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Optical Orientation of Nuclear Spins


Nuclear spins are harnessed in many important technologies, including the well established fields of magnetic resonance imaging for medical diagnostics, magnetic resonance spectroscopy in analytical chemistry as well as emerging technologies in quantum information and spintronics. All of these technologies either harness, or are subject to, the behavior of a nuclear spin ensemble. To achieve the most desirable behavior, (large spectroscopic signal or reduction of unwanted fluctuations) the nuclear spin ensemble should be prepared in a pure quantum state. In practice, this ``polarization" is typically created by allowing the energy levels of the spins in an applied magnetic field achieve thermal equilibrium. Unfortunately, even with the largest magnets available with fields greater than 20 Tesla, the separation between energy levels is much smaller than kT for all but extreme refrigerated systems. It is then desirable to achieve pure nuclear spin states which are not at thermal equilibrium with the environment. In order to do this, it is necessary to create a situation in which a pure quantum state can be created in a system other than the nuclear spin which then interacts with the nucleus to create a more pure nuclear spin state. In this work we harness the pure photon spin state of circularly polarized light as well as spin transition selection rules of a deep electronic defect in diamond to polarize nuclei.

In the first case, we use circularly polarized photons to excite spin polarized electrons in the semiconductor gallium arsenide which equilibrate with bound electronic states at recombination centers. These bound states then polarize nearby nuclear spins through the magnetic hyperfine interaction. While this hyperfine mechanism of nuclear spin polarization was previously known, we have identified a new regime of low optical absorption where the coupling of nuclear quadrupole moments to electric field gradients near recombination centers is the dominant mechanism of nuclear spin polarization. Through a combination of experiment and theory, we determine relative rates of these two mechanisms depending on the rate of optical absorption. Since optical absorption varies as a function of depth in a sample, we predicted that control of these two mechanisms is possible as a function of position in the sample. Using the stray field of a superconducting magnet to supply the gradient field for magnetic resonance imaging, we were able to directly observe patterns of nuclear magnetization on a micron length scale. When combined with in-plane control of the laser and NMR pulse sequences, this technique will give rise to fully 3-dimensional patterns of nuclear magnetization. These patterns may be created in bulk gallium arsenide without the need for lithography or other microfabrication techniques. These regions of magnetized nuclei will enable magnetic control over drifting electrons in future spintronics devices.

The paramagnetic nitrogen-vacancy defect in diamond provides a different tool to control nuclei. The ground state spin triplet of this defect may be easily polarized into the Sz=0 state with visible optical illumination. The polarization is due to the symmetry and selection rules within the defect itself and does not require polarized photons. We discovered that, with a sufficient density of defects, the 13C nuclei in the diamond lattice are spontaneously polarized upon illumination of the sample. We attribute this polarization to a highly refrigerated ``spin temperature" among the energy levels created by the magnetic dipole interaction of the many spins in the defect ensemble. This energy reservoir is in thermal contact with the 13C nuclei, which are driven to highly athermal spin states. We theoretically investigate the thermodynamics of the defect spin ensemble, first with a two-spin ``toy model" and more recently have begun a many-spin theoretical approach. The polarization of nuclei in diamond has application in the quenching of nuclear fluctuations in quantum information systems and as a platform for signal enhancement in magnetic resonance imaging and spectroscopy.

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