Switchable resonant hyperpolarization transfer from phosphorus-31 nuclei to silicon-29 nuclei in natural silicon

Silicon has been the backbone of the microelectronics industry for decades. As spin-based technologies continue their rapid development, silicon is emerging as a material of primary interest for a number of these applications. There are several techniques that currently exist for polarizing the spin-1/2 silicon-29 nuclei, which account for 4.7% of the isotopic makeup of natural silicon (the other two stable isotopes, silicon-28 and silicon-30, have zero nuclear spin). Polarized silicon-29 nuclei may find use in quantum computing (QC) implementations and magnetic resonance (MR) imaging. Both of these applications benefit from the extremely long T1 and T2 of the silicon-29 nuclear spins. However, the lack of interactions between the silicon-29 nuclei and their surroundings that allow for these long relaxation times also means that it is difficult to find a source of spin-polarization that effectively couples to the silicon-29 spin ensemble. We identify and exploit a field-dependent, frequency-matched resonant transfer process between phosphorus-31 donor and silicon-29 nuclear spins in natural silicon to efficiently hyperpolarize the bulk silicon-29 to over 6%. This all-optical technique requires no microwave irradiation, and the coupling can be switched off to recover the ultra-long nuclear spin relaxation lifetimes of silicon-29. This switchable hyperpolarization technique significantly enhances the usefulness of silicon-29 spins in QC and MR imaging applications.