Physical insights from a penetration depth model of optically pumped NMR

Abstract

A model of optically pumped NMR (OPNMR) behavior in GaAs that connects the photon energy dependence of the OPNMR signal intensity for (69)Ga with different polarizations of light has been developed. Inputs to this model include experimental conditions--external magnetic field (B(0)), temperature (T), and optical pumping parameters (tau(L), laser helicity)--as well as parameters that arise from sample-specific characteristics--electron spin lifetime (T(1e)), electron lifetime (tau(e)), electron-nuclear correlation time (tau(c)), and sample thickness (z). These various inputs affect the profile of the OPNMR signal intensity as a function of photon energy (E) in a predictable manner. Therefore, the profile can serve as a composite fingerprint by which individual parameters can be inferred when not known. Characteristics of the profile include the photon energy for maximum OPNMR signal intensity and the intensity ratio between sigma(+) and sigma(-) light.

Helicity asymmetry of optically pumped NMR spectra in 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.…”

“…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.…”

“…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.…”

“…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.…”

Optically rewritable patterns of nuclear magnetization in gallium arsenide

“…Th ese bound electrons have a strong contact hyperfi ne interaction with nearby nuclei that undergo dynamic nuclear polarization (DNP) driven by spin-exchange modulation of the hyperfi ne fi eld. Th e DNP rate decays exponentially with distance from the recombination centre, and scales linearly with probability of trapping at a recombination centre 10,11 . Both the magnitude and sign of the polarization of nuclei undergoing DNP may be controlled by the polarization of the incident light.…”