Optical pumping of dipolar order in a coupled nuclear spin system

A possible mechanism for the direct generation of dipolar order in a couple d nuclear spin system by optical pumping is investigated theoretically. Thi s work is motivated by recent optically pumped nuclear magnetic resonance e xperiments on single-crystal indium phosphide wafers, which have revealed t hat optical pumping can generate a state of nuclear spin dipolar order in a system of coupled spins without the application of radiofrequency fields o r pulse sequences. The theory proceeds by evaluating the steady-state popul ations of the eigenstates of a coupled nuclear spin system when simultaneou s nuclear and electron spin transitions are driven by fluctuating hyperfine couplings of the nuclei to an electron with a non-equilibrium spin polariz ation. In the case of two coupled spin-1/2 nuclei, substantial dipolar orde r results when the hyperfine couplings are of the dipolar, but not the Ferm i contact, type.