  The origin of helicity asymmetries in the optically-pumped NMR signal and
hyperfine shift in GaAs is derived analytically and tested experimentally. The
ratio of the optically-pumped to the equilibrium electron polarizations is a
key parameter in determining both asymmetries. Variations in asymmetry with
photon energy and laser power reflect variations in the local temperature and
the electron spin polarization, and these two quantities are extracted from the
asymmetry through a simple methodology. Other contributions to the asymmetry
are considered.
