Research ArticleCHEMICAL PHYSICS

Orientation-independent room temperature optical 13C hyperpolarization in powdered diamond

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Science Advances  18 May 2018:
Vol. 4, no. 5, eaar5492
DOI: 10.1126/sciadv.aar5492
  • Fig. 1 Experiment overview.

    (A) Polarization transfer from optically pumped NV centers to 13C in diamond particles is carried out by microwave irradiation at low field (Bpol ~ 1 to 30 mT), after which the sample is shuttled rapidly for bulk inductive readout at 7 T. We quantify the polarization enhancement with respect to the thermal signal at 7 T. (B) Polarization transfer protocol: Laser light (532 nm) is continuously applied along with swept microwave (MW) irradiation across the NV center spectrum at Bpol to hyperpolarize the 13C nuclei. The sweep time per unit bandwidth is 20 ms/MHz. (C) Envisioned nanodiamond polarizer: Optically hyperpolarized 13C diamond nuclei relay polarization to 13C spins in a frozen liquid by spin diffusion, aided by the intrinsically large surface area of nanoparticles. Subsequent rapid thaw would allow enhanced NMR detection with chemical shift resolution.

  • Fig. 2 Optical hyperpolarization in diamond microparticles.

    Hyperpolarization experiments were performed on dry 200-μm particles with 1.1% natural-abundance 13C. Solid fit lines are depicted over data points. (A) Signal gain by DNP under optimized conditions. Blue line shows the 13C NMR signal due to Boltzmann polarization at 7 T, averaged 120 times over 7 hours. Red line is a single-shot DNP signal obtained with 60 s of optical pumping, enhanced 277 times over the 7-T thermal signal (enhanced 149,153 times at Bpol = 13 mT). The signals have their noise unit-normalized for clarity. Hyperpolarization thus leads to more than five orders of magnitude gains in averaging time (inset). (B) Buildup curve showing rapid growth of bulk 13C polarization. Slow rise at longer times is due to 13C spin diffusion. (C) Hyperpolarization sign is controlled by MW sweep direction across the NV center powder pattern. Continuous family of sweeps demonstrating the idea, with extremal points representing low-to-high frequency MW sweeps and vice versa. Time t is the period of the high-to-low frequency component in one cycle of total period T. Inset: 13C signal undergoes near-perfect sign inversion upon reversal of the sweep direction. Sweeping in a symmetric fashion leads to net cancellation and no buildup of hyperpolarization. (D) Sweep rate dependence of the signal enhancement. The sweep bandwidth is 570 MHz, and the excitation laser power is ≈5 mW/mm2. The solid line is the result of a fit using the expression in the main text; we find k = 18.4 MHz/ms and Λ= 30 kHz for a Rabi field Ω =0.35 MHz. Inset: Dependence of 13C NMR signal as a function of the MW Rabi frequency. Here, the solid line serves as a visual guide. a.u., arbitrary units.

  • Fig. 3 Proposed mechanism of polarization transfer.

    (A) Energy levels of an NV electron spin hyperfine coupled to a 13C nuclear spin. Δ denotes the NV zero-field splitting, γe is the electron gyromagnetic ratio, and B (assumed much smaller than Δ) is the external magnetic field forming an angle θ with the NV axis. The quantum numbers in all kets refer to electron and nuclear spins, in that order; the notation for the nuclear spin states highlights the manifold-dependent quantization axis, in general different from the magnetic field direction. (B) Calculated energy diagram in the rotating frame corresponding to the mS = 0 ↔ mS = −1 subset of transitions [dashed rectangle in (A)] assuming a hyperfine coupling Azz = +0.5 MHz. (C) Same as in (B) but for Azz = −0.5 MHz. In (B) and (C), we assume that B = 10 mT and θ = 45° and use a transverse hyperfine constant Azx = 0.3|Azz|. Colored solid circles denote populations at different stages during a sweep in the direction of the arrow, and faint dashed circles indicate the narrower avoided crossings where population transfer takes place. LZ, Landau-Zener.

  • Fig. 4 Contributions of different NV orientations to DNP.

    (A to C) Electronic powder pattern mapped by performing DNP in a 100-MHz window, which is swept across in frequency space. This reports on the contributions of each window to the resulting signal and different orientations’ relative contribution to DNP at different magnetic fields. Note that amplifier bandwidth limitations lead to an artificial cutoff at ≈3.2 GHz. (D) Sign contributions from different NV orientations. (a and b) Every part of the powder pattern, even if corresponding to different (ms = ±1) electronic states, produces the same sign of hyperpolarization (shaded regions) that only depends on the direction of MW sweep. Solid lines are smoothened curves. Keeping the lower frequency (c) and upper frequency (d) of the DNP window fixed provides the cumulative contribution of different parts of the electronic spectrum to the polarization buildup. It shows that half of the powder pattern is sufficient to saturate the polarization enhancement. Note that we maintain the same differential sweep rate per unit spectral width equivalent to 40 MHz/ms in all experiments.

  • Fig. 5 Minimum number of particles detectable.

    (A) Single-shot SNR scaling with the number of Element6 200-μm diamond microparticles in the sample tube. The experiments are performed by careful particle counting and averaging more than 30 single-shot measurements for each accumulated collection of particles. By extrapolation, we determine that it is possible to obtain signal from a single particle with a single-shot SNR of 0.994. (B) Average of 30 single-shot signals from 20 particles. (C) SEM micrograph (Hitachi S5000) of individual Element6 high pressure–high temperature diamond particles used in majority of the experiments. The particles have a uniform size distribution (edge length, 87 ± 3.9 μm) and a truncated octahedral shape set by particle growth conditions.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar5492/DC1

    section S1. Methods and materials

    section S2. Indirect NV spectroscopy by 13C DNP

    section S3. Mechanism for orientation-independent polarization transfer

    section S4. Experimental design

    section S5. DNP electronics setup

    section S6. DNP optics setup

    section S7. Shot-to-shot variation of enhancement

    section S8. Polarization loss due to shuttling

    section S9. Data processing

    fig. S1. Enhanced 13C DNP for particles in solution.

    fig. S2. Electron powder pattern measured via 13C DNP.

    fig. S3. Single-crystal electronic lineshape measured via 13C DNP.

    fig. S4. Simulations of DNP enhancement.

    fig. S5. Detail of experimental setup schematically described in Fig. 1A.

    fig. S6. Low-field DNP setup.

    fig. S7. Schematic circuit for DNP excitation.

    fig. S8. Sweep rate dependence on laser power.

    fig. S9. DNP enhancement spread due to orientational shaking.

    References (3641)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Methods and materials
    • section S2. Indirect NV spectroscopy by 13C DNP
    • section S3. Mechanism for orientation-independent polarization transfer
    • section S4. Experimental design
    • section S5. DNP electronics setup
    • section S6. DNP optics setup
    • section S7. Shot-to-shot variation of enhancement
    • section S8. Polarization loss due to shuttling
    • section S9. Data processing
    • fig. S1. Enhanced 13C DNP for particles in solution.
    • fig. S2. Electron powder pattern measured via 13C DNP
    • fig. S3. Single-crystal electronic lineshape measured via 13C DNP.
    • fig. S4. Simulations of DNP enhancement.
    • fig. S5. Detail of experimental setup schematically described in Fig. 1A.
    • fig. S6. Low-field DNP setup.
    • fig. S7. Schematic circuit for DNP excitation.
    • fig. S8. Sweep rate dependence on laser power.
    • fig. S9. DNP enhancement spread due to orientational shaking.
    • References (36–41)

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