Optical Dynamic Nuclear Polarization of (13)C Spins in Diamond at a Low Field with Multi-Tone Microwave Irradiation

Majority of dynamic nuclear polarization (DNP) experiments have been requiring helium cryogenics and strong magnetic fields for a high degree of nuclear polarization. In this work, we instead demonstrate an optical hyperpolarization of naturally abundant [Formula: see text] C nuclei in a diamond crystal at a low magnetic field and the room temperature. It exploits continuous laser irradiation for polarizing electronic spins of nitrogen vacancy centers and microwave irradiation for transferring the electronic polarization to [Formula: see text] C nuclear spins. We have studied the dependence of [Formula: see text] C polarization on laser and microwave powers. For the first time, a triplet structure corresponding to the [Formula: see text] N hyperfine splitting has been observed in the [Formula: see text] C polarization spectrum. By simultaneously exciting three microwave frequencies at the peaks of the triplet, we have achieved [Formula: see text] C bulk polarization of 0.113 %, leading to an enhancement of 90,000 over the thermal polarization at 17.6 mT. We believe that the multi-tone irradiation can be extended to further enhance the [Formula: see text] C polarization at a low magnetic field.