Optical pumping NMR in the compensated semiconductor InP:Fe

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The optical pumping NMR effect in the compensated semiconductor InP:Fe has been investigated in terms of the dependences of photon energy (Eₚ), helicity (sigma+-), and exposure time (tauₗ) of infrared lights. The 31P and 115In signal enhancements show large sigma+- asymmetries and anomalous oscillations as a function of Eₚ. We find that (i) the oscillation period as a function of Eₚ is similar for 31P and 115In and almost field independent in spite of significant reduction of the enhancement in higher fields. (ii) A characteristic time for buildup of the 31P polarization under the light exposure shows strong Eₚ-dependence, but is almost independent of sigma+-. (iii) The buildup times for 31P and 115In are of the same order (10³ s), although the spin-lattice relaxation times (T₁) are different by more than three orders of magnitude between them. The results are discussed in terms of (1) discrete energy spectra due to donor-acceptor pairs (DAPs) in compensated semiconductors, and (2) interplay between 31P and dipolar ordered indium nuclei, which are optically induced.