Absorption spectra of Er$^{3+}$ ions in Li$_6$Y(BO$_3$)$_3$ crystals

The optical absorption spectra of Li$_6$Y(BO$_3$)$_3$:Er$^{3+}$ crystals have been studied. The absorption lines corresponding to intraconfiguration electronic transitions from the $^4$I$_{15/2}$ ground state to the levels of excited states of Er$^{3+}$ ions have been found in the spectral range of 370–700 nm. The transitions to the $^4$F$_{9/2}$, $^4$S$_{3/2}$, $^2$H$_{11/2}$, $^4$F$_{7/2}$, $^4$F$_{5/2}$, $^4$F$_{3/2}$, $^2$H$_{9/2}$, and $^4$G$_{11/2}$ levels have been investigated in detail at 2 K. The number of lines observed for these transitions corresponds to the theoretical maximum for the $f$–$f$ electronic transitions in Er$^{3+}$ ions located in the noncubic crystal field. The narrowness of the lines observed and their number indicate that erbium occupies one regular position (specifically, the yttrium position). The energy levels of the excited states have been determined for the transitions under study.