Control of relaxation properties of a macroscopic nuclear spin ensemble

Macroscopic spin ensembles in solids are powerful platforms for quantum sensing and precision metrology. A key challenge is controlling the nuclear spin population relaxation time $T_1$, which can become prohibitively long at cryogenic temperatures due to phonon freeze-out. We demonstrate optical control of the $T_1$ relaxation time of the $^{207}$Pb nuclear spin ensemble in lead-containing ferroelectric crystals PbTiO$_3$ (PT) and (PbMg$_{1/3}$Nb$_{2/3}$O$_3$)$_{2/3}$-(PbTiO$_3$)$_{1/3}$ (PMN-PT). Using X-band electron paramagnetic resonance (EPR) spectroscopy at 10 K, we characterize light-induced paramagnetic centers created by 405 nm laser illumination. In PT, we observe paramagnetic Pb$^{3+}$ centers and their hyperfine interaction with nearby nuclear spins. In PMN-PT, we identify two populations: isotropic Pb$^{3+}$ centers and anisotropic Ti$^{3+}$ centers occupying $d$-orbitals, with spin number densities of $(2.5 \pm 1.0) \times 10^{17}$ cm$^{-3}$ and $(4.1 \pm 1.7) \times 10^{17}$ cm$^{-3}$, respectively. Power-dependent EPR measurements enable extraction of spin relaxation times. We investigate the ionization and recombination dynamics of these transient paramagnetic centers. Using saturation-recovery nuclear magnetic resonance, we demonstrate that laser illumination reduces the $^{207}$Pb nuclear $T_1$ by approximately a factor of two, from $(17 \pm 2)$ s to $(7 \pm 1)$ s at 4.6 MHz, and from $(1550 \pm 40)$ s to $(850 \pm 70)$ s at 40 MHz. We develop a model relating the nuclear relaxation rate to the density of photoinduced paramagnetic centers. This optical control of nuclear spin relaxation provides a pathway toward accelerated thermal polarization and dynamic nuclear polarization in solid-state NMR-based precision measurements, including searches for axion-like dark matter.