Optical pumping in indium phosphide: 31P NMR measurements and potential for signal enhancement in biological solid state NMR

The initial results of optically-pumped, directly-detected NMR experiments on InP are reported. At low temperatures (4.2 K and above) and in a 9.39 T magnetic field, irradiation of a sample of an undoped InP wafer with 835-nm-wavelength light from a diode laser enhances the spin polarization of 31P nuclei near the sample surface in a manner that depends on the polarization of the light. The nuclear spin polarization is monitored by direct radio-frequency detection of nuclear free induction-decay signals. The maximum nuclear spin polarization (Szn> generated by optical pumping is approximately - 0.004, corresponding to a spin temperature of -0.5 K. The nuclear spin polarization may be limited in these experiments by the use of a high photon energy (1.484 eV) relative to the InP band gap (1.423 eV at low temperatures). It is proposed that optically-pumped InP may be useful as a source of enhanced nuclear spin polarizations for solid state NMR measurements on organic and biological overlayers deposited on InP substrates. Estimates are given for the magnitude of the spin polarization and the efficiency of the polarization transfer from the semiconductor substrate to the overlayer that would be required to permit solid state NMR measurements on sub-nanomole quantities of molecules in the overlayer. These estimates appear well within the range of possibility.