Helicity asymmetry of optically pumped NMR spectra in GaAs

The origin of $\sigma \pm$ asymmetries in the optically pumped nuclear magnetic resonance (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.

Figure 1

(Color online) (a) Gaussian fits to $\sigma \pm {}^{71}\text{G}\text{a}$ OPNMR spectra for $t=0.5\text{s}$, $P=123\text{mW}$, $E=1.509\text{eV}$, $B_0=9.4\text{T}$, $T=9.5\text{K}$. (b) $R_c$, (c) $R_I$, and (d) $R_{\nu }$ under these conditions. Dashed lines indicate average values.

Figure 2

(a) Power dependence of $A$ and $B$, defined in text, for $E=1.509\text{eV}$, $T_0=5\text{K}$, and $\sigma =5{\mathrm{mm}}^2$. (b) Extracted $T$ versus power. Inset: Extracted $\Lambda$ over this temperature range.

Figure 3

(Color online) (a) Excitation spectrum of $⟨S_z⟩$ in SI GaAs from ${}^{71}\text{G}\text{a}$ data (Ref. 6). $S_s$ values (open) are bounded by $S_0$ values (filled), shown for $\sigma -$ (squares) and $\sigma +$ (triangles) light. (b) Spectrum of $\tau /T_{1e}$ from the same data. Overlaid is a theoretical prediction made by our previous model for OPNMR (Ref. 5). Unphysical (negative $\tau /T_{1e}$) data points were removed and dashed lines indicate the free exciton energy. Inset: Effect of strain on $\Lambda$ extracted from ${}^{71}\text{G}\text{a}$ data (Refs. 9, 13).