Modelling of OPNMR phenomena using photon energy-dependent 〈S_z〉 in GaAs and InP

We have modified the model for optically-pumped NMR (OPNMR) to incorporate a revised expression for the expectation value of the z-projection of the electron spin, 〈S_z〉 and apply this model to both bulk GaAs and a new material, InP. This expression includes the photon energy dependence of the electron polarization when optically pumping direct-gap semiconductors in excess of the bandgap energy, E_g. Rather than using a fixed value arising from coefficients (the matrix elements) for the optical transitions at the k=0 bandedge, we define a new parameter, S_opt(E_ph). Incorporating this revised element into the expression for 〈S_z〉, we have simulated the photon energy dependence of the OPNMR signals from bulk semi-insulating GaAs and semi-insulating InP. In earlier work, we matched calculations of electron spin polarization (alone) to features in a plot of OPNMR signal intensity versus photon energy for optical pumping (Ramaswamy et al., 2010). By incorporating an electron spin polarization which varies with pump wavelength into the penetration depth model of OPNMR signal, we are able to model features in both III-V semiconductors. The agreement between the OPNMR data and the corresponding model demonstrates that fluctuations in the OPNMR intensity have particular sensitivity to light hole-to-conduction band transitions in bulk systems. We provide detailed plots of the theoretical predictions for optical pumping transition probabilities with circularly-polarized light for both helicities of light, broken down into illustrative plots of optical magnetoabsorption and spin polarization, shown separately for heavy-hole and light-hole transitions. These plots serve as an effective roadmap of transitions, which are helpful to other researchers investigating optical pumping effects.