Overtone Rabi oscillation of optically polarized triplet electron spins and nuclear hyperpolarization in powder

TL;DR

This paper demonstrates room-temperature coherent overtone Rabi oscillations of optically-polarized triplet electron spins in powders, enabling efficient nuclear hyperpolarization and significant enhancement of NMR signals.

Contribution

It introduces the coherent driving of overtone transitions to overcome orientation dependence in triplet electron spins, with analytical models and experimental validation in powders.

Findings

Overtone Rabi oscillations observed in pentacene-doped p-terphenyl and NV centers in microdiamonds.

Achieved 0.183% proton polarization at 0.2 T, 2600 times above thermal equilibrium.

Demonstrated large NMR signal enhancement in powder samples.

Abstract

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 $^1$H spin polarization of $0.183\pm0.005$\% at a magnetic field of 0.2~T. The $^1$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.