Optical pumping of Si29 nuclear spins in bulk silicon at high magnetic field and liquid helium temperature

High nuclear spin polarization is required for nuclear-spin based quantum computers and it also serves as a probe to investigate electron-nucleus cross-relaxation processes in solids. To increase the Si29 polarization in bulk silicon, we have explored the dynamic-nuclear-polarization capabilities of optical pumping at a temperature of 4K and a magnetic field of 7T with a Si29-enriched sample. The Si29 signals are observed with nuclear-magnetic- resonance spectroscopy. We have demonstrated Si29 polarizations up to 0.25%, nearly an order of magnitude larger than the previous optical-pumping record. Data analysis has stimulated the investigation of above-band-gap excitations. Our preliminary experiments in this regime look promising for further increases in Si29 polarization in the sample volume close to the exposed surface. We have also investigated the dependence of the Si29 polarization on temperature, magnetic field, and doping.