Ca. Michal et R. Tycko, Stray-field NMR imaging and wavelength dependence of optically pumped nuclear spin polarization in InP, PHYS REV B, 60(12), 1999, pp. 8672-8679
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 magneti
c held at low temperature. The depth profile of P-31 nuclear polarization r
evealed by NMR imaging is consistent with previous optical absorption measu
rements, 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 P-31 NMR signal exhibits a maximum intensity with excitation severa
l meV below the band gap, but this maximum is shown to be due to the dramat
ically increased absorption length of below-gap radiation. The efficiency o
f exciting nuclear polarization drops off quickly below the gap, and is abo
ut 1/5 of its maximum value at the energy of greatest total NMR signal inte
nsity.