Optical hyperpolarization of nitrogen donor spins in bulk diamond

We report hyperpolarization of the electronic spins associated with substitutional nitrogen defects in bulk diamond crystals. Hyperpolarization is achieved by optical pumping of nitrogen vacancy centers followed by rapid cross relaxation at the energy level matching condition in a 51 mT bias field. The maximum observed donor spin polarization is $0.9\%$, corresponding to an enhancement of 25 compared to the thermal Boltzmann polarization. A further accumulation of polarization is impeded by an anomalous optical saturation effect that we attribute to charge state conversion processes. Hyperpolarized nitrogen donors may form a useful resource for increasing the efficiency of diamond-based dynamic nuclear polarization devices.

Figure 1

(a) Basic arrangement for optical hyperpolarization of nitrogen donor spins in diamond. First, nitrogen vacancy centers (${\mathrm{N}V}^{-}$, red, spin $S=1$) are optically polarized into the $m_S=0$ spin state by exposure to a green laser beam [26]. Next, the polarization is transferred to adjacent nitrogen defects (${\mathrm{N}}^0$, blue, spin $S=\frac{1}{2}$) using cross relaxation mediated by the dipolar interaction (solid arrows). Only those ${\mathrm{N}V}^{-}$ centers whose N-V symmetry axis is aligned with the external bias field (see inset) are optically pumped and contribute to cross relaxation. Finally, the polarization is rapidly distributed in the nitrogen spin bath via spin diffusion (dashed arrows). The polarization may later be transferred to target nuclear spins outside the diamond chip. This could be achieved either directly, using the integrated solid effect [27] or the Overhauser effect [28], or indirectly, via additional spin labels on the diamond surface [29] or in the analyte. (b) Spin energy level diagram indicating the matching condition between the $m_S=0\leftrightarrow m_S=-1$ transition of aligned ${\mathrm{N}V}^{-}$ spins and the $m_S=-\frac{1}{2}\leftrightarrow m_S=+\frac{1}{2}$ transition of ${\mathrm{N}}^0$ at $51\mathrm{mT}$. The matching occurs when the Zeeman field is exactly one half of the ${\mathrm{N}V}^{-}$ crystal field (dashed line). Dotted curves indicate the transitions of ${\mathrm{N}V}^{-}$ spins not aligned with the magnetic field.

Figure 2

(a) EPR spectrum at $51\mathrm{mT}$ of diamond chip A with no laser illumination. Five ${\mathrm{N}}^0$ resonances are visible, including the central transition (solid blue circle) and the four hyperfine-shifted transitions (open blue circles) associated with the $I=1$ nitrogen ${}^{14}\mathrm{N}$ nuclear spin. The ${\mathrm{N}V}^{-}$ resonance is below the detection limit. (b) EPR spectrum with $300\mathrm{mW}$ laser illumination. The $m_S=0\leftrightarrow m_S=-1$ transition of aligned ${\mathrm{N}V}^{-}$ centers appears as an additional peak at $\sim 1440\mathrm{MHz}$ (solid red circle); other ${\mathrm{N}V}^{-}$ resonances fall outside of the spectral range (see Fig. S3). A strong polarization enhancement is observed for both ${\mathrm{N}V}^{-}$ and ${\mathrm{N}}^0$. Solid lines are fits. Spectra are recorded in a single passage with a dwell time of about 1 s per point. Units have been converted from field to frequency using the free-electron $g$ factor. Additional quantities are given in Table 1.

Figure 3

Spin polarization of ${\mathrm{N}V}^{-}$ and ${\mathrm{N}}^0$ plotted as a function of laser intensity. The corresponding laser powers are $0–800\mathrm{mW}$. Open circles represent chip A and solid circles represent chip B. The maximum observed polarization enhancement is 170 for ${\mathrm{N}V}^{-}$ and 25 for ${\mathrm{N}}^0$. Solid and dashed lines represent the different kinetic models discussed in the main text: (1) No cross relaxation, $P_{{\mathrm{N}V}^{-}}=k_0/[k_0+3{\mathrm{\Gamma }}_{{\mathrm{N}V}^{-}}]$, (2) simple cross relaxation, (3) cross relaxation and ${\mathrm{N}}^0\leftrightarrow {\mathrm{N}}^{+}$ charge state conversion, and (4) cross relaxation and ${\mathrm{N}}^0\leftrightarrow {\mathrm{N}}^{+}, {\mathrm{N}V}^{-}\leftrightarrow {\mathrm{N}V}^0$ charge state conversions. An anomalous saturation effect is observed for laser intensities above ca. $10\mathrm{mW}/{\mathrm{mm}}^2$ which is not explained by the models. Model parameters are given in Table 2 and are for chip A; the slight deviation of the chip B data for the ${\mathrm{N}}^0$ panel is due to the different ${\mathrm{N}V}^{-}$ concentration. Error bars are fit errors (2 s.d.).

Figure 4

Reservoir model of the NV-$\mathrm{P}1$ system. Green bold arrows indicate the pumping process, black bold arrows indicate rapid cross relaxation, and bold boxes represent majority populations. Dashed arrows indicate charge state conversions. Solid arrows indicate the principal direction of population flow. Rate constants are explained in the text.