NMR techniques to characterize the change in electric field associated with cycling from light-on to light-off states via a linear quadrupole Stark effect (LQSE) of the nuclear spins. Simulations of these NMR spectra in terms of the radial electric fields of either donor-bound electrons or excitons indicate differences, where the bound-exciton model provides a significantly better fit to the data. The same spin physics enabled our measurement of the heterojunction interfacial field, which we find to be less than 1.3 kV/cm at the sites responsible for optical NMR. Other simulations show the promise of optical NMR as a tool in future studies aimed at atomic-level characterization of quantum-confined systems such as quantum dots and wells