Spectral hole burning by population storage in Zeeman sublevels of LaFsub3:Nd3+

We demonstrate a new mechanism for optical spectral hole burning in which population is stored in electronic Zeeman sublevels. This is shown by optical pumping the ground-state Kramers doublet of the ${}_{}{}^4{}_{}{}^{}\mathrm{I}_{9/2}^{}{}_{}{}^{}$ ${\mathrm{\leftrightarrows }}^2$ ${\mathrm{H}}_{11/2}$ (6248.9 Å) and ${}_{}{}^4{}_{}{}^{}\mathrm{I}_{9/2}^{}{}_{}{}^{}$ ${\mathrm{\leftrightarrows }}^4$ ${\mathrm{G}}_{5/2}$ (5777.1 Å) transitions of ${\mathrm{LaF}}_3$:${\mathrm{Nd}}^{3+}$ with a narrow-band cw dye laser. Experiments were carried out at 1.6 K in external fields of ∼100 G. The Zeeman shift of the resulting antiholes and holes makes an assignment of these structures possible. Hole recovery times of hundreds of milliseconds are shorter than expected at these low temperatures and fields and provide evidence for optical spectral diffusion caused by mutual spin flips in the ground state.