Spin Transfer from an Optically Pumped Alkali Vapor to a Solid

We report enhancement of the spin polarization of ${}^{133}\mathrm{Cs}$ nuclei in CsH salt by spin transfer from an optically pumped cesium vapor. The nuclear polarization was 4.0 times the equilibrium polarization at 9.4 T and $137°\mathrm{C}$, with larger enhancements at lower fields. This work is the first demonstration of spin transfer from a polarized alkali vapor to the nuclei of a solid, opening up new possibilities for research in hyperpolarized materials.

Figure 1

NMR spectra of ${}^{133}\mathrm{Cs}$ in CsH salt at 9.4 T. The cell contains a film of CsH, a separate droplet of Cs metal to provide a vapor for optical pumping, and 11 Torr of ${\mathrm{N}}_2$ gas. The dashed trace is the NMR signal with no optical pumping; the red (positive) and orange (negative) traces show the hyperpolarized NMR signal obtained when the alkali vapor is pumped with ${\sigma }_{+}$ and ${\sigma }_{-}$ light, respectively, at the $6S_{1/2}\to 6P_{1/2}$ absorption line. Although the negative signal enhancement was larger than the positive enhancement in this case, the two were much more symmetric in other trials.

Figure 2

The experimental apparatus. The Ti:sapphire laser light was routed through an optical shutter (OS) and into multimode fiber (MMF). At the output of the fiber coupler (FC) the light was polarized by the beam-splitter cube (PBS) and quarter-wave plate (QWP). A photodiode (PD) was used to verify optical absorption prior to the NMR experiment. Inside the NMR probe (inset), two temperature sensors (TS) were used to measure the cell and probe temperatures; the oven temperature was regulated by a resistive heater (HTR) controlled by the PC.

Figure 3

Modeled diffusion currents $J_S$ and $J_I$ for a ${}^{133}\mathrm{Cs}$ vapor at a temperature of $135°\mathrm{C}$, with 11 Torr of nitrogen buffer gas in magnetic field $B=9.4\mathrm{T}$, pumped with $100\mathrm{mW}{\mathrm{cm}}^{-2}$ of ${\sigma }_{+}$ light. Note that $J_S$ is actually negative at resonance (c) at $-87.5\mathrm{GHz}$; the absolute value has been taken to show its magnitude. Inset shows the allowed transition (a) and the forbidden transitions (b), (c), and (d) for ${\sigma }_{+}$ light; these correspond to the resonances shown in the plot. Laser detuning is measured with respect to the $6S_{1/2}\to 6P_{1/2}$ transition at zero magnetic field.

Figure 4

Dependence of the measured ${}^{133}\mathrm{Cs}$ NMR enhancement (points) on ${\mathrm{N}}_2$ number density. Also plotted are $J_S$ and $J_I$, the calculated electron and nuclear spin flux to the cell walls, for which uniform optical power density $p=100\mathrm{mW}{\mathrm{cm}}^{-2}$ and a temperature of $135°\mathrm{C}$ were assumed. The ${\mathrm{N}}_2$ filling pressure has been converted to a number density in amagat, where 1 amg is equal to the number density of an ideal gas at STP. Not all data points were taken at the same temperature; instead, the maximum enhancement for each cell is plotted. Radiation trapping was not considered in the numerical model.