Optical Hyperpolarization of Donor Electron Spins in Silicon Using a Widely-Tunable DBR Laser

We report on measurements of isotopically enriched silicon samples (45 ppm Si-29) with very low donor densities (1e12-1e14 cm$^{-3})$. Pushing the donor density limit necessitates optimizing the experimental sensitivity and enhancing spin polarization. Donor spin polarization greater than Boltzmann (hyperpolarization) can be established by optically exciting the no-phonon bound exciton transitions followed by Auger recombination. We established significant donor spin hyperpolarization with a distributed Bragg reflector laser, tunable across all donors including P-31 and Bi-209. For phosphorus doped silicon we observed combined electron-nuclear spin polarizations of 100{\%}-200{\%} depending on donor density. For bismuth donors at a clock transition (B $=$ 80.6 mT), we observed about 50{\%} spin polarization, and 500{\%} away from the clock transition. This increase in spin polarization allows for single shot measurement of low density samples (4e12 P/cm$^{3}$. and 1e14 Bi/cm$^{3})$.