NMR study of optically hyperpolarized phosphorus donor nuclei in silicon

TL;DR

This study demonstrates optical hyperpolarization of phosphorus donor nuclei in natural silicon at low temperature and high magnetic field, revealing broad spectral linewidths and shorter nuclear spin relaxation times compared to enriched silicon, with implications for quantum information.

Contribution

It provides the first detailed NMR analysis of optically hyperpolarized phosphorus in natural silicon, highlighting isotope and strain effects on linewidth and relaxation times.

Findings

Spectral linewidth of 30 kHz due to isotope effects and strain variations.

Shorter nuclear spin T1 and build-up times in natural silicon compared to enriched samples.

Laser irradiation further shortens the phosphorus nuclear spin coherence time.

Abstract

We use above-bandgap optical excitation, via a 1047 nm laser, to hyperpolarize the $^{31}$P spins in low-doped (N$_D =6\times10^{15}$ cm$^{-3}$) natural abundance silicon at 4.2 K and 6.7 T, and inductively detect the resulting NMR signal. The $30$ kHz spectral linewidth observed is dramatically larger than the 600 Hz linewidth observed from a $^{28}$Si-enriched silicon crystal. We show that the observed broadening is consistent with previous ENDOR results showing discrete isotope mass effect contributions to the donor hyperfine coupling. A secondary source of broadening is likely due to variations in the local strain, induced by the random distribution of different isotopes in natural silicon. The nuclear spin T$_1$ and the build-up time for the optically-induced $^{31}$P hyperpolarization in the natural abundance silicon sample were observed to be $178\pm47$ s and $69\pm6$ s respectively, significantly shorter than the values previously measured in $^{28}$Si-enriched samples under the same conditions. We also measured the T$_1$ and hyperpolarization build-up time for the $^{31}$P signal in natural abundance silicon at 9.4 T to be $54\pm31$ s and $13\pm2$ s respectively. The shorter build-up and nuclear spin T$_1$ times at high field are likely due to the shorter electron-spin T$_1$, which drives nuclear spin relaxation via non-secular hyperfine interactions. At 6.7 T, the phosphorus nuclear spin T$_{2}$ was measured to be $16.7\pm1.6$ ms at 4.2 K, a factor of 4 shorter than in $^{28}$Si-enriched crystals. This was observed to further shorten to $1.9\pm0.4$ ms in the presence of the infra-red laser.