Optically detected magnetic-resonance study of Zn-doped InP: Nuclear-spin polarization at PIn antisites

Modelling of OPNMR phenomena using photon energy-dependent 〈Sz〉 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.

Optical pumping in indium phosphide: 31P NMR measurements and potential for signal enhancement in biological solid state NMR

The initial results of optically-pumped, directly-detected NMR experiments on InP are reported. At low temperatures (4.2 K and above) and in a 9.39 T magnetic field, irradiation of a sample of an undoped InP wafer with 835-nm-wavelength light from a diode laser enhances the spin polarization of 31P nuclei near the sample surface in a manner that depends on the polarization of the light. The nuclear spin polarization is monitored by direct radio-frequency detection of nuclear free induction-decay signals. The maximum nuclear spin polarization (Szn> generated by optical pumping is approximately - 0.004, corresponding to a spin temperature of -0.5 K. The nuclear spin polarization may be limited in these experiments by the use of a high photon energy (1.484 eV) relative to the InP band gap (1.423 eV at low temperatures). It is proposed that optically-pumped InP may be useful as a source of enhanced nuclear spin polarizations for solid state NMR measurements on organic and biological overlayers deposited on InP substrates. Estimates are given for the magnitude of the spin polarization and the efficiency of the polarization transfer from the semiconductor substrate to the overlayer that would be required to permit solid state NMR measurements on sub-nanomole quantities of molecules in the overlayer. These estimates appear well within the range of possibility.