Electronic states probed by optically-pumped NMR of semiconductors

PHYS 239

Sophia E. Hayes, hayes@wustl.edu, Kannan Ramaswamy, and Stacy Mui. Department of Chemistry, Washington University, 1 Brookings Dr., Box 1134, Saint Louis, MO 63130
It is possible to orient electron spins in semiconductors by irradiating them with circularly polarized light near the bandgap energy (Eg). The extent to which the electrons can be oriented depends on the details of the band structure, the optical absorption coefficient, and relaxation processes. By changing the laser wavelength, different parts of the band structure may be accessed, resulting in photon-dependent coupling between the oriented electrons and nuclear spins. Termed “optically pumped NMR (OPNMR),” these experiments exhibit both enhanced signals and hyperfine shifts. Measurements of the intensity of OPNMR signals in semiconductors have been found to depend on the photon energy. The intensity dependence of the OPNMR signals arises as a consequence of two competing factors: nuclear polarization (Iz), and the number of accessible nuclear spins. The magnitude of the hyperfine shift of the OPNMR signals reflects the probability of occupation of optically relevant defects in the semiconductor.