The small thermal polarization of nuclear spins currently limits the capabilities of nuclear spin based technologies such as nuclear magnetic resonance spectroscopy (NMR) and magnetic resonance imaging (MRI). Existing techniques for polarizing nuclear spins beyond their thermal equilibrium, called dynamic nuclear polarization (DNP), utilize cryogenic temperatures and expensive microwave technologies to transfer the larger thermal polarization of electron spins to targeted nuclei. Ubiquitous access to polarized nuclei through a room temperature, microwave-free alternative to DNP would revolutionize the capabilities of NMR and MRI.
Optically pumping nitrogen-vacancy (NV) defects in diamond can generate room temperature, microwave-free 13C nuclear polarization at the high magnetic fields used in NMR (7.05T, 9.4T). The mechanism of NV center electronic polarization is well understood, and 13C polarization has been observed, but the mechanism for polarization transfer from NV to 13C remains unknown. Here we present NMR and EPR results characterizing the polarization dependence of 13C and NV in diamond, as well as a quantum mechanical model describing a possible polarization transfer mechanism.
The sign and magnitude of the 13C polarization sensitively depends on the orientation of the diamond with respect to the directions of the applied magnetic field and laser polarization. The polarization magnitude further depends on the defect concentrations, magnitude of the applied magnetic field, temperature, and the illumination conditions: wavelength, power, and exposure time.
To better understand the source of polarization, the NV defects were characterized with EPR to determine relaxation times, concentrations, and homogeneity. EPR was also used to determine the orientation dependence of NV polarization. The NV polarization is constant in the defect frame, which, when rotated into the laboratory frame, results in highest polarization when aligned with the field, zero polarization at 54 degrees, and inverted polarization at higher angles. These EPR insights into the NV physics were incorporated into models for 13C polarization mechanisms.
Dipolar coupled pairs of NV centers are proposed as the source for 13C polarization in NV- diamonds at high magnetic fields. Our model shows these dipolar-coupled manifolds have transitions matching the frequency of the 13C nuclei, making them a feasible source of spontaneous polarization transfer. The model also qualitatively captures the observed polarization sign changes as a function of crystal orientation.