Optical Tweezer Phonon Laser

Optical spin pumping of color centers promises to revolutionize the practice of nuclear magnetic resonance via efficient dynamic nuclear polarization (DNP) protocols. Significant progress has been attained with systems such as the nitrogen-vacancy (NV) center in diamond, but extreme spectral broadening and nuclear spin-lattice relaxation from co-existing paramagnetic defects are hurdles still difficult to circumvent. The technique we present in this manuscript addresses both issues through the implementation of magnetic-field-sweep-induced NV cross-relaxation with ancilla point defects, here acting as proxies to mediate the transfer of polarization to the nuclear spin ensemble. The governing spin dynamics is robust to field misalignment and ideally suited for shallow NVs, indirectly coupled to outside nuclei through paramagnetic defects at the diamond surface.

Optically-pumped color centers in semiconductor powders can potentially induce high levels of nuclear spin polarization in surrounding solids or fluids at or near ambient conditions, but complications stemming from the random orientation of the particles and the presence of unpolarized paramagnetic defects hinder the flow of polarization beyond the defect’s host material. Here, we theoretically study the spin dynamics of interacting nitrogen-vacancy (NV) and substitutional nitrogen (P1) centers in diamond to show that outside protons spin-polarize efficiently upon a magnetic field sweep across the NV–P1 level anti-crossing. The process can be interpreted in terms of an NV–P1 spin ratchet, whose handedness —and hence the sign of the resulting nuclear polarization — depends on the relative timing of the optical excitation pulse. Further, we find that the polarization transfer mechanism is robust to NV misalignment relative to the external magnetic field, and efficient over a broad range of electron-electron and electron-nuclear spin couplings, even if proxy spins feature short coherence or spin-lattice relaxation times. Therefore, these results pave the route towards the dynamic nuclear polarization of arbitrary spin targets brought in proximity with a diamond powder under ambient conditions.