We report coherent overtone Rabi oscillations of optically-polarized triplet electron spins and nuclear hyperpolarization in powder samples at room temperature. The strong dependence of the single-quantum resonance on the orientation of the zero-field splitting (ZFS) interaction is overcome by coherently driving the significantly narrower overtone transition. Analytical formulas for the overtone lineshape and nutation functions for axially symmetric ZFS interactions are derived. Overtone Rabi oscillations are observed in pentacene-doped \textit{p}-terphenyl and NV− centers in microdiamonds. For the former, overtone triplet dynamic nuclear polarization using the integrated solid effect leads to 1H spin polarization of 0.183±0.005\% at a magnetic field of 0.2~T. The 1H NMR polarization is enhanced by a factor of 2600 with respect to thermal equilibrium and reaches a large portion of the randomly oriented microcrystals.
@article{arxiv.2506.10498,
title = {Overtone Rabi oscillation of optically polarized triplet electron spins and nuclear hyperpolarization in powder},
author = {Koichiro Miyanishi and Takuya F. Segawa and Makoto Negoro and Akinori Kagawa and Kazuyuki Takeda},
journal= {arXiv preprint arXiv:2506.10498},
year = {2025}
}