All-optical hyperpolarization of electron and nuclear spins in diamond

Low thermal polarization of nuclear spins is a primary sensitivity limitation for nuclear magnetic resonance. Here we demonstrate optically pumped (microwave-free) nuclear spin polarization of ${}^{13}\mathrm{C}$ and ${}^{15}\mathrm{N}$ in ${}^{15}\mathrm{N}$-doped diamond. ${}^{15}\mathrm{N}$ polarization enhancements up to $-2000$ above thermal equilibrium are observed in the paramagnetic system ${{\mathrm{N}}_{\mathrm{s}}}^0$ . Nuclear spin polarization is shown to diffuse to bulk ${}^{13}\mathrm{C}$ with NMR enhancements of $-200$ at room temperature and $-500$ at 240 K, enabling a route to microwave-free high-sensitivity NMR study of biological samples in ambient conditions.

Figure 1

(a) Atomic structures of ${\mathrm{N}}_{\mathrm{s}}$ (left), NV, and ${\mathrm{N}}_3\mathrm{V}$ . In all cases, the unpaired electron probability density is localized primarily in the carbon orbitals (gray). (b) The uv-vis absorption spectrum of the sample at 80 K. (c) Photograph of the sample used for this study. Nitrogen inhomogeneity is evident by the variation in yellow color saturation in the different growth sectors. Counterintuitively, the highest concentration of ${\mathrm{N}}_3\mathrm{V}$ is found in the clear sector: this is because the level of nitrogen aggregation is highest in the high-nitrogen sector, leading directly to a reduction of the yellow color.

Figure 2

(a) EPR spectra collected without (top) and with illumination by 80 mW of light at 532 nm (2.33 eV) with the sample at 85 K and the external magnetic field $B_0\parallel |\langle 111\rangle$. The two visible systems are ${}^{15}\mathrm{N}_3{\mathrm{V}}^0$ (shaded) and ${}^{15}\mathrm{N}_{\mathrm{s}}^0$ (all other lines, nitrogen hyperfine transitions numbered): inversion of the lines under illumination indicates electron spin polarization, and the change in relative intensity of different lines is due to nuclear spin polarization. The panel highlights nuclear polarization of ${}^{15}\mathrm{N}_3{\mathrm{V}}^0$ and ${}^{13}\mathrm{C}$ coupled to ${}^{15}\mathrm{N}_{\mathrm{s}}^0$ . (b) EPR spectra along three high-symmetry directions under illumination from 70 mW of 532 nm light at a sample temperature of 85 K.

Figure 3

(a) Dependence of EPR enhancement $\eta$ on laser wavelength for each of the ${}^{15}\mathrm{N}_{\mathrm{s}}^0$ hyperfines at 85 K [labeled as in Fig. 2]. Measurements taken at 80 mW optical power at the sample ($10.2{\mathrm{W}\mathrm{cm}}^{-2}$). (b) EPR enhancement as a function of power at 520 nm and 50 K. (c) Buildup and decay of electron polarization at 50 K when illumination is switched on and off, respectively.

Figure 4

(a) EPR spectrum taken approximately 30 s after illumination is switched off. Field-parallel hyperfine ${}^{15}\mathrm{N}_{\mathrm{s}}^0$ transitions 1 and 4 correspond to $|m_I\rangle =-1/2$ and $+1/2$, respectively (transitions given in the figure in the basis $|m_S{m}_I\rangle$): intensity difference is due to ${}^{15}\mathrm{N}$ nuclear polarization. Dotted line indicates equilibrium intensity of transitions 1 and 4. (b) Nuclear polarization of field-parallel ${}^{15}\mathrm{N}_{\mathrm{s}}^0$ hyperfines 1 and 4 as a function of time after optical excitation removed: equilibrium is reached with a characteristic lifetime of 31(1) min at 50 K. A nuclear polarization of ${\epsilon }_{{}^{15}\mathrm{N}}\approx -2000$ over thermal equilibrium is observed. Hyperfines 2 and 3 equilibrate with a lifetime of 42(3) min. The data have been corrected for a slow charge-transfer process (see [43]), and they are interpreted in terms of electron migration to a population of nuclear-polarized ${\mathrm{N}}_{\mathrm{s}}^{+}$ (see the main text).

Figure 5

(a) Single shot ${}^{13}\mathrm{C}$ NMR spectra at 7.04 T (400 MHz proton frequency). Illuminated spectra were collected following illumination at 520 nm (2.38 eV); the dark spectrum was collected after 86 h at field. (b) Room-temperature bulk ${}^{13}\mathrm{C}$ polarization buildup, collected via saturation recovery using a train of saturating $\pi /2$ pulses (to destroy any polarization between each experimental shot) and illuminating for a time $\tau$ (see the inset).

Figure 6

Difference frequencies generated by the “allowed” ($\mathrm{\Delta }{m}_S=\pm 1$; $\mathrm{\Delta }{m}_I=0$) electron transitions of a ${}^{15}\mathrm{N}_{\mathrm{s}}^0-{}^{15}\mathrm{N}_3{\mathrm{V}}^0$ pair for $B\parallel \langle 111\rangle$ at (a) 0.34 (${\omega }_{{}^{13}\mathrm{C}}=3.64\mathrm{MHz}$) and (b) 7.04 T (${\omega }_{{}^{13}\mathrm{C}}=75.3\mathrm{MHz}$) with an isotropic dipolar coupling of 0.5 MHz: stronger couplings will increase the number of frequencies generated and enhance polarization transfer. ${}^{13}\mathrm{C}$ hyperfine couplings have been ignored in the model.

Figure 7

(a) Fine structure of ${\mathrm{N}}_3{\mathrm{V}}^{-}$ excited states, including the three lowest-energy triplets (ES-1) and singlets (ES-0). The higher-energy ${}^{}{A}_1$ and ${}^{}{E}_{}$ states are marked by $*$. Excited states are resonant with the local conduction-band minimum. (b) Spin-orbit (SO) coupling effects in the closest pair of ${}^3{A}_1^{*}$ and ${}^3{E}_{}^{*}$ states. Blue arrows indicate transverse spin-orbit coupling. At room temperature, phonon-induced spin-conserving transitions may average out the spin-orbit splitting of the states driven by axial spin-orbit coupling and electron spin-spin (SS) couplings. (c) Possible model for spin-polarization generation. Continuous optical excitation and relaxation causes defect pairs to oscillate between different charge and excitation states. Spin-orbit interactions generate spin polarization in the excited state of ${\mathrm{N}}_3{\mathrm{V}}^{-}$; thermal excitation out of this state produces a spin-polarized current that is captured by ${\mathrm{N}}_{\mathrm{s}}^{+}$, leading to spin-polarized ${\mathrm{N}}_{\mathrm{s}}^0$ and ${\mathrm{N}}_3{\mathrm{V}}^0$.