Simultaneous Subsecond Hyperpolarization of the Nuclear and Electron Spins of Phosphorus in Silicon by Optical Pumping of Exciton Transitions

We demonstrate a method which can hyperpolarize both the electron and nuclear spins of ${}^{31}\mathrm{P}$ donors in Si at low field, where both would be essentially unpolarized in equilibrium. It is based on the selective ionization of donors in a specific hyperfine state by optically pumping donor bound exciton hyperfine transitions, which can be spectrally resolved in ${}^{28}\mathrm{Si}$. Electron and nuclear polarizations of 90% and 76%, respectively, are obtained in less than a second, providing an initialization mechanism for qubits based on these spins, and enabling further ESR and NMR studies on dilute ${}^{31}\mathrm{P}$ in ${}^{28}\mathrm{Si}$.

Figure 1

The neutral donor bound exciton transition and its low-field Zeeman splittings are shown, along with a schematic of the selective optical polarization mechanism. On the left are the splittings of the neutral donor ground state ($D^0$) and the bound exciton ground state ($D^{0}X$). In the center the 12 allowed optical transitions between the four $D^0$ hyperfine states and the four $D^{0}X$ states are indicated, numbered from 1 to 12 in order of increasing energy. On the right is a simplified schematic of the optical polarization mechanism for the pump laser tuned to line 6.

Figure 2

PLE spectra of the ${}^{31}\mathrm{P}$ bound exciton in an $n$-type ${}^{28}\mathrm{Si}$ sample, revealing electron and nuclear polarizations obtained by selective optical pumping. At the bottom is a spectrum without any resonant pumping, showing essentially equal populations in the four $D^0$ states. The upper spectra show the PLE signal when a strong pump field is tuned to either line 6 or line 8, or the half-height point on the high energy side of line 6, which is indicated as $6^{\prime }$.

Figure 3

The transient behavior of lines revealing the four $D^0$ populations are shown, with the pump laser at line 6 in all cases. Pump and probe are switched on simultaneously at $t=0$. The dashed curves show the transients with the populations in the fully polarized state, while the solid curves show the transients when the populations have been fully equilibrated. The middle curve in (c) shows the transient after the fully polarized sample has recovered in the dark for 20 min.