Optical dynamic nuclear polarization of 13C spins in diamond at a low field with multi-tone microwave irradiation
Abstract
Most of dynamic nuclear polarization (DNP) has been requiring helium cryogenics and strong magnetic fields for a high degree of 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 an ambient temperature. It exploits continuous irradiations of pump laser for polarizing electron spins of nitrogen vacancy centers and microwave for transferring the induced polarization to 13C nuclear spins. Triplet structures corresponding to 14N hyperfine splitting were clearly observed in the spectrum of 13C polarization. The powers of microwave irradiation and pump laser were optimized. By simultaneously irradiating three microwave frequencies matching to the peaks of the triplet, we achieved a 13C bulk polarization of 0.113 %, leading to an enhancement of about a factor of 90,000 over the thermal polarization at 17.6 mT. We believe that the multi-tone irradiation can be universally adopted to further enhance the 13C polarization at a low magnetic field.
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