Nuclear Spin Polarization Transfer with a Single Radio-Frequency Field in Optically Pumped Indium Phosphide

We describe a novel spin polarization transfer phenomenon observed in high-field, optically pumped NMR experiments on single-crystal indium phosphide. Polarization transfer from ¹¹⁵In spins to ³¹P spins occurs when a weak radio-frequency field is applied at the ³¹P NMR frequency. Unlike other known high-field polarization transfer effects, no rf field near the ¹¹⁵In frequency is required. We present evidence that the ³¹P polarization arises from a state of optically pumped dipolar order in the ¹¹⁵In spin system and propose two mechanisms that may contribute to the creation of this state.