Optical measurement of the hyperfine splitting of theD21metastable state ofPr3+in LaF3by enhanced and saturated absorption spectroscopy

The sizable ground-state nuclear polarization which accompanies the optical pumping of the ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ (16 872-${\mathrm{cm}}^{-1\mathrm{}}$) $\sigma$ transition of ${\mathrm{Pr}}^{3+}$ in La${\mathrm{F}}_3$ was utilized to observe enhanced and saturated absorptions within the inhomogeneously broadened spectral line. The observed hyperfine splittings of the lowest ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ state are 3.7 and 4.7 MHz at 2.0°K. These splittings were fitted by a spin Hamiltonian $\mathcal{H}=D[I_z^2-\frac{I(I+1\mathrm{})\mathrm{}}{3}]+\left(\frac{E}{2}\right)(I_{+}^2+I_{-}^2)$, where $D=-1.10\mathrm{}\pm 0.05$ and $E=-0.48\mathrm{}\pm 0.05$ MHz. The observed linewidths (including the exciting laser linewidth) were 0.8 MHz. The deconvoluted optical transition linewidths (full widths at half-maximum) are 0.20±0.05 MHz.