Room temperature hyperpolarization of ¹³C in diamond at 3.34 T

Electron and nuclear spins in diamond have long coherence and relaxation times at room temperature, making them a promising platform for applications such as biomedical and molecular imaging, nanoscale magnetic field sensing and diverse quantum technologies.  While the optically-active nitrogen-vacancy (NV) defect has received a great deal of attention, the substitutional nitrogen (or P1) center also exhibits long coherence and relaxation times.  Here, we use microwave-induced dynamic nuclear polarization (DNP) of the P1 centers to enhance the NMR signals of natural abundance ¹³C nuclei at 3.34 T at room temperature.  We observe a greater than 100-fold enhancement of the ¹³C NMR signal with W-band (~94 GHz) millimeter-wave excitation. The DNP spectrum (enhancement as a function of millimeter-wave frequency) shows features that broadly correlate with the electron paramagnetic resonance spectrum.  The shape of the DNP spectrum indicates that multiple physical mechanisms can give rise to DNP in diamond under these conditions including the Overhauser effect, the solid effect and the cross effect.    We also investigate DNP under optical excitation of adjacent NV centers and two-tone microwave excitation.