Stray-field NMR imaging and wavelength dependence of optically pumped nuclear spin polarization in InP

One-dimensional NMR imaging experiments with micron spatial resolution are used to investigate the penetration depth and excitation energy dependence of optical pumping in Fe-doped semi-insulating InP crystals in high magnetic field at low temperature. The depth profile of ³¹P nuclear polarization revealed by NMR imaging is consistent with previous optical absorption measurements, while the efficiency of exciting nuclear polarization is found to be a complicated function of the excitation energy. This dependence is not explained by LO phonon emission by the photoexcited carriers. The optically pumped ³¹P NMR signal exhibits a maximum intensity with excitation several meV below the band gap, but this maximum is shown to be due to the dramatically increased absorption length of below-gap radiation. The efficiency of exciting nuclear polarization drops off quickly below the gap, and is about 1/5 of its maximum value at the energy of greatest total NMR signal intensity.