Spectral hole burning ofEu2+in CaF2

Spectral hole burning on the $4f^7\left({}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}\right)\to 4f^{6}5d$ transition of ${\mathrm{Eu}}^{2+}$ is observed to occur by two mechanisms in Ca${\mathrm{F}}_2$:${\mathrm{Eu}}^{2+}$. At zero magnetic field, persistent spectral hole burning occurs by two-step photoionization where the hole burning survives cycling to room temperature. The observed hole spectrum is compared with a calculation which considers the small ${\mathrm{Eu}}^{2+}$ ground-state splitting and both the ground- and excited-state hyperfine interactions. In a magnetic field above 1 T, transient spectral hole burning occurs by population redistribution among the ground-state hyperfine levels. Long-lived (minutes) holes are observed for a half integral spin system. The resulting hole spectrum consists of holes, antiholes, and a nuclear spin-flip sideband and can be explained from the hyperfine interactions in the ground and excited states. The dominant hole decay occurs by phonon-induced transitions among hyperfine levels of the $M_S=-\frac{7}{2}$ and $M_S=-\frac{5}{2}$ electron-spin sublevels. Central hole linewidths of 200 MHz are observed at zero magnetic field. Hole linewidths at high magnetic fields are as narrow as 40 MHz but these are inhomogenously broadened by superhyperfine interactions with the $F$ nuclear spins. Estimates of hole-burning quantum efficiency are also obtained.