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 $p$-terphenyl and ${\mathrm{NV}}^{-}$ centers in microdiamonds. For the former, overtone triplet dynamic nuclear polarization using the integrated solid effect leads to ${}^1\mathrm{H}$ spin polarization of $0.183\pm 0.005\%$ at a magnetic field of 0.2 T. The ${}^1\mathrm{H}$ NMR polarization is enhanced by a factor of 2600 with respect to thermal equilibrium and reaches a large portion of the randomly oriented microcrystals.

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• Nuclear & electron resonance
• Dynamic nuclear polarization
• Electron paramagnetic resonance
• Nuclear magnetic resonance
• Optical pumping