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

Abstract

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 13C 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 13C nuclear spins. We have studied the dependence of 13C polarization on laser and microwave powers. For the first time, a triplet structure corresponding to the 14N hyperfine splitting has been observed in the 13C polarization spectrum. By simultaneously exciting three microwave frequencies at the peaks of the triplet, we have achieved 13C 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 13C polarization at a low magnetic field.

Nanomaterials for hyperpolarized nuclear magnetic resonance and magnetic resonance imaging

“…Here, T 1 was found to depend strongly on B 0 and different means to prolong T 1 were suggested (Ajoy et al, 2019). Most recently, it was found that using microwaves with three different wavelengths (matching the resonance frequency of the hyperfine interactions between the 15 N and the electron) improved the polarization, yielding to an increase of 13 C bulk polarization from 0.068% to 0.113% (Kavtanyuk et al, 2022). While the optical detection of NV centers is sensitive, the achieved polarization levels still remain significantly below what 1 H, 13 C, 15 N, or 129 Xe spin systems can provide for inductive detection.…”