Helicity asymmetry of optically pumped NMR spectra in GaAs

Abstract

The origin of σ± 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.PACS numbers: 71.55.Eq,82.56Na Optical polarization of nuclear spins in semiconductors has sparked high sensitivity NMR studies of the quantum Hall regime, 1 spatial mappings of lattice strain, 2 and proposals for biological polymers.3 In the optically-enhanced or optically-pumped NMR (OPNMR) spectrum, σ± asymmetry has been noted in both the hyperfine-induced frequency shift 4,5,6 and the NMR signal magnitude, 6,7,8,9 as shown in Fig. 1A. Variations in the asymmetry with photon energy 6,8,9 and laser power (shown herein) have not yet been explained, even for the most wellstudied semiconductor, GaAs. Varying values of τ /T 1e , where τ and T 1e are the electron recombination and spinrelaxation times, were recently invoked to explain variations in signal magnitude asymmetry in GaAs.8 It is shown herein that the standard OPNMR model for GaAs predicts that τ /T 1e only affects the hyperfine shift asymmetry.It is often assumed that the initially-excited electron spin polarization ( S z 0 ≡ S 0 ) with σ± light in bulk semiconductors is constant (±50% for cubic crystals) for photon energies E in the range E g < E < E g + ∆ (where E g is the band-gap energy and ∆ is the spinorbit splitting), 10 and there is very little literature on optical electron spin orientation with E < E g . By dropping the assumption that S 0 is a constant, we find an explanation for the varying asymmetry of OPNMR spectra. It furthermore follows that this asymmetry is related so simply to electron spin parameters that it can be used to measure the electron spin polarization. Thus, this article outlines a "spinometry" method that, because the hole spin-nuclear spin interaction is very weak, probes the electron spin polarization under optical pumping.It was shown previously that the OPNMR signal in GaAs strongly correlates with the photoconductivity, and that the excitation spectrum of OPNMR signals can be largely understood from a simple picture in which optical absorption generates free electrons which then bind to shallow donors, analogous to the way that a gas adsorbs to a solid surface with a fixed number of sites. 5Rapid spin exchange 11 maintains the steady-state polarizations of the free and donor-bound electron reservoirs to be equal and thus given by a single equation:5,12,13,14where S eq is the Boltzmann electron polarization. Once bound to shallow donors, the electrons experience a strong hyperfine interaction and can undergo mutual spin flips with nearby nuclear spins. The dimensionless nuclea...

Optically pumped InP: Nuclear polarization from NMR frequency shifts

“…As pointed out in Ref. 16, if there is no helicity dependence to the diffusion constant D, the electron's lifetime τ , or J z eq , the frequency shifts observed with the two different helicities ∆ν σ± , can be used to determine the electron's polarization. It is clear, however, for Fe-doped InP, the assumptions of helicity independence for the above quantities are not correct.…”

“…A number of authors have remarked on the hyperfine contact shift observed in some semiconductors during light irradiation [13][14][15][16] , particularly as the shift is proportional to the electron's polarization. Coles used this shift to make an absolute measurement of electron polarization in GaAs 16 .…”

“…Coles used this shift to make an absolute measurement of electron polarization in GaAs 16 . He noted that a resonance shift that persisted at long pumping times might have its origins in the nuclear polarization 17 , but no quantitative work on the origins of these long time shifts has appeared.…”

Temporal and spatial evolution of nuclear polarization in optically pumped InP

“…An upper limit is placed on J z by J 0 , usually taken to be ±0.25 depending on light helicity 22 , although recent calculations suggest that significant deviation from this value is possible.…”

“…22 Any dependence of these relaxation times on the normalized distance, d † = d/d 0 , from the surface, will be determined by the relaxation mechanism, as discussed below. The variable d 0 is the optical absorption depth and is photon energy dependent.…”